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W1

AGES AND CHEMICAL EVOLUTION OF TEPHRA SEQUENCES IN
SOUTHWESTERN NEVADA

By

Kristin T. Huysken

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Geological Sciences

1996

ABSTRACT

AGES AND CHEMICAL EVOLUTION OF TEPHRA SEQUENCES IN
SOUTHWESTERN NEVADA

By

Kristin T. Huysken

Compositional and 40Ar/39Ar ages in three tephra sequences from the
southwestern Nevada record the chemical history and timing of eruption
from the parent magmas producing these volcanoclastic sequences.
Compositional changes within two of the tephra sequences, the Post-Grouse
Canyon and the Pre-Rainier Mesa tephra sequences, correspond with
changes in 40Arl39Ar ages.

The Post-Grouse Canyon tephra sequence is divided into three main
corresponding age and compositional groups. The compositional groups are
not easily related by crystal fractionation or magma mixing and are
interpreted to represent distinct magma batches derived from separate
sources. A highly compositionally variable layer occurring within the middle
age group, is interpreted to reflect the erupiion of an additional distinct
magma type. This pattern of emplacement and eruption of unrelated
magmas recorded in the Post-Grouse Canyon tephra sequence mimics the
inferred processes operating in large-volume magmatic systems in this

region.

The Pre-Rainier Mesa tephra sequence is divided into two discrete age
and compositional groups. The upper sequence is the age and chemical
equivalent of the high-silica, low Th/Nb portion of the overlying Rainier Mesa
Tufi‘ (11.6 Ma). The sequence is the age equivalent of the underlying Tiva
Canyon Tufl' (12.7 Ma), however is compositionally unlike the Tiva Canyon
Tufi‘. The lower Pre-Rainier Mesa tephra sequence is consistent with at least
two magma miidng combinations. Both combinations involve mixing of
magma compositionally similar to the low Th/Nb, high silica group from the
Rainier Mesa Tufl'. The inferred presence of Rainier Mesa-like magma at
12.7 Ma suggests that more than one magma pulse was emplaced from the
source rocks that produced the Rainier Mesa magma. The emplacement of
multiple magma pulses is in contrast to the inferred single pulse
emplacement style for the large-volume magmatic systems in the region.

The third tephra sequence, the Pre-Ammonia Tanks tephra sequence,
does not exhibit a compositional change with respect to stratigraphic
position. In general, the entire compositional range (70-78% SiOz) is present
throughout the sequence. This compositional variation is consistent with the
overlying Ammonia Tanks ash-flow sheet. Ages for the Pre-Ammonia Tanks

tephra sequence are not well constrained and may be partially reset.

For Erik

iv

ACKNOWLEDGMENTS

Above all, I thank my advisor, Tom Vogel, for guidance, support, and
friendship that has surpassed all of my expectations. I am fortunate to have
worked closely with someone so devoted to his students and so committed to
geology. I thank Tom with my deepest respect and appreciation.

My committee, Bill Cambray, Dave Matty, and Kaz Fujita, are
thanked for their review of this dissertation and guidance throughout the
course of this study. Additional thanks to Bill Cambray for facilitating many
insightful discussions during this study.

I would like to thank Tim Flood for his help, sound advice, and
”professional” discussions in the field.

Fellow students, Ben Saltoun, and Rich Stern deserve special thanks
for the many occasions when they shared their ideas and philosophy related
to geology, graduate school, and life.

I thank John Brannen for his detailed sample collection, field notes,
and related discussions; Jason Price for his help, knowledge, and
camaraderie in the field; Ed Masters and Paul DeKonig for the wonderful job
they did preparing samples of the Pre-ammonia Tanks tephra sequence; and

Rick Warren of Los Alamos National Laboratory and Bill McKinnis of

V

Lawrence Livermore National Laboratory for their help locating outcrops of
the Pre-ammonia Tanks tephra sequence.

This research was aided by a Grant-in-Aid of Research from Sigma Xi,
The Scientific Research Society, and the Lipman Research Award for 1994
(Grant No. 5372-94) from the Geological Society of America. In addition,
Michigan State University provided fellowship funds that undoubtedly
hastened the completion of this dissertation.

Finally, I thank my family for emotional support, especially my
mother, who has diligently plugged through everything I have written (sorry
this one is so long). As always, many thanks to Maureen Walton, Alex
Guimaraes, and Matt Casselton for love and support, but mostly for

friendship.

TABLE OF CONTENTS

List of Tables ....................................................................................................... ix
List of Figures ...................................................................................................... x
CHAPTER 1
INTRODUCTION ............................................................................................... 1
Purpose of Study ...................................................................................... 1
General Geology ....................................................................................... 5
The Grouse Canyon Tufl‘ ......................................................................... 9
The Post-grouse Canyon Tephra Sequence .......................................... 10
The Deadhorse Flat, Wahmonie and Calico Hills Formations ............ 12
The Topopah Springs Tufi' .................................................................... 13
The Pah Canyon and Yucca Mountain Tufl'sl4
The Tiva Canyon Tufl‘ ............................................................................ 14
The Pre-rainier Mesa tephra sequence ................................................. 15
The Rainier Mesa Tufl' ........................................................................... 16
The Pre-ammonia Tanks tephra sequence ............................................ 17
The Ammonia Tanks Tufi' ...................................................................... 18
CHAPTER 2
METHODS ........................................................................................................ 20
Sample selection for chemical analysis ................................................. 20
Glassy pumice vs. bulk rock samples .................................................... 22
Sample selection and age calculation for “Ar/”Ar analysis ................ 23
CHAPTER 3
THE POST-GROUSE CANYON MAGMATIC SYSTEM ................................ 28
Previous Work ......................................................................................... 28
”Ar/”Ar radiometric ages ...................................................................... 28
Geochemistry .......................................................................................... 29
Chemical variation with stratigraphic position ......................... 30
Chemical groups .......................................................................... 39
Mineral Chemistry ................................................................................. 45
Correlation with regional volcanostratigraphic units .......................... 48
Summary ................................................................................................ 50
CHAPTER 4

THE PRE-RAINIER MESA MAGMATIC SYSTEM ....................................... 52
Previous Work ........................................................................................ 52
4°Ar/39Ar ages .......................................................................................... 54
Geochemistry .......................................................................................... 56
Mineralogy .............................................................................................. 62
Fe-Ti oxide temperatures ....................................................................... 64
Summary ................................................................................................. 68

CHAPTER 5

THE PRE-AMMONIA TANKS MAGMATIC SYSTEM ................................... 69
Previous Work ......................................................................................... 69
40Arl39Ar age dates ................................................................................... 70
Geochemistry ........................................................................................... 72
Summary ................................................................................................. 75

CHAPTER 6

DISCUSSION .................................................................................................... 78
The Post-grouse Canyon tephra sequence ............................................. 78

Magma Mixing .............................................................................. 79
Fractional Crystallization ............................................................ 86
Partial Melting .......................................................................... . ...88
Magmatic Sources ...................................................................... 101
The Pre-rainier Mesa tephra sequence ................................................ 103
The Pre-ammonia Tanks tephra sequence .......................................... 109

CHAPTER 7

CONCLUSIONS .............................................................................................. 1 12
The Post-grouse Canyon tephra sequence ........................................... 1 12
The Pre-rainier Mesa tephra sequence ................................................ 1 15
The Pre-ammonia tanks tephra sequence ........................................... l 18

APPENDIX 1. Analytical Techniques ........................................................... 120
X-ray fluorescence (XRF) ...................................................................... 120
Instrumental Neutron Activation Analysis (INAA) ............................ 12 1
Electron Microprobe Analysis .............................................................. 123

APPENDIX 2. Major mdde and trace element analyses (normalized to
100%) ..................................................................................................... 124

APPENDIX 3. Mineral analyses .................................................................... 162

APPENDIX 4. 40Arli’i’Ar Analyses .................................................................. 178

BIBLIOGRAPHY ............................................................................................. 204

viii

LIST OF TABLES

Table 1. Representative 40Ar/39Ar analyses and weighting factors
for weighted mean age calculation .................................................... 26

Table 2. Regression analysis of crystal fractionation for the
Post-grouse Canyon tephra sequence ................................................ 90

Table 3. Regression analysis producing sample 7F from two
mixing combinations ......................................................................... 108

Table 4. Detection limits and standard deviations for trace
element analyses ............................................................................... 12 1

Table 5. Standard errors for INAA analysele2

ix

LIST OF FIGURES

Figure 1. Map of the Southwest Nevada Volcanic Field indicating calderas.
Sampled locations are labeled ‘pgc' (Post-Grouse Canyon tephra
sequence), 'prm' (Pre-Rainier Mesa tephra sequence) and 'p at'
(Pre-Ammonia Tanks tephra sequence) ....................................................... 7

Figure 2. Generalized stratigraphy of volcanic units from the Southwest
Nevada Volcanic Field, erupted from 13.7 to 1 1.45 Ma. Ages,
volumes, and unit names are for all units except the tephra
sequences are from Sawyer et a1. (1994) ..................................................... 8

Figure 3. Bedded tephra deposits from the Post-Grouse Canyon tephra
sequence (a), and a small ash-flow layer contained within the -
tephra sequence (b) ..................................................................................... 1 1

Figure 4. Range of 40Ar/39Ar ages for individual tephra layers from the
Post-Grouse Canyon tephra sequence ...................................................... 29

Figure 5. Zr vs. Si02 for the Post-Grouse Canyon tephra sequence and the
Grouse Canyon Tufl' ................................................................................... 30

Figure 6. Variation of individual glassy pumice fragments and bulk tephra
samples from the Post-Grouse Canyon tephra sequence ......................... 3 1

Figure 7. CaO vs. FeO for the bulk tephra samples from the Post-Grouse
Canyon tephra sequence ............................................................................ 34

Figure 8. Variation of glassy pumice fragments with respect to stratigraphic
position, from the Post-Grouse Canyon tephra sequence ........................ 35

Figure 9. (a) Zn vs. Zr, and Rb/Sr vs. Zr discrimination diagrams for the
middle and upper portions of the Post-Grouse Canyon tephra
sequence ..................................................................................................... 40

Figure 10. Trace element discrimination diagrams for the highly variable
layer and the chemical groups from the upper and
middle portions of the Post-Grouse Canyon tephra sequence ................ 41

X

Figure 11. Y vs. Zr variation diagram discriminating the lower portion of the
Post-Grouse Canyon tephra sequence from other chemical groups
within the sequence ................................................................................. 44

Figure 12. K20, NazO, CaO, and BaO variation of feldspar from the Post-
Grouse Canyon tephra sequence with stratigraphic depth ................... 46

Figure 13. Variation of FeO and Sr with Depth beneath the Rainier Mesa
ash-flow sheet. (X) indicates individual glassy pumice fragments.
All other plot symbols represent small ash-flow layers within the
Pre-Rainier Mesa tephra sequence. (b) Variation of Th
discriminating the lower (older) and upper (younger) Pre-Rainier
Mesa tephra sequence ............................................................................. 53

Figure 14. Range of 4°Ar/39Ar ages for individual tephra layers from the Pre-
Rainier Mesa tephra sequence ................................................................ 55

Figure 15. Trace element variation diagrams comparing individual glassy
pumice fragments from the lower Pre-Rainier Mesa tephra
sequence, the upper Pre-Rainier Mesa tephra sequence, and the
Rainier Mesa Tufl' .................................................................................... 58

Figure 16. Trace element variation diagrams comparing the Tiva Canyon
ash-flow sheet, the lower Pre-Rainier Mesa tephra sequence, and
the upper Pre-Rainier Mesa tephra sequence ........................................ 59

Figure 17. Discrimination diagrams of Cs vs. Si02, and Rb vs. Si02, for the
lower and upper Pre-Rainier Mesa tephra sequence ............................. 6 1

Figure 18. Feldspar ternary diagram comparing feldspar compositions for
the lower and upper Pre-Rainier Mesa tephra sequence ....................... 63

Figure 19. Comparison of Temperature vs. Si02 for Tiva Canyon ash-flow
sheet, the Pre-Rainier Mesa tephra sequence, and the Rainier
Mesa ash-flow sheet ................................................................................. 66

Figure 20. Range of ”Ar/”Ar ages for individual tephra layers from the Pre-
Ammonia Tanks tephra sequence ........................................................... 71

Figure 21. Trace element discrimination of the Pre-Ammonia Tanks tephra
sequence, the Rainier Mesa ash-flow sheet, and the Ammonia
Tanks ash-flow sheet ................................................................................ 73

Figure 22. Zr vs. Y comparing the Rainier Mesa ash-flow sheet, the Pre-
ammonia Tanks tephra sequence, and the Ammonia Tanks ash-
flow sheet .................................................................................................. 76

Figure 23. (a) Trace element ratio diagram of Rb/Sr vs. Zr/Y, and companion
plot, illustrating that the middle and upper portions of the Post-
Grouse Canyon tephra sequence are not related by magma
mixing. (b) Trace element ratio diagram of Rb/Y and Zn/Sr, and
companion plot, illustrating that the lower and upper portions of
the Post-Grouse Canyon tephra sequence are not related by
magma mixing. (0) Trace element ratio diagram of Rb/Zr vs. Sr/Y,
and companion plot, illustrating that the lower and middle
portions of the Post-Grouse Canyon tephra sequence are related
by magma mixing ...................................................................................... 80

Figure 24. (a) Trace element ratio diagram of Rb/Zn vs. Sr/Y and Rb/Y vs.
Sr/Zr, and companion plots, illustrating that the middle portion of
the Post-Grouse Canyon tephra sequence cannot be produced by
mixing of the high Zr group from the highly variable layer with
any other chemical group ......................................................................... 82

Figure 25. Variation diagrams of Zr, CaO, and Sr vs. Si02, illustrating
distinct trends for the lower, middle, and upper portions of the
Post-Grouse Canyon tephra sequence .................................................... 87

Figure 26. A1203 and CaO variation diagrams evaluating potential
fractionation trends for the Post-Grouse Canyon tephra sequence ....... 89

Figure 27. Y vs. Zr discrimination diagram showing that the lower, middle
and upper portions of the Post-Grouse Canyon (PGC) tephra
sequence cannot be related by partial melting of a single source .......... 94

Figure 28. Rb vs. Si02 and Zr vs. Si02 variation diagrams, evaluating partial
melting of a single source, for the middle and upper portions of
the Post-Grouse Canyon tephra sequence ............................................... 95

Figure 29. Variation of Sr and Zr with respect to Si02, evaluating partial
melting of a single source for the origin of the lower and middle
portions of the Post-Grouse Canyon tephra sequence ............................ 97

Figure 30. Variation diagram of Rb vs. Zr, and potential melting curves
relating the Grouse Canyon Tuff (upper) and the Topopah
Springs Tufl‘ (lower) to the middle and upper portions of the Post-
Grouse Canyon tephra sequence .............................................................. 99

Figure 31. Trace element ratio diagram of Rb/La vs. Zr/Th, and companion
plot Rb/La vs. Th/La, evaluating mixing of low silica Tiva Canyon
magma and low Th/Nb Rainier Mesa magma to produce the lower
Pre-Rainier Mesa tephra sequence ........................................................ 106

Chapter 1

INTRODUCTION

Purpose of Study

Chemically and mineralogically zoned ash-flow sheets provide good
evidence that the source magmas were also zoned (Lipman et al., 1966;
Smith 1979; Hildreth 1981; Mahood 1981; and Baker 1985). Because of this,
zoned ash-flow sheets have been studied extensively to evaluate the origin
and evolution of zoned magma bodies. In almost all of these studies, the.
inferred trends towards the top of the magma bodies are increasing silica and
HzO, and decreasing temperature and phenocrysts.

Tephra sequences often associated with zoned ash-flow sheets receive
less attention than the associated ash-flow sheets. Typically, these deposits
are considered to be merely pre-eruptive “burps” from the uppermost portion
of the magma body, and are often neglected in studies of large-volume
magmatic systems. However, smaller volume tephra sequences may be far
more useful in interpreting the petrolog'c history of an area than is generally
recognized. For example, a recent ”Ar/”Ar age study on sanidine

phenocrysts from the Laacher See Tephra by van den Bogaard (1995) records

a complex history of emplacement, crystallization, and eruption of a smaller
volume (< 5 km3) tephra deposits.

In the Southwest Nevada Volcanic Field, major caldera forming
eruptions led to the formation of large-volume, chemically zoned, ash-flow
sheets with smaller volume tephras intercollated between the large ash-flow
deposits. These small units may provide valuable information central to our
understanding of the origin and evolution of these voluminous magmatic
systems. This study focuses on these smaller volume tephra sequences, and
their relationship to the large-volume ash-flow sheets.

These small-volume tephra sequences may be used to determine the
processes that modified magmatic systems and ultimately led to the eruption
of the large volume ash-flow sheets. Furthermore, they may provide
information related to the time scales over which these processes occurred
(that is, the rates of magmatic evolution).

The central hypotheses of this study are: To investigate the chemical
and age variations of individual tephra sequences in order to determine the
processes and rates of these processes that formed the larger magmatic
systems.

The Southwest Nevada Volcanic Field contains four very large volume,
chemically zoned ash-flow sheets (>900 km3) and at least eight other ash-flow
sheets larger than 100 km3. Although closely related in time and place,

many of these very large eruptive deposits have distinct chemical

compositions. Associated with many of these ash-flows, are sequences of
smaller volume tephra deposits that record numerous, small-volume,
eruptive events. These tephra sequences consist of pumice and ash-fall
deposits, and small volume ash-flow layers. Some of the tephra sequences
have significant chemical variation, indicating that they represent more than
merely the uppermost portion of a large fractionating magma body (Huysken
et al., 1994). Instead, these tephra sequences record magmatic changes that
took place in the system between eruption of the major ash-flow sheets. They
provide a unique glimpse into a large, dynamic magmatic system.

Compositional changes, related to changing stratigraphic position,
occur in the tephra sequences. Because the tephra sequences represent a
time sequence, it is inferred that these chemical changes reflect magmatic
changes occurring as these tephras were erupted. Relating chemical changes
in these sequences to magmatic processes (e.g., crystal fractionation, magma
mixing), provide insight into the magmatic development of the associated
large-volume ash-flow sheets. Chemical changes along with changes in
radiometric ages throughout these tephra sequences, provide information
concerning the rates of the petrologic processes that chemically modified
these systems.

Crystal fractionation, magma mixing and assimilation, or partial
melting of a single source are models that can be tested by the variation in a

tephra sequence. If these major petrologic processes (or combinations of

them) cannot account for the observed variation from layer to layer, the null
hypothesis (that these layers are unrelated and therefore must represent
melts from a different sources) must be concluded. Understanding the
processes that control chemical variation of these sequences is just one step
in understanding the dynamics of magmatic systems. Equally significant,
are the rates at which magmatic processes occur.

Theoretical and experimental work establishing rates of magmatic
processes (Spera and Crisp, 1981; Crisp, 1984; Trial and Spera, 1990; Wolff
et al., 1990; Christiansen and DePaolo, 1993; de Silva and Wolff, 1994; and
others) provide only first order estimates on the timing of these processes.
Variables used in theoretical and experimental models such as heat
capacities, diffusion coefficients, amount of undercooling, temperature,
pressure, volume, and chamber shape, may be poorly constrained. In natural
systems, more direct means for measuring rates of change must be employed.
40Ar/39Ar radiometric age dating, provides a method of direct measurement
for the timing of chemical changes taking place in these systems.

In this study, stratigraphically detailed chemical and age
measurements were obtained for three tephra sequences. They are, the Post-
Grouse Canyon tephra sequence, the Pre-Rainier Mesa tephra sequence, and
the Pre-Ammonia Tanks tephra sequence. These data were used to test

whether these tephra sequences could be used to decipher magmatic changes

that took place between large eruptive events; and, to determine the rates of

these magmatic processes.

General Geology

The Southwest Nevada Volcanic Field (Figure 1) was most active from
approximately 15.25 Ma to 6.3 Ma and was related to igneous activity that
occurred throughout the Basin and Range Province from approximately 37 to
5 Ma ago (Eaton, 1984). Six major volcanic centers produced more than
thirty major ash-flow sheets and lava flows (Byers, 1976). An extensional
tectonic regime has operated in this region throughout the Cenozoic and, in a
general way, volcanism increased from 15.25 Ma to 1 1.45 Ma, and then .
waned to 6.3 Ma (Sawyer et al., 1994). Volumetrically, ash flows are the
major eruptive product of this volcanic activity. Many of the ash-flow sheets
are chemically zoned and some are very large in volume (>900 km3). The
composition of major eruptive units also changes with time, and with erupted
volume.

Earlier ash-flow deposits were dominated by peralkaline compositions.
The eruption of the Grouse Canyon Tufl‘ at 13.7 Ma and Deadhorse Flat 'I‘ufi'
at 13.5 Ma, mark the end of this stage of peralkaline volcanism. Eruption of
the large-volume, metaluminous ash-flow sheets of the Paintbrush and
Timber Mountain Groups followed. Together, these units total more than

4000 km3 in volume, marking the most intense eruptive activity in this area.

This stage of eruption ended after the eruption of the Ammonia Tanks Tufi‘

(1000 km3) at 11.45 Ma. Later, peralkaline ash flows were again the

 

 

  
  
 

 
    
      
   

 

 

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Figure 1. Map of the Southwest Nevada Volcanic Field indicating calderas

Sampled locations are labeled ‘pgc' (Post-Grouse Canyon tephra .

sequence), 'prm' (Pm-Rainier Mesa tephra sequence) and ‘pat'
(Pm-Ammonia Tanks tephra sequence)

 

‘ u noma an.s 113
11.45 Ma
1000 km3

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'aimer ' esa 113
11.6 Ma
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’ tephra sequence

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1000km3

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Figure 2. Generalized stratigraphy of volcanic units from the Southwest
Nevada Volcanic Field, erupted from 13.7 to 11.45 Ma. Ages, volumes, and
unit names are for all units except the tephra sequences are from Sawyer et
al. (1994).

9

dominant ash-flow type. A generalized regionally stratigraphy (after Sawyer
et al., 1994) is used here to correlate units regionally, based on 40Ar/39Ar ages
of sanidines and chemical composition of individual units (Figure 2).

The present study focuses on tephra sequences associated with large-
volume ash-flow sheets erupted during the period of metaluminous
volcanism. The three tephra sequences used for this study lie respectively
above the Grouse Canyon Tufi', beneath the Rainier Mesa Tuff, and beneath
the Ammonia Tanks Tufl‘. The following is a summary of work done to date
for the tephra sequences and associated ash-flow sheets used in this study.
All ages from previous work (and from this study) are reported with a 20

error.

The Grouse Canyon Tuff

The Grouse Canyon Tufi'is one of the last peralkaline units to erupt
prior to the onset voluminous metaluminous volcanism in this area. It was
erupted from the Silent Canyon Caldera (Noble et al., 1968), and is smaller
(2 10 km3) than the following metaluminous eruptions from the Claim Canyon
Caldera and Timber Mountain-Oaisis Valley Caldera Complex (Figure 1).
”Ar/39A: ages on sanidines from the Grouse Canyon Tufl‘ yield an age of 13.7

i 0.08 (Sawyer et al., 1994).

10

The Post-Grouse Canyon Tephra Sequence

Above the Grouse Canyon ash-flow sheet lies a thick sequence of
tephra fall and interbedded small ash-flow layers. It is very well exposed on
Pahute Mesa, where it is approximately 175 meters thick, and is truncated
above by the surge deposit of the Rainier Mesa ash-flow sheet. At this
location, the sequence consists of bedded pumice- and ash-fall deposits
interbedded with small ash-flow layers (Figure 3a). The pumice falls consist
of moderately to very well sorted, light gray and brown pumice fragments.
Individual pumice fragments range in size from sand to cobbles. These
layers are commonly well stratified, exhibit normal or reverse grading, and
commonly contain minor amounts of lithic and light gray obsidian fragments
up to 0.5 centimeters in diameter. The ash layers are light to medium gray
and brown and range in thickness from less than 0.25 centimeters to several
meters.

Interbedded with the tephra-fall layers are numerous small ash-flow
layers. These layers are very poorly sorted, nonstratified to poorly stratified
mixtures of pumice, lithics, and ash (Figure 3b). They are often
discontinuous within an individual exposure, and contain basal zones (2-5
centimeters) of reworked material. Many small ash-flow layers exhibit a
color change from white or very light gray at the base of the layer, to brown
at the top. Most of the color change occurs in the ashy matrix and in the
smaller pumice fragments, while the larger pumice fragments are

consistently light gray throughout the layer. Some tephra-fall and small ash-

 

Figure 3. (a) Bedded tephra deposits from the Post-Grouse Canyon tephra
sequence. (b) small ash-flow layer contained within the tephra sequence.

flow layers contain organic remains (probably plant roots) in the upper few
centimeters of the layer. In addition, at least two 1 centimeters thick layers
of very dark brown and black organic rich layers are present within the
sequence.

Two very localized channel deposits are present within the sequence.
One is a ‘V’ shaped deposit, approm'mately 1 meters wide and 1.5 meters
high, consisting of coarse gravel and pebbles. The deposit is two to three
meters across and less than a meter in height. In this case, the channel fill
consists of ash-flow material.

The Post-grouse Canyon tephra sequence contains a 50 meters thick
mass flow unit within it. Unlike the smaller ash-flow layers in the sequence,
this unit exhibits no color change. There is a 0.3 meters thick surge deposit
of white ash at the base, overlain by a white ash-flow containing boulder-
sized lithic and pumice fragments. Though thick, this unit is extremely

localized and probably fills a small paleochannel or depression.

The Dead Horse Flat Formation, Crater Flat Group, Wahmonie
Formation, and Calico Hills Formation

Regional stratigraphic units overlying the Grouse Canyon Tufl' include
the Dead Horse Flat Formation (13.5 i 0.04 Ma), the Crater Flat Group
which includes the Bullfrog Tufl' (13.25 3: 0.04), the Wahmonie Formation

(13.0 i 0.18 Ma), and the Calico Hills Formation (12.9 i 0.04 Ma). 40Ar/3v9Ar

l3

ages for these units were determined by Sawyer et al. (1994). In the revised
stratigraphic framework proposed by Sawyer et al. (1994), The Dead Horse
Flat M is the youngest formation in the Belted Range Tufl and is the last
major peralkaline unit to have erupted prior to the voluminous
metaluminous deposits making up the Paintbrush and Timber Mountain
Groups. Three formations make up the Crater Flat Group. The Bullfrog Tuff
consists of two metaluminous ash-flow sheets and is the most voluminous
formation of the Crater Flat Group. The Wahamonie Formation consists of
andesitic and dacitic lavas and tephras that are generally widespread but are
not present in the area where the Post-grouse Canyon tephra sequence

outcrops.

The Topopah Springs Tuff

Eruption of the Topopah Springs Tufl‘followed the eruption of the
Post-Grouse Canyon tephra sequence. The Topopah Springs Tufi‘ is a 1200
km3 (Byers, 1976; Scott et al., 1984) compound cooling unit (Lipman et al.,
1966), erupted at 12.8 :I: .04 Ma (Sawyer et al., 1994). It is compositionally
zoned with high silica rhyolite overlain by a crystal-rich quartz latite; and,
inferred to be the result of eruption from a layered magma body with a sharp
compositional interface between the high silica rhyolite and quartz latite

magmas (Schuraytz et al., 1989).

l4

Yucca Mountain and Pah Canyon Tuffs

Two smaller eruptions followed the eruption of the Topopah Springs
Tufl'. The Pah Canyon Tuff (35 km3) and the Yucca Mountain Tufl (25 km3)
make up two metaluminous formations of the Paintbrush Group. Major and
trace element composition of the Pah Canyon Tufl‘is consistent with mixing
of the lower and upper portions of the Topopah Spring Tuff (Flood et al.,
1989). The Yucca Mountain Tufflikely represents initial eruption of the Tiva

Canyon magma chamber (Flood et al., 1989).

The Tiva Canyon Tuff

Underlying the Timber Mountain Group is the Tiva Canyon Tufl‘
(Figure 2). The Tiva Canyon 'I‘uff is the youngest formation in the
Paintbrush Group, and was erupted from the Oasis Valley-Timber Mountain
Caldera complex (Figure l). Radiometric 40Ar/39Ar ages on sanidines from the
Timber Mountain Tufl' gives an age of 12.7 i 0.06 Ma (Sawyer et al., 1994).
The Tiva Canyon Tufi'has an erupted volume of 1000 km3 and together with
the other members of the Paintbrush Group have an erupted volume of 2200
km3. Like many of the large-volume southwestern Nevada ash-flow sheets,
the Tiva Canyon M is chemically zoned. The highest silica portion is
interpreted to represent a hybrid mixture of magmas, combined with crystal
fractionation, of the underlying Pah Canyon Formation and quartz latite

from the Tiva Canyon Tufi’ (Flood et al., 1989).

15

The Pre-Rainier Mesa Tephra Sequence

The Pre-Rainier Mesa Tephra Sequence lies stratigraphically between
the Tiva Canyon and Rainier Mesa ash-flow sheets. It is well exposed at two
locations on the Nevada Test Site and one location west of Yucca Mountain in
the Crater Flats area (Figure 1). In all locations this sequence is truncated
above by the Rainier Mesa ash-flow sheet. The section on Rainier Mesa
conformably overlies the Tiva Canyon ’I‘ufi'. At the other two locations, the
base is fault bounded.

The Pre-Rainier Mesa tephra sequence ranges in thickness from about
30 meters to 60 m. Like the Post-grouse Canyon tephra sequence, the Pre-
rainier Mesa tephra sequence consists of bedded pumice- and ash-fall
deposits interbedded with small ash-flow layers. The pumice falls consist of
moderately to very will sorted, light gray (almost white) to brown pumice
fragments that range from approximately sand- to boulder-size. These layers
are often well stratified and exhibit normal or reverse grading; they
commonly contain minor amounts of lithic fragments. Ash-fall layers range
in thickness from 0.25 centimeters to 4 meters in thickness. Their colors
range from light to medium gray and brown.

Interbedded with the tephra-fall layers are numerous small ash-flow
layers. These layers range in thickness from less than a meter thick to about

6 meters or more in thickness. Though no volume measurements have been

16

made, individual ash-flow layers generally are thicker towards the top of the
sequence; this is interpreted to indicate a related increase in the erupted
volume. The layers are very poorly sorted, nonstratified mixtures of pumice,
lithics, and ash. They are often discontinuous, contain basal zones (2-5
centimeters) of reworked material, and have been interpreted to represent
mass flow deposits by Warren and Valentine (1990). Many small ash-flow
layers exhibits a color change from white or very light gray at the base of the
layer, to brown at the top. Most of the color change occurs in the ashy matrix
and smaller pumice fragments, while the larger pumice fragments remain
light gray throughout the layer. A few tephra-fall layers and small ash-flow
layers contain organic remains (probably plant roots) in the upper few

centimeters of the layer.

The Rainier Mesa Tuff

The Timber Mountain Group is composed of two of the largest volume
ash-flow sheets in the Southwest Nevada Volcanic Field. The Rainier Mesa
Tufi' (11.6 i 0.06) is 1200 km3 in volume and is the older of the two
formations. Eruption of the Rainier Mesa Tufl‘led to the initial collapse of
the Timber Mountain Caldera (Byers et al., 1976; Broxton et al., 1989;
Farmer et al., 1991). The Rainier Mesa ash-flow sheet is chemically and
mineralogically zoned (55 to 78% silica) and was produced by the eruption of

a chemically stratified magma body (Mills, 1991). The ash-flow sheet

consists of low- and high-silica trends defined by a compositional gap at
approximately 72% silica (Cambray et al., 1995). The high silica portion of
the of the ash-flow sheet represents approximately ninety percent of the total
erupted volume (Mills, 1991). The high-silica portion of the ash-flow sheet is,
in turn, subdivided into two distinct high-silica groups based on differences
in Th and Nb content; and, are interpreted to represent separate magmas
that shared the same magma chamber and erupted simultaneously (Cambray
et al., 1995; Saltoun, 1995). The low Th/Nb chemical group comprises the
bulk of the high silica portion of the ash-flow sheet and is widespread. The
high Th/Nb group is also widespread yet it represents only a minor volume of
the high silica portion of the ash-flow sheet.

Prior to this study, the relationship of the Rainier Mesa M to its
associated tephra sequences (both the Pre-Rainier Mesa tephra sequence and

the Pre-Ammonia Tanks tephra sequence [see below]) were largely unknown.

Pre-Ammonia Tanks Tephra Sequence

Situated between the Rainier Mesa and Ammonia Tanks Tufis is the
Pre-Ammonia Tanks Tephra Sequence. It is exposed in at least 5 locations
on the Nevada Test Site, (Figure 1) and outcrops range from approximately
3.5 meters to 9 meters thick. The Pre-Ammonia Tanks Tephra Sequence is
primarily composed of ash- and pumice-fall layers. At two locations, small

ash-flow layers and possible paleosols are also present. The Pre-Ammonia

18

Tanks Tephra Sequence is the thinnest of the tephra sequences in this study
and in many locations is not present between the two large ash-flow sheets.
As noted for the Pre-Rainier Mesa Tephra Sequence, many of the small ash-
flow layers within the Pre-Ammonia Tanks Tephra Sequence display a
progressive color change from base to top.

Unlike the other sequences in this study, there is evidence of
widespread hydrothermal alteration in the Pre-Ammonia Tanks tephra
sequence. In many areas, this is shown by vividly colored pink, purple, and

orange layers in the sequence.

The Ammonia Tanks Tuff

The youngest unit of the Timber Mountain Group is the Ammonia
Tanks Tufl'. This 1000 km3 ash-flow sheet is composed of two cooling units
(Byers et al., 1976) and, like the underlying Rainier Mesa Tufl', is
accompanied in most locations by the associated tephra sequence discussed
above. Eruption of the Ammonia Tanks ash-flow sheet resulted in the
terminal collapse of the Timber Mountain Caldera (Byers et al., 1976;
Christiansen et al., 1977; Broxton et al., 1989), approximately 11.45 million
years ago. Like the Rainier Mesa Tufi‘, the Ammonia Tanks ash-flow sheet
has a chemical range that spans from 57 to 78 wt.% Si02. The low and high
silica compositions are separated by a compositional gap at about 7 2% Si02

(Rose, 1988; Mills, 1991; Cambray et al., 1995). The high-silica portion of the

19

Ammonia Tanks Tuff is consistent with crystal fractionation of the lower-
silica portion combined with some mixing of the high silica portion of the
Rainier Mesa Tufl' (Rose, 1988). However, it has recently been suggested
that the high-silica portion may be a product of mixing of the two high-silica

groups from the Rainier Mesa Tufi (Cambray et al., 1995).

Chapter 2

METHODS

Sample Selection for chemical analyses

Site selection for this study was based on the quality of exposure and
the completeness of the sequence at any one locality. For the tephra
sequences associated with the Timber Mountain Group, attempts were made
to collect samples from a variety of locations around the caldera. Limited
exposures, however, restricted the number of sites sampled. For the Post-
Grouse Canyon tephra sequence, one thick section was sampled. The
location of this site is shown in Figure 1. Samples were collected from three
locations of the Pre-Rainier Mesa tephra sequence, from locations northeast,
north, and southwest of the caldera. The Pre-Ammonia Tanks tephra
sequence was sampled at four locations. All of these locations were along the
northern portion of the caldera (Figure 1).

Careful stratigraphic control and detailed sample collection were
crucial to the success of this study. Once a sample site was selected,
lithologic changes observed in the sequence and heterogeneity observed

within a given layer determined the collection of samples.

20

21

Within an individual tephra layer, collection of individual pumice
samples was based on size, color, and crystal content. Because glassy pumice
fragment variations represent the variations in magmatic compositions at the
time of eruption, an attempt was made to select as wide a variety of pumice
fragments as possible from each layer.

Sample selection among layers throughout the tephra sequence was
based on textural and color changes from layer to layer. Some lithologic
changes in the tephra sequences take place on the millimeter scale. These
probably do not reflect individual eruptions but rather small changes in wind
direction, wind intensity, or settling velocities of different particle sizes after
a single eruption. In such cases, samples were taken at regular intervals,
provided that samples large enough for analyses could be found.

In addition to individual glassy pumice fragments, bulk tephra
samples were collected from individual small ash-flow layers, and from thick
ash-beds and laminated or thinly bedded ash beds where pumice fragments
large enough for chemical analyses were not present. Glassy pumice
analyses were used to facilitate comparisons between the tephra sequences
and the associated large volume ash-flow sheets. The reasons for selecting
bulk rock samples vs. individual glassy pumice fragments are discussed

below.

22

Glassy pumice vs. bulk rock samples

Glassy pumice fragments most closely represent the geochemical and
mineralogical compositions of the original magma (Wolff, 1985). This is
because glassy pumice fragments are a direct sample of the crystal and liquid
portions of the magma. It is for this reason that individual glassy pumice
fragments are used in this study.

Unfortunately, individual glassy pumice fragments from small ash-
flow layers within the tephra sequences used in this study, do not represent
the entire chemical range of the ash-flow layer. Most of the chemical
variation takes place in pumice fragments too small for analysis, and in the
ashy matrix. In these cases, bulk samples were collected for analysis. Here,
bulk samples have proved useful in comparing differences that take place
within and among small ash-flow layers and among more fine grained
deposits within a given tephra sequence.

Chemical analyses used for comparison from previous studies on the
large ash-flow sheets (Schuraytz, 1988; Flood et al., 1989a, 1989b; Schuraytz
et al., 1989; Mills and Rose, 1991; Cambray et al., 1995; Saltoun, 1996; Mills
et al., in review) employ glassy pumice fragments exclusively. For this
reason, only glassy pumice analyses from the tephra sequences are used
when making comparisons between tephra sequences and ash-flow sheets.

Because all the tephras from this study are secondarily hydrated, analyses

23

are normalized on a volatile free basis to facilitate comparisons among

diflerent units.

Sample selection and age calculation for “Ar/”Ar analysis

4°Ar/39Ar radiometric ages were determined for samples collected from
all seven sections in this study. The selection of samples for age dating was
based on the freshness of tephra layers, stratigraphic position in the
sequence, and known compositional variations within the sequence (based on
a preliminary chemical study). In addition, samples were selected from
above and below small unconformities with the expectation that the hiatus
represented by unconformities might be distinguishable by 40Ar/39Ar
methods.

Both the Post-Grouse Canyon and Pre-Rainier Mesa tephra sequences
contained compositional changes from base to the top of the sequences.
Consequently, layers throughout the sequence were dated. The Pre-
Ammonia Tanks tephra sequence, though it contains a wide chemical
variation, did not vary systematically, from base to top. Therefore, fewer
layers were dated from the Pre-Ammonia Tanks tephra sequence. These
dates are mainly from the uppermost and lowermost portions of the
sequence. In all, 47 separate tephra layers were dated.

Laser ablation 40Ar/39Ar analyses were conducted on between five and

eleven, hand picked sanidine grains were obtained for each tephra layer. In

24

selecting grains for analysis, care was taken to sample fresh parts of the
outcrop and to select fresh grains, void of large inclusions for 40Ar/39Ar
analysis. Many grains, however, did contain small glass inclusions.
Residual glass was dissolved from grain surfaces using fluoboric acid,
followed by a distilled water rinse.

Ideally, grains were analyzed individually, however, because the
amount of argon released is proportional to the size of the crystal, two or
more crystals were fused together when crystal size was very small
(approximately less than 1mm in length).

Radiometric 40Ar/39Ar ages are calculated based on the equation:

t = ll}. ln{J- (4°Ar*/39Ar)+ 1}
where 4°Ar* is the amount of radiogenic 40A: present in the sample, it is the
decay constant for 4°K—->4°Ar*, and J is a study specific, irradiation parameter
based on the neutron flux during irradiation. All samples and standard Bern
4B were irradiated together at the McMaster nuclear reactor. The weighted
average of J, based on the equation:

J: (ell total. istandard -— l) ,

(40Ar./39Ar)

 

3 tan da rd
is 0.000486 i0.000008.

To minimize the contribution of 400a, samples with large Ca/K ratios
(>0.l), were eliminated from the database before ages for individual tephra

layers were calculated. Samples containing very little 4°Ar* (radiogenic

argon 40), were also eliminated. An acceptable range of “Ar" was
determined in the same manner as a recent study by Best et al. (1995). For
each tephra sequence, there appeared to be a natural break in the data
where the amount of 4°Ar* in most samples was either above a certain level
or far below it. For the Post-Grouse Canyon and Pre-Ammonia Tanks tephra
sequences, this break occurred at 94% 4°Ar*. 4°Ar* values for the Pre-Rainier
Mesa tephra sequence were less tightly grouped as the other sequences.
Radiogenic 4°Ar values as low as 75% were used for age calculations of this
sequence.

The radiometric 4°Ar/39Ar age of individual tephra layers are calculated

as inverse variance weighted means, 1, and standard deviation, 0: , based on

the equation:

 

where the weight w: = l/oaz, and ti is an individual age measurement.
Calculated ages from individual layers are then pooled to give an average age
for that group. Uncertainties for pooled averages are calculated as the mean
of the weighted deviates. 40Ar/5‘9Ar analyses used for the calculation of the
upper portion of the Pre-Rainier Mesa tephra sequence, are reported in Table

l.

26

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sample % 4°Ar* 4°Ar/39Ar 37Ca/39K 40Ar/39Ar Age (Ma) weighting
factor

12R8-0-21 98.25 13.514 0.0 10 1 1.572 t 0.054 342.936
12R8-0-23- 79.20 16.624 0.026 1 1.465 i 0.513 3.800
1

12R8-0-23- 99.61 13.482 0.010 1 1.704 at 0.057 307.787
2

12R8-0-24- 92.03 14.278 0.013 1 1.454 : 0.057 307 .7 87
l

12R8-0-24- 99.83 13.393 0.009 1 1.652 t 0.067 222.767
2

12R8-0-25 98.19 13.507 0.009 1 1.559 i 0.063 251.953
12R8-0-27- 92.02 14.177 0.010 11.373 i 0.593 2.844
2

Weighted mean age and uncertainty (Ma)
1 1.58 i 0.03
Sample % 4°Ar* 40Ar/39Ar 37Ca/39K 4°Ar/39Ar Age (Ma) weighting
factor

12R8-3-12 81.30 16.334 0.005 1 1.578 t 0.063 251.953
12R8-3-2 97.96 13.680 0.015 1 1.679 t 0.058 297.275
12R8-3-5 96.13 13.946 0.008 1 1.684 i 0.059 287.274

 

 

 

 

 

 

Weighted mean age and uncertainty (Ma)
1 1.65 i 0.04

 

 

Pooled weighted mean and uncertainty (Ma)

1 1.62 i 0.04

 

Table 1. Representative 40Ar/39Ar analyses and weighting factors for

weighted mean age calculation.

Care should be taken when interpreting uncertainties from radiogenic

age analyses. Three main sources of uncertainty exist. They are, 1) the

precisional error of the instrumentation used in acquiring the analyses (in

this case a laser mass spectrometer), 2) variation caused by sampling and

 

27

preparation techniques, and 3) variation caused by geological factors (e.g.

Geyh and Schleicher, 1990).

Chapter 3

THE POST-GROUSE CANYON MAGMATIC SYSTEM

Previous Work

The 200 meters thick tephra sequence overlying the Grouse Canyon
ash-flow sheet on Pahute Mesa, has not been previously studied. The
Rainier Mesa ash-flow sheet truncates this sequence above. Chemically, the
tephra sequence closely resembles the high silica portion of the
metaluminous Rainier Mesa Tufl’. This sequence is not chemically related to
the peralkaline trend of the Grouse Canyon Tufl‘. In fact, the current
designation, ‘Post-Grouse Canyon’ is based solely on new ages for the tephra

sequence and not on its chemistry.

“Ar/”Ar radiometric ages

Based on non-overlapping ages for individual tephra layers within the
Post-grouse Canyon tephra sequence, 40Ar/39Ar radiometric ages on sanidines
record at least three different age groups as shown in Figure 4. The pooled
ages for the three groups are 13.52 10.04 for the base of the sequence, 13.33
i006 for the middle portion of the tephra sequence, and 13.05 10.04 for the

upper portion of the tephra sequence. The age break between the lower and

28

29

 

 

§

 

8
0
..L

 

Height above GCAF (m)
8

8

 

 

 

O . Y +
12.5 12.7 12.9 13.1 13.3 13.5 13.7 13.9

Age (Nb)

 

 

 

 

Figure 4. Range of 40Ar/39Ar ages for individual tephra layers from the Post-
Grouse Canyon tephra sequence.

middle portions of the sequence is tightly constrained. However, the two
layers underlying the break for the upper age group overlap the both middle
and upper age groups. Because of geochemical considerations (see later)

these layers are grouped with the middle portion of the sequence.

Geochemistry

The Post-Grouse Canyon tephra sequence has the largest chemical
range of the three sequences in this study. Si02 ranges from 67 to 78%, and
Zr, from 60 to 686 ppm (Figure 5). It is distinct from the underlying
peralkaline Grouse Canyon ash-flow sheet based on both major and trace

element chemistry of glassy pumice fragments collected for comparison.

 

1000 I

 

 

 

 

i
YWY _

Y ~?’
800 — Y Y _.
L— A —i

Y Y
600 — —
Zr — ‘ _
A
400 - n
_ A ..
‘ AX
200 ~ ...
0 1
65 7O 75 80
SiO

2
A = Post-grouse Canyon tephra sequence (glassy pumice fragment)

X = Post-grouse Canyon tephra sequence (bulk tephra)

Y = Grouse Canyon T ufl‘ (glassy pumice fragment)
Figure 5. Zr vs. Si02 for the Post-Grouse Canyon tephra sequence and the Grouse
Canyon Tufl‘.
CHEMICAL VARIATION WITH STRATIGRAPHIC POSITION

As discussed previously, bulk samples were collected mainly where
individual pumice fragments large enough for chemical analysis, and
representing the entire chemical variation of the tephra, were not
available(sma11 ash-flow layers, for example). Bulk tephra analyses from the

middle portion of the tephra sequence, are chemically similar to analyses of

individual glassy pumice fragments from corresponding portions of the

31

coursed otqdmfinens sAneIsu “OW-“0‘1 3iqd913ll‘ms 95991921

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

S, a 2 c: S (:3: E 3::
I I v I 1 so
4., 3 J! 3 I“
x
F ‘ xx t‘ m
1 4 “
x {1
. O 5
x x ‘N g:
u i‘ ‘
x O
.. x it. ~ ..
V X X
.- fi ‘ t—
. 433:0“.
‘ d
¢
1 1 o l L 8
I r a g I I M
1‘
r- ‘-o’
1
- c .0. ‘
4 o ‘
1 - 1N
i' x§‘ 4‘2 1
v- x‘ 1‘ Q"
4 ° 2
x
_ C gm
X ‘X c _ _
X ‘
. .33.”.
x C
‘ x «g X
_ d ‘ ‘..—:
l t 3! °
1
‘ T ‘ q 4 l O
o :3 cc o c o o
N '— M N '—
uonpod stqdufinans annals}; nonysod :3quQO arms};

Figure 6. Variation of individual glassy pumice fragments and bulk tephra
samples from the Post-Grouse Canyon tephra sequence (A) = glassy pumice
fragment; x = bulk tephra sample).

SiO

C so

IIoIIIsod oqueifintms cartels};

32

 

 

 

 

 

 

 

C C
N -- :3:
I I 8
‘
‘
+— ‘ —8
m
‘
4 8‘-

— — m

N
“
‘3
l‘

- 14-8
X *H ‘ '—
x§x“

X C C‘
G
1 l o
I I :
C
— u—ie
‘

 

 

d
d
l l l‘
c c o
N —-
uoyusod omduflmns alums};

 

 

30

225*;
m “3;“
“fl. 3

 

1..— .
c

l l
O G
N u—

nwod WWW semen

A = glassy pumice fragment; x = bulk tephra sample.

Figure 6 (continued).

300

200

Rb

100

sequence. Two small ash-flow layers from the upper portion of the tephra
sequence, however, are more variable than glassy pumice fragments from
adjacent tephra layers (Figure 6). This is evident in plots of Si02, Sr,

N a20+K20, Fe0, and Ca0. In addition, Rb and K20 concentrations for most
of the zoned ash-flow layers throughout the sequence are significantly lower
than individual pumice fragments from nearby layers.

One explanation for the greater variability of the zoned ash-flow layers
is that, lower silica magma may produce smaller fragments than the higher
silica magma. As a result, pumice samples represent only one end of the
chemical spectrum and contain smaller chemical ranges. Another possibility
is that secondary weathering products and soil development in the bulk
samples accounts for the observed chemical variation. Although some
weathering of the tephra layers has certainly occurred, it probably does not
account for the observed chemical variation.

First, glass shards to at least 250 um, making up the ashy matrix of
the ash-flow layers, are still vitreous. It is reasonable to assume that small
glass fragments should be the first constituent to break down in a
weathering ash-flow. It should be noted that some poorly developed soil
horizons were observed, and avoided, during sampling of bulk tephra
samples.

In addition, the Ca0 content of bulk tephra samples increases with

increasing FeO (Figure 7). Prior to chemical analysis, samples were leached

34

 

 

 

 

1 l l 1 l
10 xx X XX
. ._ xx 1
’S<
S3 .
U x 32 901‘
’ix x
x>It x
0.0 1 l l 1 l
o 1 2 3
Foo

Figure 7. Ca0 vs. FeO for the bulk tephra samples from the Post-Grouse
Canyon tephra sequence, where bulk samples were collected. This is
evidence that the increase in Ca0 is primary, corresponding to more mafic
magmatic compositions.
to remove any secondary carbonates. Therefore, if iron enrichment in these
samples reflects secondary processes, the CaO enrichment should not
increase correspondingly. The observed trend, however, is one of increasing
CaO with increasing Fe0 for the middle and upper portions of the tephra.
A third possibility for the large chemical variation of the ash-flow

layers, is that crystals are concentrated in these layers due to gravity and to

wind effects during eruption and deposition. Field observations of the small

ash-flow layers, however, show no indication of crystal concentrations in the
sampled zoned ash-flow layers.

Rb and (N a20 + K20) values from bulk tephra samples are
significantly lower than values for individual glassy pumice fragments
(Figure 6). The high mobility of Rb and K20 may have contributed to the
difference in concentration between the two types of samples.

Because some elements display significant differences between bulk
tephra samples, and because the cause of the variation between bulk tephra
samples and glassy pumice fiagments is not completely understood,
individual glassy pumice fragments are used exclusively for further
interpretations and comparisons of the Post-Grouse Canyon tephra sequence.

Because a stratigraphic sequence represents a time sequence,
evaluation of magmatic changes as a function of time is best accomplished by
comparing chemical variation with stratigraphic position (Figure 8). Two
prominent chemical changes are present within the Post-Grouse Canyon
tephra sequence. The first, and most distinct, is the dramatic increase in
compositional variation in the tephra layer at 1 16 meters. Below the 1 16
meters layer, Sr (as an example) varies from only 45 to 131 ppm. In contrast,
Sr values fiom the tephra layer at 1 16 m, varies from 18 to 367 ppm.
Chemical ranges of Zr, Sr, A1203, and Si02, all exceed those from the
underlying tephra layers.

The other distinct characteristic of this sequence, is the dramatic

decrease in chemical variation in the upper portion of the tephra sequence

36

 

 

 

 

 

 

 

 

 

 

(In) iseqs Meg-qss 39 sAoqe tqfiIsH ([11) 199118 .uou-qse 39 axoqe IqfiIsH
5. o .. (é é o
x c
I 3 I oo
4
4 8‘ ‘ a"
i- ‘ - 8 a ‘
("i
4
4 4
4 o T N
- a g r‘i‘ - - I? a
‘ 4
‘ 4
4 4 ‘
- 4 4 - 2
I ' I '
41‘ if "
l C J 8
e _§_ . 22
4
F I c
8 . 1 .. g
- ‘ -

 

I n
A A
1 1
400
Zr
I
u
“A AA
A A“
L“
1
14
A10
2

 

 

 

 

 

 

 

b ‘ -
i g - I g
- «‘ 1‘ ‘l ‘1 - ‘
1 7 o 1 2
.3 ° § 5. °
(W) 399113 MOB-m 39 aAoq' "13151-1 (111) ”ms MOB-Ilsa 39 QAOQB IqalQH

Figure 8. Variation of glassy pumice fragments with respect to stratigraphic
position, fi'om the Post-Grouse Canyon tephra sequence.

(In) Issqs mou-qse 39 sAoqe Iqfltsn
8

—

0
I 50

 

I

‘An
1
100

Zn

AA
“A
A.“
A
1‘:
50

'3‘ ‘
‘L

 

 

 

 

 

A

50

its:
A
AA
AA
A
A

 

 

 

 

 

 

 

4 8 a c
I- ‘ ‘ '1 N a I- ‘ ‘ - Q
‘ ‘ 1 4|
2| . 1 g
a ‘ g $3
4 4 a
i " ‘ i
4
4
A l 8 l S
.2 ° a .3. °
(m) Wis non-II“ so most mien (a!) 199115 M01141” 30 Most MPH

Figure 8 (continued).

38

with respect to trace element and major oxide concentrations. In addition,
there are lower concentrations of Zn and Zr, and higher concentrations of Rb,
than the underlying tephra layers. The abrupt change in chemistry,
corresponds exactly to the change in radiometric ages fiom the 13.3 Ma age
group to the 13.0 Ma age group.

Another change that occurs in the lower portion of the sequence,
though less distinct, is the change in Y concentration at approximately 30 m.
This tephra layer contains the lowest Y concentrations of any in the sequence
(less than 14 ppm). Two samples from the layer directly overlying, have
widely varying Y concentrations (22 and 49 ppm 3: 6%). However, the
remaining tephra layers, up to about 150 m, have Yttrium concentrations no
lower than about 27 ppm. The layers with the lower Y concentrations fall in
the oldest age group of the tephra sequence (13.5 Ma). Overlying layers, up
to 150 m, correspond to the 13.3 Ma age group.

In summary, chemical changes in the Post-Grouse Canyon tephra
sequence correspond to age and stratigraphic changes throughout the
sequence. The exception, is the highly variable tephra layer at 1 16 m. The
age of this layer falls in the 13.3 Ma age group. However, the chemical
variation of this tephra layer is much larger than any other layer in the
sequence.

The correlation of radiometric age dates to observed chemical changes

in the sequence, marks this sequence as the best candidate for testing the

39

main hypothesis of the study. If the magmatic processes governing changes
in the Post-Grouse Canyon tephra sequence can be distinguished, rates of

these processes can be determined.

CHEMICAL GROUPS

Three groups can be defined based on the corresponding age, chemical,
and stratigraphic changes that occur in the Post-Grouse Canyon tephra
sequence. These three groups are referred to as the lower, middle, and upper
portions of the Post-Grouse Canyon tephra sequence, respectively.

Contained within the middle age and stratigraphic group, is the highly
variable tephra layer at l 16 m. Because this layer has a much larger
chemical range than the rest of the tephra layers in the middle portion of the
tephra sequence, a separate plot symbol is assigned to this layer to facilitate
comparisons. It is referred to as the highly variable tephra layer.

Almost all major and oxides and trace elements distinguish the upper
and middle portions of the Post-Grouse Canyon tephra sequence (Figure 8).
In a diagram of Zn vs. Zr (Figure 9), the middle portion of the tephra
sequence has lower Zn concentrations (51 to 97 ppm), higher Zr
concentrations (1 15 to 188 ppm), and does not overlap the upper portion of
the tephra sequence (Zn = 21 to 53 ppm; Zr = 77 to 101 ppm). Additionally,
the ratio of RblSr vs. Zr defines two distinct trends for the middle and upper

portions of the Post-Grouse Canyon tephra sequence.

 

 

 

 

 

 

80— _
A
60r AAA‘ '7
AAA ‘ A
t:
N 40- 1
20— 3 —
0 l L
0 150 200 250
12 I l
10- AA 3 -
gA‘ “
8- QA ~
6~ so!“ i
a A
.0
°‘ 4- ‘A -
2L- _
250

 

 

Zr

Figure 9. Zn vs. Zr, and RblSr vs. Zr discrimination diagrams for the middle
(5) and upper (A) portions of the Post-Grouse Canyon tephra sequence.

41

 

 

 

 

I I I 1 n I I
o o O
100 e ‘ 4
O
N
50 +- 0 ~
0
0 l 1 1 I 1 l 1
0 100 200 300 400 500 600 700 800
Zr
12 T I I 1 I I I

 

Rb/Sr

A

I
890
9

2-
0‘ Co
0
0 mic l 0 P l 1
200 300 400 500 600 700 800
Zr

A = upper Post-grouse Canyon tephra sequence
= middle Post-grouse Canyon tephra sequence
0 = highly variable layer within the middle Post-grouse

Canyon tephra sequence.

Figure 10. Trace element discrimination diagrams for the highly variable
layer and the chemical groups from the upper and middle portions of the

Post-Grouse Canyon tephra sequence (PGC).

 

 

 

300

200

Sr

100

200

lib

100

110
90
70
50
3O

10

Figure 10 (continued).

42

 

 

 

 

 

 

 

 

 

 

 

 

 

 

O
O .i
O
O ._
0
er
0 c) d
0
Middle,
Go 1 A 1 1 1 1
100 200 300 400 500 600 700 800
I I I f
0
0
1 1 [p 1 1 1 1
100 200 300 400 500 600 700 800
I I I I T I I
-i
O ..
0 I1
1 1 1 1 1 1 1
100 200 300 400 500 600 700 800
Zr

43

Seven samples from the highly variable layer occur in the same
chemical group as the middle portion of the Post-Grouse Canyon tephra
sequence. In Figure 10, analyses fiom the highly variable layer are
superimposed onto the diagrams of Zn vs. Zr, and Rb/Sr vs. Zr. The
difference between the groups are best illustrated on the diagram of RblSr vs.
Zr, where chemical groups representing the middle and upper portions of the
tephra sequence form distinct chemical trends, with five samples from the
highly variable layer consistently plotting with the middle portion of the
Post-Grouse Canyon tephra sequence.

Eight samples fi'om the highly variable layer exhibit chemical behavior
that is distinct from other chemical groups in the tephra sequence and from
all other samples collected fiom the same tephra layer. Zn and Zr
concentrations in these samples range fiom 60 to 1 15 ppm and 2 10 to 690
ppm, respectively (Figure 10). These samples account for most of the
chemical variation within the highly variable layer. These eight samples will
be referred to as the high Zr group from the highly variable layer. The
remainder of the samples from the highly variable layer plot near the middle
and upper portions of the tephra sequence but do not consistently plot with
any other chemical group. These samples will be referred to as the low Zr
group from the highly variable layer. Trace elements discriminating the
highly variable layer fi-om the middle and upper portions of the tephra

sequence are also shown in diagrams of Sr, Rb, and Y vs. Zr (Figure 10).

ci

44

 

 

 

 

10

 

400 500 600 700 800

O = low Zr samples from the highly variable layer

. = samples from tephra layers lying stratigraphically between the lower and middle portions of
the tephra sequence.

= lower portion of the Post-grouse Canyon tephra sequence
Figure 1 1. Y vs. Zr variation diagram discriminating the lower portion of the
Post-Grouse Canyon tephra sequence from other chemical groups within the
sequence.

The lower portion of the Post-Grouse Canyon tephra sequence is also
chemically distinct from the middle and upper portions of the sequence, and
from the highly variable layer. Chemical groups representing the middle and
upper portions of the tephra sequence, as well as the highly variable layer,
are compared with the lower portion of the tephra sequence (Figure 1 1).
Most samples from the lower portion of the tephra sequence are
characterized by lower Y concentrations (less than 14 to 25 ppm) than the

other chemical groups and Zr concentrations that fall between the chemical

groups from the middle and upper portion of the tephra sequence. Three
samples from the lower portion of the tephra sequence (filled diamonds)
consistently plot with the middle portion of the tephra sequence. These
samples were collected from layers situated between the lowermost dated
layer fi'om the middle portion of the tephra sequence, and the uppermost
dated layer from the lower portion of the tephra sequence. Chemical
correlation places these samples with the middle portion of the tephra
sequence, though the age of these samples is unknown.

In summary, three distinct chemical groups within the Post-Grouse
Canyon tephra sequence correspond to three distinct 40Ar/39Ar radiometric
ages. The highly variable layer contains samples that belong to the middle
portion of the tephra sequence, in addition to samples that are distinct from

any of the age and stratigraphic groups fiom the tephra sequence.

Mineral chemistry

Mineral chemistry also distinguishes major chemical and age groups
present in the Post-Grouse Canyon tephra sequence. Alkali feldspar
compositions, especially BaO concentrations, reveal difi'erences between the
middle and upper portions of the tephra sequence (Figure 12). BaO
concentrations are also distinct for the highly variable layer. For this layer,
Ba0 wt.% concentrations in alkali feldspars range from 0.15 to 0.52. The
remainder of the middle portion of the tephra sequence has BaO wt.%

concentrations that range from 0.4 to greater than 0.6. Ba0 wt.% values in

46

(m)~unl 39 sAoqe :qSIsH (“111101 ()0 moqc 111319”

3
O

2()()

0
- l(l()

 

 

 

 

 

 

 

 

 

 

r E
I- I . .4 o o
8 o L m:
1— . ~00 O
0 II!
- - O
O ‘0 I- ‘6'
O
L- - a"
ogog z _ q
i- O -<r O
O O
N
I- -‘ h- 0 do-
. .. °§e °
1- -i .
1 go 1 . C,
O O
I «C31 ' “o"
I- -oo

 

 

 

 

 

0.... O a 809

 

1 ‘ O o
a o E °
((11) 1m .1. 39 moms :qSIoH (m) 1.1“]. ()0 94099 WSPH

Figure 12. Edge analyses of plagioclase (O) and alkali feldspar (0) from the
Post-grouse Canyon tephra sequence showing K20, Na20, Ca0, and BaO
variation with stratigraphic depth.

830

CaO

47

the upper portion of the sequence range from nearly zero to just 0.4 wt. N a20
ranges from 3.9 to 5.1 wt.% in alkali feldspars from the middle portion, and
from 3.1 to 3.9 wt.% in the upper portion, with the exception of one alkali
feldspar that has a higher Na20 (4.6-4.9 wt.%). Likewise, there is a shift to
greater concentrations of K20 in the alkali feldspar from the upper portion of
the tephra sequence. N a20 and K20 values do not distinguish alkali
feldspars from the highly variable layer as well as BaO values. Plagioclase
feldspars do not change from the middle to the upper portion of the sequence.
Most feldspar grains are homogeneous or only slightly zoned. Two
plagioclase grains from the middle portion of the tephra sequence display
normal zoning, M1522 and An13.32, respectively. One zoned alkali grain was
observed (Oreo-59).

Additional major phenocryst phases fi'om the Post-Grouse Canyon
tephra sequence include hornblende, biotite, pyroxene, magnetite, ilmenite,
and quartz. Of these, very few phenocrysts display any compositional
zoning. One exception, is a biotite phenocryst with a compositional range
from 33 to 65% Si02 , 0.08 to 0.19 Cl and 2.5 to 8.2 wt.% F.

With the exception of the alkali feldspars discussed above, the highly
variable layer does not exhibit highly variable or significantly difl'erent
mineral chemistry when compared to other layers from the middle portion of

the sequence, except for alkali feldspar concentrations discussed above. In

48

fact, the composition of individual minerals in this layer are slightly less

variable than in other layers.

Correlation with Regional Volcanostratigraphic Units

Complex stratigraphic relationships among volcanic units make in
depth correlation difficult. 4°Arl39Ar ages obtained in this study for the Post-
Grouse Canyon tephra sequence closely correspond with revised ages
obtained by Sawyer et al., (1994), for units in the Southwest Nevada Volcanic
Field. In a revised volcanic stratigraphy by Sawyer et al. (1994), an 40Ar/39Ar
age of 13.7 :i:0.08 Ma was calculated for the Grouse Canyon Tuff of the Belted
Range Group. This is followed by an age of 13.5 i004 Ma for the Deadhorse
Flat Formation (also part of the Belted Range Group) (Figure 2), and an age
of 13.25 10.08 Ma for the Bullfrog Tufi' of the Crater Flat Group. Above the
Crater Flat Group, 40Ar/39Ar ages of 13.0 $0.2 Ma and 12.9 i008 Ma were
obtained for the Wahmonie Formation and Calico Hills Formation,
respectively.

Care must be taken when comparing radiometric ages fi-om different
laboratories. For all ages determined herein and all ages reported fiom
previous work, 20 error range is used.

Radiometric age ranges from the lower portion of the Post-Grouse
Canyon tephra sequence overlap those of the Deadhorse Flat Formation

[13.52 i0.04 (20 error) Ma] for the lower portion of the Post-Grouse Canyon

49

tephra sequence, compared with 13.5 $0.04 (20 error) Ma for the Deadhorse
Flat Formation. However, the Deadhorse Flat Formation is part of the
peralkaline volcanics of the Belted Range Group (Sawyer and Sargent, 1989;
Sawyer et al., 1994), and cannot be chemically correlated with the
metaluminous Post-Grouse Canyon tephras. Zr concentrations from the
lower Post-Grouse Canyon tephra sequence are more closely related to the
Crater Flat Group and the Calico Hills Formation (13.25 10.04 Ma and 12.9
i004 Ma respectively (Sawyer et al., 1994)) than to the peralkaline Belted
Range Group.

The middle portion of the Post-Grouse Canyon tephra sequence has a
radiometric age of 13.33 i 0.06 Ma. This corresponds to a 13.25 i .04 Ma
4°Ar/39Ar age determined for the Bullfrog Tufi‘ of the Crater Flat Group by
Sawyer et al., 1994. The middle portion of the Post-Grouse Canyon tephra
sequence possess a significantly larger range of Zr variation than tephras
from the Crater Flat Group, ranging from 100 to 750 ppm Zr, compared with
a range of 100 to 250 ppm for the Crater Flat Group. Much of this variation,

however, is caused by the highly variable layer that falls within this age

 

group. The ratio of MgglioFeO of biotite grains from the middle portion of

the Post-Grouse Canyon tephra sequence are more variable than those for

the Crater Flat Group (0.1 to 0.6 versus 0.35 to 0.6, respectively). This

50

variation takes place throughout the middle portion of the Post-Grouse
Canyon tephra sequence and is not restricted to the highly variable layer.
The upper layers of the Post-Grouse Canyon tephra sequence,
corresponding most closely in age with the Wahmonie Formation and the
Calico Hills Tufl (13.0 and 12.9 Ma, respectively). The Wahmonie Formation
consists of andesitic and dacitic lavas and tephras and are not present in the
Post-Grouse Canyon sequence. Zirconium concentrations of the upper layers
from the Post-Grouse Canyon, do correspond to those from the Calico Hills

Tufl‘.

Summary

In summary, the Post-Grouse Canyon tephra sequence can be divided
into three major groups based on age, compositional variation, and
stratigraphic position. The lower tephra sequence has an 40Ar/39Ar age of
13.53 i .04 Ma. It is characterized by lower Y concentrations than any other
group and can be differentiated fi'om the other groups based on the variation
of Y vs. Zr (Figure 10). The middle tephra sequence has an 4°Ar/39Ar age of
13.33 + .06 Ma and is distinguished fi‘om the lower tephra sequence by
higher Y and lower Sr concentrations. It is distinguished from the upper
tephra sequence by higher Zn concentrations, lower Rb concentrations, and
significantly larger variation than any other group (Figure 8). Within the

middle age and stratigraphic group of the tephra sequence, there is a highly

51

variable layer that can also be divided into distinct chemical groups. The
highly variable layer contains samples that belong to the middle portion of
the tephra sequence, in addition to samples that are distinct from any of the
age and stratigraphic groups from the tephra sequence. The upper tephra
sequence has an 4°Ar/39Ar age of 13.05 i .04 Ma. It has the highest Rb
concentrations, the lowest Zn and Zr concentrations, and the least

compositional variation of any group.

Chapter 4
THE PRE-RAINIER MESA MAGMATIC SYSTEM
Previous Work

Large chemical variations characterize the Pre-Rainier Mesa tephra
sequence. For example, silica ranges from 66 to 78 wt.% (Huysken et al.,
1994). This variation is greater than the chemical variation in the high
silica portion of the overlying Rainier Mesa ash-flow sheet (73-78%) noted by
Cambray et al., 1995. The high silica portion comprises about 90% of that
unit, volumetrically.. Relatively large chemical variation also occurs in
individual ash-flow layers within the tephra sequence (Huysken et al., 1994).
These layers are normally zoned (Figure 13) with as much as 5 wt.% Si02
variation. Because the entire tephra sequence and the smaller ash-flow
layers within are characterized by such large chemical variations, it is
unlikely that they were erupted from the uppermost portion of a large (at
least 1200 km3), compositionally zoned, Rainier Mesa magma body.

In addition, layers from the lower portion of the tephra sequence are
generally less silicic and less chemically evolved than those from the upper
portion (Huysken et al., 1994) (Figure 13). Based on these observations, and
the fact that the ash-flow layers are thicker towards the top of the sequence,

Huysken et al. (1994) proposed that the Pre-Rainier Mesa tephra sequence

52

53

 

‘8. O l I

 

 

 

 

 

 

 

 

 

 

 

 

3’2 x
E . 0
a 10 r- 0 ‘
§
X
.3 20 - x x '*
€- >& I II
a {mu . .e
30 l l
0 1 2 3 4
F60
0 I. ..I 1 I
5 l
3 x
2% .’
310 Q . .
3.
0 X
g 20 - x x 4
3. I!" ' I;
30 l l I
0 100 200 300 400
Sr
0 ‘ I
’a‘ 0’ ’0
E X uppcrsequcnce
3 10 I x ' '
g Iowa'soquence x
320- x x q
X
X
E . aid-1":
30 L
10 20 30
Th

. O I 1' A A = bulk samples from small ash-flow layers (each
symbol represents a different layer)
X = glassy pumice fragments

Figure 13. (a) Variation of Fe0 and Sr with Depth beneath the Rainier Mesa
ash-flow sheet. (X) indicates individual glassy pumice fi'agments. All other
plot symbols represent small ash-flow layers within the Pre-Rainier Mesa
tephra sequence. (b) Variation of Th discriminating the lower (older) and
upper (younger) Pre-Rainier Mesa tephra sequence.

was formed by periodic eruption of a continually evolving magma body that
grew larger with time.

Preliminary 40Ar/39Ar ages from the lower- and uppermost layers of the
sequence reveal a 1.31 :04 Ma radiometric age difference between the lower
and upper portions of the Pre-Rainier Mesa tephra sequence (Huysken et al.,
1994). These radiometric age dates, and the observation of changing
compositions from the base to the top of the sequence, are the basis for the
original proposal that rates of magmatic evolution of the Rainier Mesa
magma body could be determined from the Pre-Rainier Mesa tephra

sequence.

“Ar/”Ar ages

The only complete section of the Pre-Rainier Mesa Tephra Sequence is
located on Rainier Mesa and is characterized by a bimodal age distlibution
(Figure 14). At this location, the entire lower 13 meters of the Pre-Rainier
Mesa Tephra Sequence has a radiometlic 40Ar/39Ar age of 12.79 i007 Ma.
This age is consistent with ages obtained for the underlying Tiva Canyon
ash-flow sheet (12.7 i 0.06 Ma, Sawyer et al., 1994). The upper 12 meters of
this section have a radiometric 40Ar/39Ar age of 1 1.62 10.04 Ma, which is
consistent with the age obtained for the overlying Rainier Mesa ash-flow

sheet [11.6 :06 Ma (Sawyer et al., 1994)].

Western side of Yucca Mtn.

 

 

 

 

 

 

 

 

 

 

 

Rainier Mesa (Northeast oi (Southwest 0111.11“, Mt“.
Timber Mtn Caldera) Caldera) North of Timber Mtn. Caldera
o 0-—m—— o
r—H
5,, 5._ 5..
E10 10
fi " n ‘5 10”
2
g 1541- H 15“ 15¢
g H—1
3 20 101 20.. , zoi.
g 1'91 1"——'."1
3 25 14" 25+ 25 : :H'!
M 11.5 12 12.5 13 13.5
30 . 30.. A9915“)
10 12 14 m
AseiMa) 35 a : :
11 11.5 12 12.5 13
A99 (Ma)

Figure 14. Range of 40Arl39Ar ages for individual tephra layers from the Pre-
Rainier Mesa tephra sequence.

In two other sections, located on the west side of Yucca Mountain, and
on Buckboard Mesa Rd, ”Ar/”Ar ages also fall into two groups. The ages of
the lower portion of these sections are consistent with those from the
complete stratigraphic section on Rainier Mesa. Ages obtained for the upper
layers in these sequences are l 1.91 $0.07 Ma and 11.94 $0.04 Ma,
respectively. Conceivably, ages from the sequence on Buckboard Mesa Road,
may represent a portion of the sequence that is absent on Rainier Mesa.

However, excellent stratigraphic correlation between the upper portion of the

sections on Rainier Mesa and the west side of Yucca Mountain, indicate that
samples from these two locations come from the same portion of the tephra
sequence. The most likely cause for the discrepancy in 40Ar/39Ar radiometric
ages, is that geologic factors, such as partial resetting of 4"Ar/”Ar ages by
hydrothermal processes or heating from the overlying Rainier Mesa ash-flow

sheet, have afl'ected 4°Ar/39Ar ratios.

Geochemistry

The determination of two distinct, non-overlapping, ages for the upper
and lower portions of the Pre-Rainier Mesa tephra sequence, rather than a
continuous age progression, was unexpected. Because of this, chemical
variation as a function of stratigraphic position in the sequence has been
examined in detail. The variation of Th, Sr, and FeO with stratigraphic
position (Figure 13) best illustrate that the Pre-Rainier Mesa tephra
sequence can be divided into a lower and upper portion that correspond with
the age groups. The individual layers in the lower portion of the sequence
are compositionally similar, even though each layer is characterized by large
chemical variation. The upper portion of the sequence has smaller average
Sr and FeO values (50 ppm Sr, and 1.0 wt.% FeO, compared with 150 ppm
Sr, and 2.0 wt% FeO, for the lower portion of the sequence), and is much less
chemically variable. This new grouping is in contrast to the previous

interpretation (Huysken et al., 1994) that the tephra sequence becomes

progressively more chemically evolved from base to top. For a clear
illustration of how bulk samples and individual glassy pumice fragments
difi‘er within the same unit, individual glassy pumice fragments represented
as X’s, and bulk tephra samples from small ash-flow layers are represented
as a different symbol for each small ash-flow layer. Variation of the bulk
rock samples is larger than that of individual glassy. pumice fragments, and,
as a group, individual glassy pumice fragments are more silicic.

The Pre-Rainier Mesa tephra sequence can be divided into two
discrete, chemical groups that correspond exactly to the two age groups. They
are, the lower (older) tephra sequence, and the upper (younger) tephra
sequence. For the following comparisons, only individual glassy pumice
fragments are included from the Pre-Rainier Mesa tephra sequence so that
the lower (older), and upper (younger) portions of the tephra sequence are
easily distinguished. Samples from the upper tephra sequence are plotted as
open circles, and those from the lower sequence, are plotted as Open squares.

Chemical variation of the upper Pre-Rainier Mesa tephra sequence is
equivalent to the high silica portion of the overlying Rainier Mesa ash-flow
sheet (Figure 15). Major and trace element variation of the tephra sequence
compared to the overlying Rainier Mesa Tuff, indicates that the upper
portion of the tephra sequence plots consistently with the high-silica, low
Th/Nb group defined by Cambray et al. (1995), and Saltoun (1996). The

same comparison for the lower portion of the Pre-Rainier Mesa tephra

 

DJ

Th/Nb
I

 

 

 

 

400

 

 

 

300 -

200'-

Rb

100 '-

 

 

 

 

 

 

 

 

 

 

 

 

Yb
N

 

 

 

 

 

 

A = Rainier Mesa ash-flow sheet
0 = upper Pre-rainier Mesa tephra sequence
D = lower Pre-rainier Mesa tephra sequence

)-

 

 

400

300

200

100

50

30

10

Figure 15. Trace element variation diagrams comparing individual glassy
pumice fragments from the lower Pre-Rainier Mesa tephra sequence, the
upper Pre-Rainier Mesa tephra sequence, and the Rainier Mesa Tufi'.

Th.’Nb

Rb

Yb

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

40 . fi 40
30- -I - «30
5.8
D
20- 0“qu l “D -20
.-:=. a fi
‘ 10 f ‘10
u- q )- D
0 4 L 0
400 u r 400
' o
300- o- - -3oo
. O
a ”or o ‘ i “0.200;:
a: 0
° 4’ an “‘
100? o T r- ‘100
0 4 0
5 u
coi— '5 5
p 6 . ..

4 o 6 o L 0°83 4
3P ? '4 i- auaq3
g 2:- d b .2;

D
1" " i- all
0 ' a
6 I U
5— . -I r— -5

0
4L 6 . . .4
3)- : t- -3
[3 2i- .. . D .123
O
1. % .. . .1
0 Egg
0 ‘ L l E-I—lo
50 70 8050 70 80
$02 SiO2

O = upper Pre-rainier Mesa tephra sequence
D = lower Pre-rainier Mesa tephra sequence
6 = Tiva Canyon ash-flow sheet

Figure 16. Trace element variation diagrams comparing the Tiva Canyon
ash-flow sheet, the lower Pre-Rainier Mesa tephra sequence, and the upper
Pre-Rainier Mesa tephra sequence.

60

sequence reveals that this group is chemically distinct from the Rainier Mesa
’I‘ufi (Figure 15).

A chemical comparison of the lower portion of the Pre-Rainier Mesa
tephra sequence (12.79 i 0.07 Ma) to underlying Tiva Canyon ash-flow sheet
(12.7 i 0.03 Ma), (Figure 16) reveals that the lower portion of the tephra
sequence is distinct from both the underlying Tiva Canyon ash-flow sheet
(even though it is consistent in age with this ash-flow tufi), and the overlying
Rainier Mesa ash-flow sheet.

In addition, differences in concentrations of Cs and Rb show that no
reasonable crystal fractionation model can account for the chemical variation
of the lower and upper portions of the Pre-Rainier Mesa tephra sequence
(Figure 17). The lower tephra sequence is characterized by very little change
in Cs (<5 ppm) over a large range in Si02. However, the upper Pre-Rainier
Mesa Tephra Sequence is characterized by a higher Cs values (8-12 ppm)
with very little increase in Si02. Rb variation is also characterized by little
change in Rb with increasing Si02. However Rb values are significantly
larger in the upper Pre-Rainier Mesa tephra sequence.

One sample from the lower Pre-Rainier Mesa tephra sequence has a
silica content higher than any other individual glassy pumice fragment. This
sample consistently plots with the upper Pre-Rainier Mesa tephra sequence
with respect to silica but contains Cs and Rb values consistent with the lower

tephra sequence. The reason for this difi'erence in silica content is unknown,

 

Fig1
Iowe

Cs

400

300

200

Rb

100

61

 

 

 

 

 

 

 

 

o
h— 0 -1
0
Cl DEF D Cl [I] D
D D
l l l l l l l l
69 7o 71 72 73 74 75 76 77 78
2
T l l l j l l l
_ D [:1 D _
[nu Q] D
C]
[:1 E] D
i- _
l l l l l l l l
69 7o 71 72 73 74 75 76 77 78

SiO

0 = upper Pre-rainier Mesa tephra sequence
D = lower Pre-rainier Mesa tephra sequence

Figure 17 . Discrimination diagrams of Cs vs. Si02, and Rh vs. Si02, for the
lower and upper Pre-Rainier Mesa tephra sequence.

62

however, this sample also consistently plots away from the lower tephra
sequence in petrologic model comparisons (see below).
Mineralogy

Phenocryst phases present in the Pre-Rainier Mesa bulk tephra and
pumice samples include plagioclase, orthoclase, biotite, hornblende,
pyroxene, ilmenite, magnetite, and sphene. The main mineralogical contrasts
between the upper and lower portions of the Pre-Rainier Mesa Tephra
Sequence are, 1) compositional differences in phenocryst phases between the
upper and lower portions of the sequence; and, 2) disequilibrium feldspars in
the lower portion of the sequence.

The lower and upper portions of the Pre-Rainier Mesa Tephra
sequence contain different feldspar compositions (Figure 18). The upper
portion of the tephra sequence is characterized by an alkali feldspars range
of 01‘57-71, and a plagioclase compositions ranging from Amen. Most feldspars
from the upper portion of the sequence are homogeneous, however, some
orthoclase grains display normal zoning.

Feldspars from the lower portion of the Pre-Rainier Mesa Tephra
Sequence have alkali feldspar ranges from Onoss, with the most intensely
zoned feldspars ranging from One-69. Plagioclase feldspars range from ADM-45.
Most plagioclase grains are zoned, with some grains displaying reverse
zoning. Three grains have plagioclase cores and orthoclase rims. These
grains are likely in disequilibrium and may represent influx of a new magma

into the system.

63

 

 

 

Ab Or

. = upper Pre-rainier Mesa tephra sequence
I = lower Pre-rainier Mesa tephra sequence

Figure 18. Feldspar ternary diagram comparing feldspar compositions for the lower
and upper Pre-Rainier Mesa tephra sequence.

Alkali feldspars from the upper portion of the Pre-Rainier Mesa tephra
sequence are similar in composition to those of the overlying Rainier Mesa
Tufl'. Alkali feldspars from the lower portion of the Pre-Rainier Mesa Tephra
Sequence display a large compositional range with most compositions falling
between alkali feldspar compositions from the Rainier Mesa ash-flow sheet
those from the Tiva Canyon ash-flow sheet.

Very few plagioclase grains were found in the upper portion of the Pre-

Rainier Mesa Tephra Sequence. Three analyses from one of the small ash-

64

flow layers within the upper sequence shows a very narrow compositional
range (An 16-17). This small range probably reflects of the small sample size
since An compositions from the overlying high-silica portion of the Rainier
Mesa ash-flow sheet have a larger compositional range.

Plagioclase grains from the lower portion of the tephra sequence have
a considerably greater An compositional range than the upper portion of the
tephra sequence. However, plagioclase compositions from the Tiva Canyon
ash-flow sheet, Rainier Mesa ash-flow sheet, and Pre-Rainier Mesa Tephra

Sequence all overlap.

Fe-Ti Oxide Temperatures

Temperatures were calculated for the Pre-Rainier Mesa Tephra
Sequence in order to determine whether the upper and lower tephra
sequence could be divided into thermometric groups corresponding to the
chemical and radiometric age divisions already observed. In addition,
thermometry provides an additional basis for comparison among the two
subunits making up the Pre-Rainier Mesa tephra sequence to the much
larger underlying and overlying ash-flow sheets.

Magmatic temperatures from the upper and lower portions of the Pre-
Rainier Mesa Tephra Sequence were obtained using magnetite/ilmenite
pairs. When possible, magnetite and ilmenite grains in contact were used to

ensure equilibrium between the grains. However, some samples did not

provide grains in contact, so other means of testing equilibrium were
employed.

One test for equilibrium is Mg/Mn partitioning between magnetite and
ilmenite (Bacon and Hirschmann, 1988). The rationale for this test is that
the exchange reaction

Mgu + Mnmt = Mnu +Mgmt
will produce a linear array of points (within analytical uncertainties) for
equilibrium magnetite and ilmenite, on a plot of the log(Mg/Mn)m vs.
log(Mg/Mn)u (Bacon and Hirschmann, 1988). A positive result does not
confirm equilibrium, however, a negative result does eliminate the possibility
of equilibrium between a magnetite/ilmenite pair.

Magnetite and ilmenite grains in contact were considered as pairs and
tested for equilibrium. Grains not in contact with an Fe-Ti cidde counterpart
were paired with other separate grains from the same sample and tested for
equilibrium. Magnetite/ilmenite pairs that fell outside of the analytical
limits defined by Bacon and Hirschmann, were disregarded in subsequent
temperature determinations.

Edge analyses give the best approximation of magnetite and ilmenite
compositions being produced at the time of eruption. For this reason, only
edge analyses of flesh grains were used for temperature determinations.

Temperatures were calculated using the “QUILF4. 1” model developed

by Andersen et al. (1993). Five temperature estimates from the lower Pre-

66

 

 

 

 

 

950 l l r T I O
900 L -
A A A A A
350 _ A dare-
Temperature
(Celsius) 800 r
750 ’
700 _
650 “
600 1 1 1 1 i
50 55 60 65 70 75 80
A Rainier Mesa ash flow SiQ
0 Upper Pre-Rainieerr Mesa ttephra sequence
D Lower Pre- hra s eqceuen

— Range of temps. cantdp silicaeq contents for the Tiva Canyon ash flow

Figure 19. Comparison of Temperature vs. Si02 for Tiva Canyon ash-flow
sheet, the Pre-Rainier Mesa tephra sequence, and the Rainier Mesa ash-flow

sheet.

Rainier Mesa Tephra Sequence range from 694-77 5°C (Figure 19). Two
temperatures were obtained fi'om the upper Pre-Rainier Mesa Tephra
Sequence. These were the lowest and the highest from the entire sequence
(669°C and 958°C). The lower of these temperatures was obtained from a
mt/il pair in contact. The higher of the two temperatures was obtained from
two separate grains that passed the test for equilibrium. Absence of
abundant Fe-Ti oxides in the upper portion of the sequence restricted efforts
to obtain a more satisfying and statistically significant number of
temperature analyses. From the temperatures obtained, it is not clear

whether one, or both of the temperature measurements are incorrect.

67

A plot of temperature vs. Si02 (Figure 19), relates the four lower silica
samples with temperatures ranging from 749-775 °C, and two of the three
higher silica samples with temperatures ranging from 669-674 °C, The
comparison of temperatures to silica content indicates a very general
relationship between lower silica and higher temperatures. However, one
high silica sample yields an extremely high temperature determination (958
°C). The overall relationship of decreasing temperature with increasing Si02
suggests that this temperature estimate is not correct. However, with only
two temperature determinations from the upper portion of the sequence, any
interpretations using temperature estimates should rely only on those from
the lower tephra sequence.

Temperature determinations from the Rainier Mesa ash-flow sheet
were initially conducted by Mills (1991). Temperatures were recalculated
(Vogel, pers. comm., 1996), using the QUILF4.1 (Andersen and Lindsay,
1993). Additional temperature determinations were recently made by
Saltoun (1995). These workers found that the three thermometric groups
could be defined, coinciding with the three compositional groups of the
Rainier Mesa 'I‘ufi‘ (Cambray et al., 1995; Saltoun, 1995; Mills et al., in
review). Compared with the Rainier Mesa Tufi’, the Pre-Rainier Mesa tephra
sequence possesses significantly lower temperatures for the same silica
content. The same statement holds true when the tephra sequence is

compared to the range of Fe-Ti Oxide temperatures and silica contents from

68

the Tiva Canyon ash flow sheet. Ranges of temperatures are given for the
Tiva Canyon ash-flow because temperature estimates for the Tiva Canyon
Tufi‘ are uncertain due to the presence of sphene, rather than ilmenite, as the
main Ti oxide phase (Flood, 1989). Temperatures from the Tiva are rough

estimates only.

Summary

The Pre-rainier Mesa tephra sequence can be divided into two distinct
groups based on 40Ar/39Ar ages and compositional differences between the
lower and upper portions of the tephra sequence. The upper portion of the
tephra sequence (1 1.62 i .04) is equivalent in age and composition to the low
Th/Nb, high silica group of the overlying Rainier Mesa ash-flow sheet (1 1.6 i
.06; Sawyer et al, 1994). The lower portion of the tephra sequence has the
same 40Ar/39Ar age as the underlying Tiva Canyon ash-flow sheet [12.79 + .07
Ma for the lower Pre-rainier Mesa tephra sequence compared to 12.7 i .06
Ma (Sawyer et al., 1994) for the underlying Tiva Canyon ash-flow sheet].
Compositionally, the lower portion of the tephra sequence is distinct from
both the overlying Rainier Mesa ash-flow sheet and the underlying Tiva

Canyon ash-flow sheet (Figure 15).

Chapter 5
THE PRE-AMMONIA TANKS MAGMATIC SYSTEM
Previous Work

Both the Rainier Mesa and Ammonia Tanks ash-flow sheets are
chemically zoned with large variation (55—7 8 and 57-78 wt.% Si02
respectively) (Rose, 1988; Mills, 1991). In addition, both the Rainier Mesa
and Ammonia Tanks ash-flow sheets erupted from the same caldera. The
difference in eruption time between the two ash-flow sheets is only about
200,000 years, yet chemical difi‘erences between the Ammonia Tanks Tufi
and underlying Rainier Mesa Tufl’ precludes their being related by
differentiation of the same source magma body (Cambray et al., 1995). Based
on these chemical difl'erences, Cambray et al., (1995) proposed that the
Rainier Mesa and Ammonia Tanks Tufis represent separate lower crustal
melts that have been emplaced along the same fracture system.

A compositional gap occurs between the low and high silica portions of
the Ammonia Tanks Tufi‘ (Mills, 1991; Cambray et al., 1995). In their recent
study, Cambray et al., (1995) suggest that the high-silica portion of the
Ammonia Tanks ash-flow sheet may represent a mixture of the two high-

silica batches from the Rainier Mesa M. This suggestion is based on the

69

70

fact that chemically, the high-silica portion of the Ammonia Tanks ash-flow
sheet is intermediate between the two high-silica batches of the Rainier Mesa
Tufl‘ with respect to most trace elements. This contrasts with earlier studies
(Rose, 1988) that indicate that the entire compositional range of the
Ammonia Tanks can be produced by a combination of crystal fractionation
and magma mixing. If Cambray et a1. (1995) are correct, then magma mifing
plays a far more important role in producing the higher silica portion of the
Ammonia Tanks Tufi' and crystal fractionation is relatively insignificant. It
is important to note the diflerence in time scales for the two mechanisms. In
the crystal fractionation/magma mixing model, the magma must be stored
long enough for extensive fractionation to occur, however, the Cambray et al.
(1995) model stresses brief residence time in the magma chamber before

eruption.

“Ar/”Ar age dates

40Ar/39Ar radiometric ages for the Pre-Ammonia Tanks tephra range
from 11.5 $ 0.04 to 11.9 $.02 Ma. The Rainier Mesa 'I‘ufi' has a radiometric
40Ar/E‘E’Ar age of 11.6 $0.06 Ma (Sawyer et al., 1995). Since the Pre-Ammonia
Tanks tephra sequence lies stratigraphically above the Rainier Mesa Tufi‘,
one or both ages are in error. Coincidentally, the oldest ages from the Pre-
Ammonia Tanks tephra sequence coincides with the youngest ages from the

two incomplete sections of the Pre-Rainier Mesa tephra sequence that were

71

Pahute Mesa (North Rainier Mesa (NONI Of

Dead Horse Flat orthwest
Aqueduc‘Mesamm" (N Timber Mtn. Caldera)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

of‘fjmbcr Mtn. Caldera) ofTimber Mtn. Caldera) ot‘Timbcr Mm. Caldera)
0 0
0 | . I 0
1 .. 1 .1 l—O-—-l 1 -- 1 .. l-O-l
2 - 2 -~ ..
2 H 1 [—9—] 2
3 sl- 3 3 ” 3 7r H
R? "'
411- m 4 "" 4 “'- 4 i
11 11.5 12
5 I ' 5 5 .1.-
Age (Ma)
11 11.512 12.5 6“ 6.. 1491"
Age (Ma) 7 ,, 7 1
8 I . I 11 11.5 12
Age (Ma)
9 i—m—+—~
11.4 11.6 11.8 12
Age (Ma)

Figure 20. Range of 4°Ar/39Ar ages for individual tephra layers fi'om the Pre-
Ammonia Tanks tephra sequence.
discussed previously; this includes the section at Yucca Mountain. It is
possible that whatever processes affected the Pre-Rainier Mesa tephra
sequence, also affected the Pre-Ammonia Tanks tephra sequence. One
possible explanation for the discrepancy in ages is the apparent
hydrothermal alteration that has taken place within the Pre-Ammonia Tanks
tephra sequence.

Two of the four dated sections were useful in age interpretations of the
Pre-Ammonia Tanks tephra sequence. Both of these sections have a 200,000
to 300,000 year average age difference between the base and the top of the

sequence. In Figure 20, the age range for each layer is illustrated with

1951

ha

eh

72

respect to stratigraphic position. Radiometric ages from the base and the top
of a both sections overlap slightly, however still may give some indication

that the upper portion of the sequence is younger than the lower portion.

Geochemistry

There is a large chemical variation in the Pre-Ammonia Tanks tephra
sequence. The variation is illustrated by Zr vs. Si02 (Figure 2 1), and
compared with the overlying Ammonia Tanks ash-flow sheet. Silica ranges
from 69.1 to 77.5 wt.% and Zr ranges fi'om 100 to 400 ppm.

For silica concentrations less than 70 wt.%, the Rainier Mesa and
Ammonia Tanks ash-flow sheets are distinguishable by trace element
variation (Figure 21). Only a few trace elements however, distinguish the
higher silica portions of these ash-flow sheets. Of these, Rb, Y, and Nb
provide a the greatest distinction between the two ash-flow sheets.

The chemical variation of individual glassy pumice fragments show
that, with respect to Rb, the Pre-Ammonia Tanks tephra sequence is nearly
identical to the overlying Ammonia Tanks Tufi‘. Four samples, collected from
one location at various stratigraphic positions within the sequence, fall into
the lower Rb, high-silica group of the underlying Rainier Mesa Tufl'.

Contrasting this behavior, is the variation of Nb. Nb values from high
silica samples hour the Pre-Ammonia Tanks tephra sequence span the
chemical ranges of both the underlying Rainier Mesa Tufi', and the overlying

Ammonia Tanks Tufi‘. In addition, this plot shows that Nb values for the

73

 

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300 -

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100*-

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J M? 1 300
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o o
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i=Ammonia Tanks ash-flow sheet (individual glassy pumice fi'agments)
+=Pre-ammonia Tanks tephra sequence (bulk tephra)

O=Pre-ammonia Tanks tephra sequence(individual glassy pumice fragments)
A=Rainier Mesa ash-flow sheet (individual glassy pumice fragments)

Figure 2 1. Trace element discrimination of the Pre-Ammonia Tanks tephra
sequence, the Rainier Mesa ash-flow sheet, and the Ammonia Tanks ash-flow sheet.

74

 

 

60 I I' 1 I 70

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 21 (continued).

ba

111

(We

75

high silica Ammonia Tanks Tuff do not fall between the two high-silica
batches of the Rainier Mesa Tufl, suggesting that the high silica portion of
the Ammonia Tanks Tufi'is not a simple mixture of the high silica batches
from the Rainier Mesa 'I‘uff. Thus, the evolution mechanism suggested by
Cambray et al. (1995) seems unlikely.

Most of the lower silica samples from the tephra sequence plot
consistently with the low silica portion of the Ammonia Tanks Tuff (Figure
2 1). Three samples span a compositional gap that exists between the low and
high silica portions of the Ammonia Tanks M, and seem to be more closely
related to the high silica group with respect to Y. Two samples fiom the
lower silica portion of the Pre-Ammonia Tanks tephra sequence, that fall into
the low silica Ammonia Tanks group, contain slightly higher Y
concentrations than samples hour the Ammonia Tanks Tufi'. The different
compositions of these samples is not caused by any amount of mixing of the
Ammonia Tanks magma with Rainier Mesa magma. In a diagram of Zr vs. Y
that includes chemical groups fi-om the Rainier Mesa ash-flow sheet (Figure
22), inspection indicates that no mixing combination could produce the
observed higher values of Y. One possible explanation is that wall rock

assimilation produced this slight compositional difl'erence.

Summary
The Pre-Ammonia Tanks tephra sequence is chemically similar to the

overlying Ammonia Tanks 'l‘ufi'. Nb concentrations from the high silica Pre-

76

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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X=High Si02 Ammonia Tanks ash-flow sheet

i=Low Si02 Ammonia Tanks ash-flow sheet

O=Low 8102 Pre-ammonia Tanks tephra sequence (glassy pumice)

Figure 22. Zr vs. Y comparing the Rainier Mesa ash-flow sheet, the Pre-
ammonia Tanks tephra sequence, and the Ammonia Tanks ash-flow sheet.
Ammonia Tanks tephra sequence span the entire range of compositions of
the high silica portion of the underlying Rainier Mesa ash-flow sheet and the
overlying Ammonia Tanks ash-flow sheet. This behavior, and the high Nb
concentrations of high silica samples from the Ammonia Tanks ash-flow
sheet, strongly suggests that the high silica Ammonia Tanks pumice samples

do not represent a mixture of the two Rainier Mesa high silica groups. Lower

Sr

77

silica samples from the tephra sequence fill the compositional gap between
the low and high silica groups from the Ammonia Tanks ash-flow sheet.
Ages for the Pre-Ammonia Tanks tephra sequence are not well
constrained, possibly due to hydrothermal alteration of the tephra sequence.
However, there is a 250,000 radiometric age difference between the base and

top of one section.

Chapter 6
DISCUSSION
The Post-grouse Canyon tephra sequence

The three main chemical, stratigraphic, and radiometric age groups in
the Post-Grouse Canyon tephra sequence are, the lower, middle, and upper
portions of the Post-Grouse Canyon tephra sequence. There is a fourth major
compositional group, the highly variable layer, that falls into the same age
group as the middle portion of the tephra sequence. This group has the
largest variation in the sequence, and can also be divided into smaller
chemical groups (see above). The large range of chemical variation in this
tephra layer is evidence for the influx of new magma into the system prior to
the eruption of this layer.

The focus of this section is, in part, to determine the petrologic
relationships among the chemical groups of the Post-Grouse Canyon tephra
sequence. However, permissible petrologic relationships among any of the
chemical groups must also be consistent with age and stratigraphic

constraints in the tephra sequence.

78

79

MAGMA MIXING

One powerful test of magma mixing is the comparison trace element
ratios (e.g. Langmuir et al., 197 7; Cox et al., 1979). Ifmagma mixing is the
only process relating a suite of rocks, the ratios of any four trace elements
will plot on a hyperbola. Likewise, any of the initial ratios plotted against
the ratio of the denominators will fall on a straight line. If both conditions
are not satisfied, magma mixing as a dominant process must be rejected.

Magma mixing cannot relate the middle and upper portions of the
Post-Grouse Canyon tephra sequence to each other, based on a diagram of
the ratio Rb/Sr vs. Zr/Y (Figure 23). Trace element ratios for the two
chemical groups produce trends that cannot plot on any mixing curve.
Moreover, the accompanying plot, Rb/Sr vs. Y/Sr indicates that these groups
do not fall on the same straight line.

Similarly, plots of Rb/Y vs. Zn/Sr and Rb/Y vs. Sr/Y reveal that magma
mixing cannot relate the lower and upper portions of the Post-Grouse Canyon
tephra sequence. Also, plots of Rer vs. Sr/Y and Rb/Zr vs. Y/Zr eliminate
magma mixing as the main mechanism relating the lower and middle
portions of the Post-Grouse Canyon tephra sequence.

As discussed above, the highly variable layer contains pumice
fi'agments that consistently fit into the same chemical group as the rest of the
middle portion of the tephra sequence. In Figure 24, these samples have
been assigned the same as those from the middle portion of the tephra

sequence. The highly variable layer also contains pumice fiagments that are

80

(a)

 

RbI/SI

 

 

 

 

Rb/Sr

 

 

 

 

Y/Sr

A = upper Post-grouse Canyon tephra sequence
A = middle Post-grouse Canyon tephra sequence

0 = lower Post-grouse Canyon tephra sequence

Figure 23. (a) Trace element ratio diagram of RblSr vs. Zr/Y, and companion plot,
illustrating that the middle and upper portions of the Post-Grouse Canyon tephra
sequence are not related by magma mixing. (b) Trace element ratio diagram of Rb/Y
and anSr, and companion plot, illustrating that the lower and upper portions of the
Post-Grouse Canyon tephra sequence are not related by magma mixing.

(c) Trace element ratio diagram of Rb/Zr vs. Sr/Y, and companion plot, illustrating
that the lower and middle portions of the Post-Grouse Canyon tephra sequence are
related by magma mixing.

8]

(c)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

l 0 L I 1 l
6 7 0 2 4 6 8 10
Sr/Y
1 4 I I I 1 F I
. 8A
. 3 L ME ‘
A
1 A i
. ..2 7 A 7
. s r o
-4 m o
l 7 '
l 0 4 l L l l J
6 7 0.0 0.2 0.4 0.6
Y/Zr

A = upper Post-grouse Canyon tephra sequence
A = middle Post-grouse Canyon tephra sequence
0 = lower Post-grouse Canyon tephra sequence

Figure 23. (continued)

Rb/Zn

82

 

 

 

 

12

 

 

 

 

 

 

I j I I I I I I

 

 

  

1 L 1 1 1 l 1 1 1 1

u ~i \O :
Rb/Y

W

 

~

 

N

 

I I I I I I I I I I

 

1111114111

 

 

O

 

A = upper Post-grouse Canyon tephra sequence
A = middle Post-grouse Canyon tephra sequence
0 = low Zr samples from the highly variable layer
0 = high Zr samples from the highly variable layer

= lower Post-grouse Canyon tephra sequence

N
C

Figure 24. (a) Trace element ratio diagram of Rb/Zn vs. Sr/Y and Rb/Y vs. Sr/Zr,
and companion plots, illustrating that the middle portion of the Post-Grouse
Canyon tephra sequence cannot be produced by mixing of the high Zr group from
the highly variable layer with any other chemical group.

83

 

10- -

Rb/Zn
is" .
8
4%

 

 

 

 

 

 

 

O L 1 l J 1
0 2 4 6 8 10 12
Sr/Y
12 r a i
10 - -
A A A
A
A .1
Q
21 -
I
2 3

 

Y/Zn

A = middle Post-grouse Canyon tephra sequence
0 = low Zr samples from the highly variable layer
0 = high Zr samples from the highly variable layer

0 = lower Post-grouse Canyon tephra sequence

Figure 24.(b) Trace element ratio diagram of Rb/Zn vs. Sr/Y, and associated
comp anion plot, illustrating that the lower portion of the Post-Grouse
Canyon tephra sequence is not produced by mixing of the high Zr group from
the highly variable layer and any other chemical group from the tephra
sequence.

84

distinct from all other chemical groups. These samples may also be divided
into a high Zr and low Zr chemical groups (distinguished here as solid and
open circles, respectively).

Plots of Rb/Zn vs. Sr/Y, and of Rb/Zn vs. Y/Zn best illustrate the
relationship between the highly variable layer and the other chemical groups
from the Post-Grouse Canyon tephra sequence (Figure 24a). Data for glassy
pumice samples from the underlying Grouse Canyon Tufl‘ (this study) are
included to evaluate its potential as an end-member for mixing. The Grouse
Canyon Tufl', however, can be eliminated as a potential end member upon
inspection. Sr/Y ratios for the Grouse Canyon Tufl' are lower than any
samples fiom the overlying tephra sequence. Also, Rb/Zn concentrations are
lower than all but a few samples from the highly variable layer. These low
ratios eliminate the Grouse Canyon Tuff as an end-member for any mixing
trends.

The middle portion of the tephra sequence could not have been
produced by mixing any chemical group with the highly variable layer. In
trace element ratio diagrams of Rb/Zn vs. Sr/Y, and Rb/Y vs. Sr/Zr, the
average Sr/Y and Sr/Zr values for the middle tephra sequence are lower than
any other chemical group (except the Grouse Canyon ’l‘ufl). Any mixing
model for the middle portion of the tephra sequence, must produce Sr/Y and

RblY values intermediate between the end member concentrations.

85

It is also unlikely that magma mixing produced the lower portion of
the tephra sequence. Figure 24b is the same as Figure 24a, with the symbols
representing the middle portion of the tephra sequence removed. Rb/Zn
values from the upper portion of the tephra sequence overlap nearly the
entire range of Rb/Zn values fiom the lower portion of the tephra sequence,
yet contain significantly lower Sr/Y values. Moreover, Sr/Y concentrations
from the highly variable layer also overlap the Sr/Y range of the lower
portion of the tephra sequence. There is no reasonable mixing process that
can account for this variation.

The low Zr chemical group fi'om the highly variable layer (open
circles), also could not have formed by simple mixing of the upper portion of
the tephra sequence and the high Zr chemical group fi-om the highly variable
layer (solid circles). The trends of the upper Post-Grouse Canyon tephra
sequence and the high Zr group fiom the highly variable layer are nearly at
right angles to one another. There is no simple mixing process or
combination of mixing and crystal fraction that can account for the observed
trend. It is interesting to note, that one samplefrom the low Zr group from
the highly variable layer has a SrlY value lower than any sample from the
upper portion of the tephra sequence. This would seem to indicate that the
low Zr group could not have been produced by mixing of any chemical group

with the upper portion of the tephra sequence.

86

The fact that the middle portion of the tephra sequence has a lower
Sr/Y content than the upper portion of the sequence, and that the lower Zr
group varies more widely than all of the other chemical groups combined
(contains both higher and lower values) with respect to Sr/Y, is also good
evidence that crystal fractionation combined with magma mixing could not
have produced the middle or lower Post-Grouse Canyon chemical groups, or

the high Zr group fiom the highly variable layer.

FRACTIONAL CRYSTALLIZATION

Fractional crystallization, as a mechanism for compositional
modification, is tested below. As with magma mixing, certain fractionation
schemes can be eliminated by inspection before numerical models are
employed. In addition, an attempt is first made to relate the major
compositional groups, then to evaluate the relationship of the highly variable
layer to other compositional groups.

The variation of Zr best indicates that the lower, middle, and upper
portions of the Post-Grouse Canyon tephra sequence are not related by
crystal fractionation (Figure 25). In a diagram of Zr vs. Si02, the lower,
middle, and upper portions of the tephra sequence are defined by three
separate linear trends. Each is characterized by decreasing Zr with
increasing Si02. The upper portion of the tephra sequence has the lowest Zr
concentration (mean = 80 ppm), and the middle portion has the highest

(mean = 160 ppm). The lower portion of the tephra sequence has Zr

87

 

150 —

Zr

 

 

 

 

 

 

 

 

 

 

 

 

 

100 "' .1

50 ' l l l
74 75 76 77 78

I I I

A

150 —- -
125 - 1
100 F -
a 75- —
50 ~ 4
25 ~ -

0 1 1 1
74 75 76 77 78

L0 1 1 I

A
0.8 - AA A -
‘ A ‘ 0A o 00 A
A M A
o 05- ‘ #‘ a)“ d
a
o

03 - -

0.0 l 1 1
74 75 76 77 78

SK)
2

A = upper Post-grouse Canyon tephra sequence
A = middle Post-grouse Canyon tephra sequence
0 = lower Post-grouse Canyon tephra sequence

Figure 25. Variation diagrams of Zr, CaO, and Sr vs. Si02, illustrating
distinct trends for the lower, middle, and upper portions of the Post-Grouse
Canyon tephra sequence.

88

concentrations intermediate between the lower and middle portions of the
sequence (mean = 120 ppm). This pattern is not compatible with crystal
fractionation relating these groups.

Within any of these chemical groups, the decrease of CaO and Sr with
increasing Si02 (Figure 25), suggests that crystal fractionation dominated by
plagioclase can account for the observed variation.

The high Zr group from the highly variable layer cannot be produced
by fractionation of any other chemical group. This group is the least evolved
(66-74% Si02) of any group fiom the Post-Grouse Canyon tephra sequence.
However, any one of the other chemical groups can be produced by crystal
fractionation of the low Zr group from the highly variable layer. Numerous
regression models can produce an excellent fit for any of the chemical groups
by fractionating combinations of present phenocryst phases. Crystal
fractionation models for any group, however, are dominated by plagioclase,
and alkali feldspar fractionation (Figure 26). The best multiple linear

regression model for each chemical group is reported in Table 2.

PARTIAL MELTINQ

Reasonable models of magma mindng or crystal fractionation cannot
relate the lower, middle, and upper portions of the Post-Grouse Canyon
tephra sequence. The presence of the highly variable layer suggests that new
magma was introduced into the Post-Grouse Canyon magmatic system at

approximately 13.3 Ma. Three possible relationships exist for the series of

89

 

 

 

 

 

 

 

 

 

 

0
° 1 "’ ° °°Aggg___
70 75 80
310,
Q: o co. II
J
0 L l
65 7O 75 80
SK)
2

A = upper Post-grouse Canyon tephra sequence
A = middle Post-grouse Canyon tephra sequence
0 = low Zr samples from the highly variable layer
0 = high Zr samples from the highly variable layer
0 = lower Post-grouse Canyon tephra sequence

Figure 26. A1203 and CaO variation diagrams evaluating potential
fractionation trends for the Post-Grouse Canyon tephra sequence.

90

Table 2. Regression analysis of crystal fractionation for the Post-grouse
Canyon tephra sequence.

Evaluation of crystal fractionation to produce the lower tephra sequence from the high Zr group of the
highly variable layer.

Parent = RM-l9R65-21 lG

 

Cocf SA: Cun1_,_,__,mb/Lingr_§l/1399k 6ND- - .- --

H v-w.,

 

 

 

 

0.044 6.8 m21-7 biotite
0.049 7.6 m21-7 hornblende -
0.002 -0.4 m19-10 ilmenite
0.012 1.8 m21-7 magnetite
0.136 20.9 m21-27 orthoclase
0.410 63.2 m21-27 plagioclase
0.349 RM-19R65-4D1=Daughter

8102 T10; A1293 FeO MnO MgO CaO NazO K20 P293“
Daughter 77.15 0.13 12.75 0.94 0.05 0.04 0.64 2.32 5.98 0.00
ParentRM-19R65-
leG
ObsParent 66.21 0.39 17.51 3.30 0.16 0.76 2.13 5.02 4.44 0.08
Cale. Parent 66.23 0.39 17.22 3.30 0.13 0.79 2.11 5.18 4.49 0.00
Difference (wt %) -0.01 0.00 0.15 0.00 0.03 -0.03 0.02 0.16 -0.05 0.08

Sum of the squares of the residuals = 0.057

91

Table 2 (continued)

Evaluation of crystal fractionation to produce the low Zr group from the high Zr group of the highly
variable layer

Parent = RM-l9R65-21 lG
Coef %Cum Mineral/Rock

 

 

 

0.022 3.3 m21-7 biotite

0.069 10.3 m21—7 hornblende

0.000 0.0 m19-10 ilmenite

0.010 1.5 m21-7 magnetite

0.196 29.6 m21-27 orthoclase

0.366 55.2 m21-27 plagioclase

0.336 RM-l9R65-211E=Daughter _

Daughter 77.51 0.08 12.73 0.89 0.08 0.18 0.47 2.88 5.17 0.01
Parent RM-19R65-

leG

Obs. Parent 66.21 0.39 17.51 3.30 0.16 0.76 2.13 5.02 4.44 0.08
Cale. Parent 66.23 0.39 17.11 3.30 0.16 0.80 2.10 5.25 4.51 0.00

Diflerence (Wt %) -0.01 0.00 0.20 0.00 0.00 -0.04 0.03 -0.23 0.07 0.08
Sum ofthe squares ofthe residuals = 0.107

 

Evaluation of crystal fractionation to produce the middle tephra sequence from the low Zr group of the
highly variable layer.

Parent = RM-19R65-21 lG
Coef %Cum Mineral/Rock

0.037 5.6 m21-7 biotite
0.061 9.2 m21-7 hornblende
0.001 -0.1 ml9-10 ilmenite
0.009 1.3 m21-7 magnetite

0.160 24.3 m21-27 orthoclase
0.393 59.7 m21-27 plagioclase

 

 

0.340 RM-19R65-236B=Daughter

8102 T10; A1203 F60 MRO M50 C30 N820 K20 P205
Daughter 77.42 0.07 12.45 1.00 0.07 0.01 0.44 2.76 5.77 0.01
Parent RM-l9R65-236B
(1)5. Parent 66.21 0.39 17.51 3.30 0.16 0.76 2.13 5.02 4.44 0.08
Cate. Parent 66.22 0.38 17.07 3.30 0.14 0.81 2.10 5.28 4.52 0.00

Difference (wt%) -0.01 0.01 0.22 0.00 0.02 -0.05 0.03 -0.26 -0.08 0.08
Sum of thesquaresof the residuats=0.107

Table 2 (continued)

92

Evaluation of crystal fractionation to produce the upper tephra sequence from the high Zr group of the

highly variable layer.

_l:arent = RM-l9R65-2110

 

Coef /oCuL-.MILesr_a!(1399!<-_

.wwh-‘_~. ---.......-_._ --Ahi .. ._

 

 

 

0.043 6 6 m21-7 biotite
0.054 8 1 m21-7 hornblende
0.002 -0 3 ml9-10 ilmenite
0.011 1.7 m21-7 magnetite
0 159 24.2 m21-27 orthoclase
0.393 59.7 m21-27 plagioclase
0.339 RM-19R65-25-l-2 = Daughter

...sro:..--.:.io;.___412.ot -. reg.-. M119... -.MgQ---.§a.,Q... N829.-.1$2Q-_---P2_Q§.-...
Daughter 77.68 0.07 12.48 0.79 0.06 0.00 0.64 2.73 5.63 0.01
Parent RM-l9R65-236B
Obs.Parent 66.21 0.39 17.51 3.30 0.16 0.76 2.13 5.02 4.44 0.08
Cale. Parent 66.23 0.39 17.10 3.30 0.14 0.80 2.10 5.26 4.52 0.00
Difference (wt%) -0.01 0.00 0.21 0.00 0.02 -0.04 0.03 -0.24 -0.07 0.08

Sum of the squares of the residuals = 0.107

93

spatially related melts that produced the chemical groups of the Post-Grouse
Canyon tephra sequence: 1) the chemical groups represent melts related by
various degrees of partial melting from a single source that were
subsequently erupted to the surface; 2) the chemical groups represent partial
melts of the same source that resided separately and subsequently followed
separate differentiation paths; 3) the major chemical groups of the Post-
Grouse Canyon tephra sequence represent melting of different sources.

It is possible to test whether melts are related by partial melting by
comparing observed trace element behavior to predicted behavior of trace
elements between liquid and mineral phases in the magma. For this, it is
crucial that the partitioning of trace elements between phases is well
understood.

Experimental determinations of partition coefficients has been
conducted mainly for basaltic systems. However, in recent years, more has
been published on the partitioning of trace elements in more silicic systems
(Ewart and Griffin, 1994; Sisson, 1994; Sisson, 1991; Michael, 1987; Nash
and Crecraft, 1985; Mahood, and Hildreth, 1982;). The consensus is that the
partition coeficients in highly silicic systems are significantly more variable
than those for basaltic systems. Thus, the application of partition coeflicients
determined for a particular silicic System to another system is more difficult
than in basaltic systems. Because analytically determined partition

coemcients are not available for the Post-grouse Canyon magmatic system,

94

70

 

 

 

 

 

l l l T l
60 1- equilibrium _
melting theoretical parent
' I.)
50 1—
Y 40 _
30 1-
A upperPGC
20 1—
‘ middlePGC
0 lowerPGC
10
0 50 100 150 200 250 300

Figure 27 . Y vs. Zr discrimination diagram showing that the lower, middle
and upper portions of the Post-Grouse Canyon (PGC) tephra sequence cannot
be related by partial melting of a single source.
reasonable ranges of partition coefficient values were selected from the
literature to determine if the chemical groups could be related by and
reasonable means of partial melting. If none of the selected partition
coeflicient values produced a reasonable fit, the model was rejected.

Rocks related by different degrees of melting of the same parent must
lie on a curvilinear trend. A diagram of Y vs. Zr, shows that the chemical
groups making up the lower, middle, and upper portions of the tephra

sequence three chemical groups do not lie on a single curve (Figure 27) and,

therefore, cannot be related by melting of the same source.

95

 

250 — _

D

D

D’DJ>
DD

D D
,fi.»
13 tF>
DD

 

 

 

 

 

 

 

zoo _
22 AL AL
‘ at 5
AL
150 F‘ A A“ “w I
100 l 1 1
74 75 76 77 78
SKDZ
300 l l l
250 ~ _
200 + -
11 1‘ill‘ “ ‘1
A ‘
15° ” .isunsaga. AL ‘
§
‘:§“
100 - z: —
. A‘sAqu A
AA A
50 ~ _
0 l l l
74 75 76 77 73

810
2

A = upper Post-grouse Canyon tephra sequence
A = middle Post-grouse Canyon tephra sequence

Figure 28. Rb vs. Si02 and Zr vs. Si02 variation diagrams, evaluating partial
melting of a single source, for the middle and upper portions of the Post-
Grouse Canyon tephra sequence.

96

Conceptually, any two of the chemical groups can be related by partial
melting of a common source for any given trace element. A comparison of the
chemical behavior of more than one trace element, however, indicates that
some combinations of melting are unreasonable. The behavior of Zr and Rb
with respect to SiOz demonstrate this for the chemical groups representing
the middle and upper portions of the tephra sequence (Figure 28).

In high-silica systems, Zr acts compatibly. That is, Zr favors the solid
phase during crystallization or melting events. The reverse is true for the
incompatible behavior of Rb which favors the liquid phase. It is thus
reasonable to assume that if the middle and upper portions of the tephra
were sequentially produced by partial melting of the same source, that the
most primary rocks from the middle tephra sequence would have lower Zr
and higher Rb concentrations than the upper tephra sequence. That is, if the
middle portion of the tephra represents the first melt produced, it should
have the lowest Zr values, and the highest Rb values. The reverse behavior
is observed.

These relationships imply that the middle and upper portions of the
tephra sequence can be related by partial melting of a single source, but only
if the upper portion of the tephra sequence represents an earlier melt than
the middle portion of the tephra sequence. Thus, the magma represented by
the upper portion of the tephra sequence would have had to reside in the
crust until production and eruption of the magma that produced the middle

portion of the tephra sequence occurred. This equates to a minimum

97

 

150 — -

100

 

 

 

 

 

 

 

 

 

r5
50
0 1 1 1
74 75 76 77 78
SK)
2
300 1 1 I
250 — _
200
150
H
N
100
50 - ‘
0 1 1 1
74 75 76 77 78
SK)

2

A = middle Post-grouse Canyon tephra sequence
0 = lower Post-grouse Canyon tephra sequence

Figure 29. Variation of Sr and Zr with respect to Si02, evaluating partial
melting of a single source for the origin of the lower and middle portions of
the Post-Grouse Canyon tephra sequence.

98

residence time of at least 200,000 years based on the average age differences
between the two.

The lower and middle portions of the Post-Grouse Canyon tephra
sequence cannot be related by partial melting based on the chemical behavior
of Sr, and Zr with respect to Si02 (Figure 29). In very high silica systems,
both Zr and Sr act compatibly with respect to silica. In fact within a
chemical group, the concentrations of both elements decrease with increasing
silica.

If the chemical groups representing the lower and middle portions of
the tephra sequence were related by partial melting of the same
homogeneous source, the first melt produced should have the lowest Zr
concentrations. This would equate with the chemical group that comprises
the lower portion of the tephra sequence. However, Sr concentrations should
also below in the earlier melt. In the lower tephra sequence, Sr
concentrations are higher than they are in the middle tephra sequence
(Figure 25). This behavior is not consistent with the two chemical groups
being related by partial melting of the same homogeneous source.

Partial melting of the same source is a permissible relationship
between the lower and upper portions of the tephra sequence (Figure 30). In
high silica systems, Rb is highly incompatible, and the first melt should be
highly enriched in Rb. Likewise, Zr is compatible in high silica systems and

the first melt should have lower concentrations than subsequent related

99

 

300 j 1 n 1 l 1 1 1 1

F-start—‘-.01 F-cnd=.99 F-incr.=.l

 
   
    

 

 

 

 

 

 

 

 

 

_ Y _
22 20° D(Rb)=.86 D(Zr)=10 Y 9.;
%Y
D(Rb)=1
Rb)=1.13 D(Zr)=4.9
100 l l I l l I J I l
0 200 400 600 800 1000
Zr
400 1 1 1 1 l 1 1 1 1
D RD =.5 D Zr '8
300 _ ( ) ( l _
A
Rb 200 - ‘
100 - -
0 1 1 1 1 l 1 1 1 1
0 200 400 Zr 600 800 1000

U = Topopah Springs Tufl‘

= upper Post-grouse Canyon tephra sequence
A = middle Post-grouse Canyon tephra sequence
Y = Grouse Canyon Tuff

Figure 30. Variation diagram of Rh vs. Zr, and potential melting curves
relating the Grouse Canyon ’I‘ufi' (upper) and the Topopah Springs Tufl'
(lower) to the middle and upper portions of the Post-Grouse Canyon tephra
sequence.

100

melts. A diagram of Rb vs. Zr shows that the upper portion of the tephra
sequence contains higher Rb values and lower Zr values than the lower
portion of the tephra sequence. This is consistent with the upper and lower
portions of the tephra sequence being related by partial melting, with the
upper portion of the tephra sequence representing an earlier melt.

The behavior of other trace elements in the two groups indicate that
sequential partial melting could produce the upper and lower portions of the
tephra sequence (Figure 30).

Partial melting of a single source cannot relate all three of the lower,
middle, and upper portions of the Post-Grouse Canyon tephra sequence.
However, partial melting of the same source can relate either the middle and
upper portions of the tephra sequence, or the lower and upper portions of the
tephra sequence. Either situation requires that the upper portion of the
sequence melt first.

Relating the origin of the middle and upper portions of the tephra
sequence by partial melting of the same source is far less complex than
relating the origin of the lower and upper portions of the sequence by partial
melting. If the upper portion of the tephra sequence was related to the lower
portion of the tephra sequence, then the upper portion of the sequence would
have to form first, and then be stored for 500,000 years. During this period,

the lower portion of the tephra sequence, and the unrelated middle portion of

101

the tephra sequence would have erupted. This sequence of events is very
complex and seems unlikely.

Relating the middle and upper portions of the tephra sequence is
slightly less complex. In this situation, the storage time for the magma that
produced the upper portion of the tephra sequence, is 300,000 years. In
addition, an unrelated lower portion of the tephra sequence would erupt
before the related middle and upper portions of the sequence, instead of

between them.

MAGMATIC SOURCE’S

Compositions for potential sources are unknown. To date, there are no
studies relating xenolith chemistry to potential sources for Nevada ash-flow
tufl's. Numerous workers suggest that the Nevada volcanics are ultimately
related to melting of the lower crust or lithospheric mantle (Farmer et al.,
1991; Hildreth and Moorbath, 1988; Menzies et al., 1983; Farmer, 1989;
Cambray et al., 1995). Nd and Sr isotopes and whole rock compositions
suggest that intermediate and silicic magmas from the Paintbrush and
Timber Mountain Groups may be derived from co-erupted basalts (Farmer et
al., 1991). There are no basaltic layers, however, present in the Post-Grouse
Canyon tephra sequence.

The specific sources of the Post-Grouse Canyon magmas remain

unknown. The middle and upper Post-Grouse Canyon tephra sequence may

102

be related to either the Grouse Canyon Tuff (erupted prior to the tephra
sequence) or the Topopah Springs Tuff (erupted just after the tephra
sequence).

There is no reasonable equilibrium or fractional melting model that
can relate the Grouse Canyon Tufi and both the middle and upper portions of
the tephra sequence (Figure 30). With any combination of bulk distribution
values for Rb and Zr, calculated melting paths cannot include these three
groups.

Equilibrium partial melting models can produce the Topopah Springs
Tuff and the upper and middle portions of the Post-Grouse Canyon tephra
sequence. In a diagrams of Rb vs. Zr and Sr vs. Zr (Figure 30b), melting
curves are constructed using bulk distribution coefficients (DRb = 0.5; Dzr = 8;
DSr = 10). In this model, partition coefficients consistent with these values
are reported for Rb and Sr in Nash and Crecraft (1985), and Hildreth (1977),
for sanidine and plagioclase in high-silica rhyolite. As long as sanidine and
plagioclase are the main melting phases, reasonable bulk distribution
coefficients could be calculated from these partition coefficients. The highest
partition coefiicients found for Zr, however, are not more than 3.9 (Ewart and
Griflin, 1994) for magnetite in dacite, and even less in high silica rhyolite. Zr
could have a larger value if one of the main melting phases is zircon. This is
unlikely, however, as zircon is a refractory phase and would not participate

in initial partial melting events.

103

The Pre-rainier Mesa tephra sequence

It is clear that the lower (older) portion of the Pre-Rainier Mesa tephra
sequence is chemically distinct from the overlying upper (younger) portion of
the sequence. Geochemical considerations provide strong evidence that the
lower and upper portions of the l’re-Rainier Mesa Tephra Sequence are not
related by crystal fractionation (Figure 17). The lower portion of the tephra
sequence is also distinct from the underlying Tiva Canyon ash-flow sheet,
and the overlying Rainier Mesa ash-flow sheet, although, radiometric
4°Ar/39Ar ages are indistinguishable between the lower Pre-Rainier Mesa
tephra sequence and the Tiva Canyon ash-flow sheet.

From chemical considerations, the lower Pre-Rainier Mesa tephra
sequence, is consistent with two models of magma mixing. The best
numerical fit is achieved by mildng the low M Rainier Mesa magma and
the low silica Tiva Canyon magma to produce the lower Pre-Rainier Mesa
magma. Alternatively, regression analyses indicate that the low silica
Rainier Mesa magma could be mixed with high Th/Nb Rainier Mesa magma
to produce another good fit. All other potential mixing models can be
eliminated on inspection of trace element ratio diagrams.

Trace element ratios were examined in order to evaluate magma
mifing (Langmuir et al., 1977; Cox et al., 1979). This test is not exclusive

and a comp anion plot of one of the original ratios plotted against the ratio of

l 04

the denominators provides an additional test for mixing. Here, the end
members and the supposed hybrid magma must plot on as a straight line
(Langmuir et al., 1977). Rollinson (1993), has shown that plotting elemental
ratios often forces trends in unrelated rock suites. This behavior limits the
value of interpretations made for rock suites that fit this pattern. However,
such plots are extremely useful for eliminating potential solutions.

Most potential mixing combinations for the lower Pre-Rainier Mesa
tephra sequence can be eliminated upon inspection by using ratio/ratio plots
for comparison. However, two combinations can produce the lower portion of
the Pre-Rainier Mesa tephra sequence: 1) a low silica Tiva Canyon magma
minng with low Th/Nb Rainier Mesa magma; and, 2) low silica Rainier Mesa
magma mixing with the high Th/Nb Rainier Mesa group, to produce the
lower portion of the Pre-Rainier Mesa tephra sequence.

Regression analyses provides another quantitative means by which to
evaluate magma mixing. For each test of magma mixing, multiple linear
regressions were performed on two parent samples, one from the Tiva
Canyon ash-flow sheet, and one from the Rainier Mesa ash-flow sheet, to
produce a representative hybrid magma selected from the lower Pre-Rainier
Mesa tephra sequence. Sample selection for each regression was based on
potential mixing trends observed on chemical variation diagrams. More than
one potential mixing trend exists for production of a Pre-Rainier Mesa hybrid

magma from a mixture of Tiva Canyon and Rainier Mesa magmas.

105

Regressions were performed for all possible parent combinations to produce
the lower portion of the tephra sequence.

Major oxide concentrations were regressed based on the following
equation:

1.00 A = bB + cC
where A is the hybrid daughter magma, B and C are parent magmas; and, b
and c are the coefficients representing the fraction of magmas B and C to be
mixed. Goodness of fit is measured by the sum of the squares of the residuals
and by r2. The sum of the squares of the residuals should be less than one for
geologic systems, and r2 should approach unity.

The best regression results were obtained by mixing 12.2% of the low
silica Tiva Canyon (Tiva- 1) and 87.8% of the low Th/Nb Rainier Mesa magma
(R26-20) to produce the lower Pre-Rainier Mesa magma, 7F (1.00 7F = 0.122
Tiva-l + 0.878 R26-20). The sum of the squares of the residuals for this
regression is 0.171 and r2 = 0.98 (Figure 31).

A good regression analysis was also obtained by mixing 18.6 % of the
low silica Rainier Mesa group (R8-16) and 81.4 % of the low Th/Nb Rainier
Mesa magma group (R8-21) to produce the lower Pre-Rainier Mesa sample,
7F (1.00 7F = 0.186 R8-16 + 0.814 R8-21). For this regression, the sum of the

squares of the residuals is 0.264 and r2 = .96.

106

 

 

 

 

 

 

 

 

 

I I I I I T I
8- 1
6*- ‘
é
.0 4_ F
a:
2_ -I
0
0 80
10 1 I I F
A
8__ —
a
{I .—
.o
a:
_,
1.0

 

A = low Th/Nb group from the Rainier Mesa ash-flow sheet
U = lower Pre-rainier Mesa tephra sequence

0 = low silica group from the Tiva Canyon ash-flow sheet

Figure 31. Trace element ratio diagram of Rb/La vs. Zr/Th, and companion
plot Rb/La vs. Th/La, evaluating miidng of low silica Tiva Canyon magma
and low Th/Nb Rainier Mesa magma to produce the lower Pre-Rainier Mesa
tephra sequence.

107

An independent test of magma mixing is the comparison of calculated
trace element concentrations determined by regression analysis to the
observed trace element concentrations of the hybrid magma. Trace element
values calculated for both mixing models, do not difi'er significantly from the
observed values for glassy pumice fragments from the lower Pre-Rainier
Mesa Tephra Sequence. Calculated trace element abundance compared with
observed trace element values are reported in Table 3.

Two samples were excluded from the comparison because they
consistently plot away from the rest of the group making up the lower portion
of the tephra sequence. One sample, 8A, is the same sample high silica
sample that consistently plotted away from the lower tephra sequence in
trace element variation diagrams (Figure 17). The behavior of the other
sample, 10A, is not easily explained except to say that weathering has most
likely modified its composition.

Although two magma mifing models can account for the chemical
variation of the lower Pre-Rainier Mesa tephra sequence, mixing of low silica
Tiva Canyon magma with low Th/Nb Rainier Mesa magma is a more
geologically viable solution. Because both the Tiva Canyon and Rainier Mesa
'I‘ufi's erupted from the same nested caldera, it is reasonable that unerupted
low silica Tiva Canyon magma could mix with incoming high silica, low
Th/Nb Rainier Mesa magma to produce the lower Pre-Rainier Mesa tephra

sequence. This is also consistent with the fact that the upper Pre-Rainier

108

Table 3. Regression analysis producing sample 7F from two mixing
combinations.

Mixture of low Si02 Tiva Canyon magma with high silica low Th/Nb Rainier Mesa magma
Si02 TiOz A] Q3 F69 :MnQ“ MgQ CaO NagQ K20 P305”

-m- -w-.

 

 

 

 

 

 

Tival 65. 92 0.61 17.14 199 0.1 0.77 207 4.33 666 0.14
R26-20 76.67 0.15 13.25 0.84 0.06 0.27 0.38 3.23 5.14 0.01
Observed 7F 75.16 0.2 13.55 1.13 0.09 0.7 0.54 3.32 5.3 0.01
Calculated 7F 75.12 0.21 13.71 0.98 0.06 0.33 0.58 3.35 5.31 0.03
Difference (wt %) 0.02 -0 -0.08 0.15 0.03 0.37 -0 -0.03 -0 -0.02
lSzum of the squares of the residuals (SSR) = 0.171

= .86

Tiva l R26-2_0 Observed 7F CEIEEIELGQ 7F Residual

Rb 76.5 215.5 204 197.9 6.1
Zr 887.7 96.0 195.4 191.9 3.5
La 218.3 25.5 45.5 48.9 -3.4
Lu 0.3 0.5 0.5 0.5 0.0
Yb 3.0 3.3 2.9 3.3 -0.4
Ce 386.9 60.2 84.8 99.8 -15
Eu 5.2 0.2 0.8 0.8 0.0
111' 16.5 4.4 6.9 5.8 1.0
Tb 0.9 0.7 1.0 0.8 0.3
Th 12.6 23.4 23.2 22.0 1.2

 

Mixture of low silica Rainier Mesa magma with high silica, high Th/Nb magma
S102 T102 A1203 FeO MnO MgO NazO CaO K20 P205

 

 

 

R8-l6 76.88 0.13 13.40 0.77 0.08 0.54 0.43 2.62 5.13 0.02
R8-21 67.35 0.47 16.86 2.38 0.09 0.76 2.14 4.52 5.28 0.15
(liserved7F 75.16 0.20 13.55 1.13 0.09 0.70 0.54 3.32 5.30 0.01
Calculated7F 75.15 0.19 14.05 1.07 0.08 0.58 0.75 2.98 5.16 0.04
Difl'erence (wt%) 0.00 0.01 -.0.25 0.06 0.01 0.12 -0.21 0.34 0.14 -0.03
Sumofthesquaresoftheresiduals=0264
=.96
R8-16 R8-21 Observed 7F Calculated 7F Residual
Rb 252 90 204 222 -18
Sr 14 453 61 95.8 -34.8
Y 46 21 35.5 41.4 -5.9
Zr 107 497 195.4 179.7 15.7
Nb 26 16 23.9 24.2 -0.3
La 16.3 125 45.5 36.6 8.9
Lu 0.5 0.2 0.5 0.4 0
Yb 4.0 1.7 2.9 3.6 -0.7
Ce 61.2 194.1 84.8 86 -1.2
Eu 0.3 1.9 0.8 0.6 0.2
Hf 4 11 6.9 5.3 1.5
Sc 3.6 3.7 2.6 3.6 -1.0
Tb 0.6 0.6 1 0.6 0.4
Th 24.4 29 23.2 25.3 -2.1

 

109

Mesa tephra sequence is chemically equivalent to the low Th/Nb group from
the Rainier Mesa Tufi'.

It is far more unlikely that low silica Rainier Mesa magma and high
Th/Nb Rainier Mesa magma would mix and erupt immediately after the Tiva
Canyon with no contribution from Tiva Canyon magma. It would also be
difficult to account for the fact that the upper Pre-Rainier Mesa tephra

sequence is chemically equivalent to the low Th/Nb Rainier Mesa Tufi‘.

The Pre-Ammonia Tanks tephra sequence

The Pre-Ammonia Tanks tephra sequence is the only one that does not
vary chemically with stratigraphic position. In this sequence the entire
chemical variation (66 to 78 wt.% Si02) is observed throughout the sequence.
Any model explaining the magmatic history of the Pre-Ammonia Tanks
magmatic system must account for the full chemical variation present
throughout the tephra sequence.

The chemical variation of the Pre-Ammonia Tanks tephra sequence is
very similar to that of the overlying Ammonia Tanks 'I‘ufi'. Nb concentrations
of the high silica portion of the Pre-Ammonia Tanks tephra sequence span
the compositional range of both the Ammonia Tanks Tufl‘, and the underlying
Rainier Mesa Tufl‘. The lower-silica portion of the tephra sequence
consistently fills a compositional gap between the low silica and high silica

portions of the Ammonia Tanks ash-flow sheet.

110

Radiometric 40Ar/39Ar ages for the Pre-Ammonia Tanks tephra
sequence are not able to distinguish a clear age difference between the base
and the top of the tephra sequence due to overlapping ages and to large
errors associated with absolute age values. The latter is most likely due to
secondary hydrothermal alteration of the sequence. Higher average age
values for the base of the sequence and very small overlaps between the base
and the top of the sequence, suggest of an age difference of approximately
200,000 years between the base and the top of the sequence. This age
difference, if real, and the chemical similarity of the Pre-Ammonia Tanks
tephra sequence to the overlying Ammonia Tanks ash-flow sheet, may
indicate that the Ammonia Tanks magma is also a long lived system.

The best examples of the chemical variations and inferred age
differences noted between the base and top of the sequence, are where high
silica and lower silica magma (down to about 66 wt.% silica) were apparently
emplaced immediately after the eruption of the Rainier Mesa ash-flow sheet.
In this scenario, there are no restrictions on the time of emplacement for the
Ammonia Tanks low silica compositional range. If the entire compositional
range was emplaced at this time, subsequent eruptions tapped only down to
66 wt.% Si02. It is unclear whether the entire chemical range of the Pre-
Ammonia Tanks tephra sequence was tapped periodically, until the eruption
of the overlying Ammonia Tanks ash-flow sheet occurred, or all eruption

ceased until just before the eruption of the Ammonia Tanks Tufi'. Age

Ill

determinations from only the base and top of this tephra sequence limit the
possible interpretations of this sequence.

The absence of changing compositions from the base to the top of the
Pre-ammonia Tanks tephra sequence, and inconclusive radiometric ages did
not allow magmatic evolution rates to be calculated for this sequence .
However, chemical and age data are interpreted to represent a long lived
Ammonia Tanks magma body that contained highly evolved, high silica,
Ammonia Tanks magma, and lower silica magma injected into the system

near or at the time of emplacement.

Chapter 7
CONCLUSIONS
The Post-Grouse Canyon tephra sequence

Although radiometric age differences match chemical changes within
the Post-Grouse Canyon tephra sequence, rates for specific magmatic process
that produce chemical change within the pre-eruptive magmatic system
cannot be calculated. This is because at least three out of the four chemical
groups present were likely produced by the eruption of unrelated magmas.

It is possible to relate a) the middle and upper portion of the tephra
sequence, or b) the lower and upper portions of the tephra sequence by
partial melting of a single source, provided that in both cases the upper
portion of the tephra sequence represents an earlier melt. The former is
geologically more reasonable. In this scenario, the upper portion of the
tephra sequence must have resided in the crust for at least 300 ka prior to
eruption, based on average radiometric age differences between the middle
and upper portions of the sequence. For this to occur a) the crust into which
the melt resided was hot enough so that the closure temperature of Ar in
sanidine was not reached, b) the magma was still fluid enough for eruption to
occur, and c) the magma did not accumulate more than about 5% phenocrysts

(the total phenocryst content of glassy pumice from the sequence).

112

113

The eruption of the Post-Grouse Canyon precedes the most intense
period of eruptive activity in the history of Cenozoic volcanism in the
southwest Nevada volcanic field. One hundred thousand years after the
eruption of the Post-Grouse Canyon tephra sequence, more than 4400 km3 of
volcanic material was erupted over a time span of less than 2 million years.
An elevated thermal budget was most likely already present in the crust
produced by the Grouse Canyon and preceding magmatic systems. If the
Paintbrush magmas were already working their way through the crust, they
would also contribute to elevated crustal temperatures.

The alternative is that the middle and upper portions of the tephra
sequence, like the other chemical groups within the sequence, represent
partial melting of separate sources. This is possibly the most reasonable
explanation as it eliminates problems of a long storage period for the upper
sequence while the middle sequence is emplaced and erupted.

Whether or not the upper and middle portions of the tephra sequence
are related by partial melting of a single source, the dominant relationship is
that numerous unrelated magmas erupted one after another in the same
place. One reason for focusing on the tephra sequences in this study was the
belief that the small volume eruptive deposits represented a series of
intermediate steps in the evolutionary history of a larger-volume magmatic
system. It is remarkable to consider that a sequence of smaller-volume

deposits, closely associated in time and place, are unrelated to one another.

114

This is the same relationship that has been interpreted to relate the chemical
groups in some of the very large-volume ash-flow sheets (Saltoun, 1996;
Cambray et al., 1995; Mills, 1991).

Prior to the eruption of the Post-Grouse Canyon tephra sequence,
elevated temperatures in the crust coupled with the possible emplacement of
Paintbrush magma may have produced a series of partial melts from the
melting of separate sources or of a single heterogeneous source. Separate
sources may have been tapped if partial melting occurred at various levels in
the crust as the Paintbrush magma was emplaced at higher levels. Once
formed, if these melts were isolated, each would follow its own difl'erentiation
path. Cambray et al. (1995) suggested that magma bodies which produced
the large-volume ash-flow sheets of the Paintbrush and Timber Mountain
Tufl's were stored along series of releasing steps created during periods of
extension. This model provides a mechanism for magmas to be transported
through the crust as well as a simple method for producing separate
evolutionary paths. The interpreted relationships among the chemical
groups in the Post-Grouse Canyon tephra sequence support this model.

The purpose of this study was to determine rates of magmatic
processes leading to the chemical variation of large-volume magmatic
systems in the Southwest Nevada Volcanic Field. This efl'ort results in the
general conclusion that magmatic processes accounting for chemical

variation of the tephra sequences, are dominated by, (1) the emplacement of

115

often unrelated melts and (2) magma mixing that takes place over time

scales too short to be measured by 40Ar/39Ar dating.

The Pre-Rainier Mesa tephra sequence

Radiometric age, stratigraphic position, and chemical groups divide
the Pre-Rainier Mesa tephra sequence into two distinct magma types. The
upper Pre-Rainier Mesa tephra sequence is the compositional and age
equivalent of the high silica, low Th/Nb magma group defined by Cambray et
al. (1995) and Saltoun (1996) from the overlying Rainier Mesa ash-flow
sheet. It is interesting to note that no evidence of the high Th/Nb Rainier
Mesa group is observed in this sequence.

Ages of the lower portion of the tephra sequence and the underlying
Tiva Canyon ash-flow sheet are the same. However, the lower portion of the
Pre-Rainier Mesa tephra sequence is chemically distinct from the Tiva
Canyon ash-flow sheet. Regression analysis indicates that the lower portion
of the Pre-Rainier Mesa tephra sequence is most consistent with mim'ng of
12.8 % of the low silica Tiva Canyon magma with 87.2% of the high silica,
low Th/Nb Rainier Mesa magma.

There is no evidence that any Rainier Mesa magma was present
during the eruption of the Tiva Canyon Tufl'. Samples with the composition
of the Rainier Mesa Tuff have not been observed in the Tiva Canyon ash-flow

sheet. However, age relationships presented here indicate that magma with

116

Rainier Mesa Tuff composition must have entered the magma chamber
almost immediately after eruption of the Tiva Canyon ash-flow sheet, to mix
with low silica Tiva Canyon magma.

In this situation, the magma that produced the Rainier Mesa Tufi‘,
immediately filled the same chamber from which the Tiva Canyon magma
erupted. Rapid mixing of these two end member magmas must have
occurred to form the completely mixed magma of the lower Pre-Rainier Mesa
tephra sequence. As space was created by the eruption of the Tiva Canyon
Tufl', and Rainier Mesa magma entered the chamber, high energy, turbid
conditions for complete mixing of the two end member magmas would be
produced.

The most important implication of mixing Rainier Mesa and Tiva
Canyon end members, to form the lower Pre-Rainier Mesa magma, is that the
high silica Rainier Mesa magma (or a Rainier Mesa-like magma) must have
existed at the time of eruption of the lower Pre-Rainier Mesa tephra sequence
( 1.1 million years before the Rainier Mesa Tufl' was erupted). This is
evidence for a long-lived, already developed, high silica, low Th/Nb Rainier
Mesa magma body that was emplaced from deeper levels in the crust when
the Tiva Canyon ash-flow sheet was erupted.

The lack of evidence that the high Th/Nb Rainier Mesa magma was
erupted with the upper Pre-Rainier Mesa tephra sequence, suggests that it

was not present at the time the upper portion of the tephra sequence was

117

erupted (11.6 Ma). In this case, the high Th/Nb magma must have been
emplaced very quickly to have been erupted with the overlying Rainier Mesa
Tufi.

Alternatively, the high Th/Nb magma may have resided at deeper
levels in the chamber. This is consistent with Fe-Ti oxide temperatures for
the two Th/Nb groups from the Rainier Mesa Tufl' (Figure 19) [Saltoun, 1996;
Mills et al. (in review)], where temperatures from the low Th/Nb group have
an average Fe-Ti oxide temperature 54°C lower than the high Th/Nb group.
If Fe-Ti oxide temperatures in the Rainier Mesa Tufl‘ reflect temperature
gradients in the pre-eruptive magma chamber, the low Th/Nb group would
have resided at a higher level than the high Th/Nb group. Eruptions that
produced the smaller volume, upper Pre-Rainier Mesa tephra sequence, may
not have tapped deep enough into the chamber to sample the high Th/Nb
magma.

A thin layer of basaltic ash, present at the top of the sequence,
suggests that the low silica Rainier Mesa magma was present during the
final eruption of the Pre-Rainier Mesa tephra sequence. Mills (1991),
suggests that injection of low silica magma may have triggered eruption of
the Rainier Mesa ash-flow sheet. However, the low silica portion of the
Rainier Mesa ash-flow sheet is present only in the upper 10% of the ash-flow
sheet. It is difficult to reconcile how basaltic ash was able to erupt just prior

to the Rainier Mesa ash-flow sheet, and not again until 90% of the ash flow

118

had been erupted. As no chemical analyses of this basaltic layer have been
made, and there is some uncertainty as to how (or whether) this layer is
related to the low silica portion of the Rainier Mesa ash-flow sheet, no
attempt is made here to resolve the dynamics of its eruption or its
relationship to the rest of the sequence.

Evidence for magma mixing recorded in the Pre-Rainier Mesa tephra
sequence occurs over a time scale that is too short to be measured by 4°Ar/39Ar
age dating techniques. However, the interpretation that the low Th/Nb
Rainier Mesa magma was present at 12.7 Ma, and the fact that the high
Th/Nb Rainier Mesa magma is not observed in the Pre-Rainier Mesa tephra
sequence, places constraints on the origin of the high silica portion of the
Rainier Mesa Tufi‘. This evidence also indicates that the low Th/Nb magma
was long lived, and was highly evolved before emplacement into the crustal

level from which it was erupted.

The Pie-ammonia Tanks tephra sequence

The Pre-Ammonia Tanks tephra sequence, though characterized by a
large chemical range, does not display systematic compositional changes from
the base to the top of the sequence. This suggests little, if any, chemical
modification of the Pre-Ammonia Tanks system between the first and last
eruptions. Chemical variation of the Pre-Ammonia Tanks tephra sequence is

consistent with the overlying Ammonia Tanks Tufl‘.

119

Ages for the Pre-Ammonia Tanks tephra sequence are not well
constrained. This is most likely due to hydrothermal alteration of the
sequence. However, the base of the Pre-Ammonia Tanks tephra sequence is
generally more consistent with the age of the Rainier Mesa Tuff than with
the age of the overlying Ammonia Tanks Tuff. This lends support to the idea
that emplacement of Ammonia Tanks magma occurred very quickly during,
or just after, eruption of the Rainier Mesa Tufi‘. The Pre-Ammonia Tanks
tephra sequence is interpreted to represent a series of small eruptions from a
long-lived Ammonia Tanks magma body that was emplaced almost

immediately after eruption of the Rainier Mesa ash-flow sheet.

APPENDICES

APPENDIX 1

120

APPENDIX 1

ANALYTICAL TECHNIQUES
X-ray fluorescence (XRF)

Whole rock abundance for ten major element oxides and eleven trace
elements were obtained by X-ray fluorescence (XRF) at Michigan State
University. Fused disks were prepared for analysis at Michigan State
University using methods described, in detail, by Mills, 1991. Major and
trace element analysis abundance’s were collected with a Rigaku S-max X-
ray fluorescence spectrometer. Matrix absorption was corrected for all major
elements were corrected using Criss parameters (Criss, 1980). Trace
elements were calculated using multiple linear regression of USGS bulk rock
standards. Standards were run as known calibration standards and as
unknown. Analyses of ‘unknown’ standards were compared with known
concentrations to determine if analyses were within acceptable error limits.
Standard deviations for major oxides is i 1%. Detection limits and standard

deviations for trace elements are reported in Table 4.

121

Table 4. Detection limits and standard deviations for trace element analyses.

Element Detection limit (ppm) Relative standard
deviation (%)

Ba 100 10

La 48 10

Cr 63 5

Ni 25 4

Zn 14 6

Rb 13 6

Sr 12 5

Y 14 6

Nb l5 6

Zr 14 6

Instrumental Neutron Activation Analysis (INAA)

Trace element analyses for rare earth elements and 6 additional trace
elements, were obtained by instrumental neutron activation analysis (INAA)
at the University of Michigan, Ann Arbor.

Samples were prepared for INAA by encapsulating between 0.2 g and
0.25 g of rock flour in high purity quartz vials. Samples were irradiated with
a blank, internal standards from the Phoenix Lab at the University of
Michigan, and four USGS whole rock standards, coded as unknowns.
Standard errors for trace element analysis and deviations of INAA analyses

from USGS published concentrations are reported in Table 5.

Table 5. Standard errors for IN AA analyses.

Element l-sigma % error
Ba 8.16
La 9.54
Lu 16.26
Nd 18.58
Sm 8.85
Y 9.78
Ce 4.18
Cs 7.36
Cr 3.69
Eu 5.67
Hf 5.42
Sc 7.69
Ta 1 1.96
Tb 13.06
Th 1.81

Zn 5.75

Electron Microprobe Analyses

Phenocryst compositions were analyzed on a Cameca Microprobe at
the University of Michigan, Ann Arbor.

Grain mounts were prepared for analyses by course crushing the
sample with a mortar and pestle, and then separating heavy minerals from
feldspar, quartz, and glass by tungsten salt, heavy liquid separation.
Feldspars were further separated from glass by hand picking individual
crystals.

During analysis, grains were checked for homogeneity by analyzing
three spots accross the grain. If grains were zoned, three to eight spots were

analyzed across the grain, depending on the intensity of zonation.

APPENDIX 2

124

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129

APPENDIX 2. (Continued)

 

 

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APPENDIX 2. (Continued)

 

 

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3

APPENDIX 2. (Continued)

 

 

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7.
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APPENDIX 2. (Continued)

 

 

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a)
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134

APPENDIX 2. (Continued)

 

 

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135

APPENDIX 2. (Continued)

 

 

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136

APPENDIX 2. (Continued)

 

 

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137

APPENDIX 2. (Continued)

 

 

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138

APPENDIX 2. (Continued)

 

 

 

 

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139

APPENDIX 2. (Continued)

 

 

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140

APPENDIX 2. (Continued)

 

 

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14]

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00.0 00.0 00.0 ...0 0.0 00.0 .00 00.0 000
0 00.0 .00 0~.0 0.0 00.0 00.0 0 00.2
00.0 00.0 00.0 .00 00.0 00.0 00.0 00.0 00.2
00.0 00.0 .00 00.. 0... 00.0 ~00 00.0 000
00.0. 0.~. 00.~. 00.0. 00.0. 0.~. ~..~. .0~. EOE_<
00.0 ..0 ..0 ...0 ~.0 00.0 00.0 .00 .~.0:
00.0. .... 00... 00.0. 00.0. 00... 00... 00... 0.0.0

 

one- 500 Tim .on T 500 Tim

3. .00 0006 0.8.00 0.0.02,

QTEQGTim 9T50¢Tim .oiTwoaoTim uNHTFomaTim ”QT—baguim o”~-_.on_m_.-im

A8000. 0:09 00000000 0.000. 000000 000%....000

142

APPENDIX 2. (Continued)

...NF «.3 0m

 

 

0 0 0..
~.0. 0... 02
0.. ~0. .N
0.0~ 0.~ >
0. .. ..00 .0
0.00. 0.... 0m
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00.0 .00 0.~0.
00.0 ~.0 0.0.2
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00. .00 0006 0.00.00 0.0.02.
0.0.0-5032”. 06-300720
.8000. 20030000000 0.000. 08.000 00080-00...

 

APPENDIX 2. (Continued)

 

Pre-rainier Mesa tephra sequence (bulk tephra)

 

 

 

 

 

 

 

17A;1 17A;13 17A;12 17A;11 17A;7 17A;2 17A;24 17B;11
Weight Percent Oxide ME
Si02 69.47 69.35 68.72 69.85 70.4 70.67 74.12 70.78
Ti02 0.37 0.54 0.59 0.48 0.38 0.35 0.22 0.4
A1203 16.17 15.98 17.09 15.85 15.61 15.65 14.39 15.69
FeO 2.25 3.21 3.03 2.66 2.07 2.02 1.19 1.71
MnO 0.08 0.13 0.14 0.12 0.08 0.08 0.12 0.11
M90 0.77 1.11 1 1.09 0.84 0.76 0.37 0.45
CaO 2.01 1.7 1.44 1.64 1.9 1.7 0.49 1.06
Na20 3.81 3.54 3.6 3.76 3.63 3.63 3.49 3.52
K20 5.04 4.41 4.35 4.51 5.05 5.1 5.61 6.21
P205 0.03 0.03 0.04 0.03 0.03 0.03 0.01 0.05
X-ray Fluorescence (ppm)
Cr 1 .31 0 0 0 0 0 0 2.63
Ni 6.81 1 1.43 6.83 0 4.78 0 16.82
Cu 0.54 0 0 0 0 0 0 0
Zn 47.92 79.61 72.6 73.59 44.23 48.37 66.42 62.37
Rb 133.89 136.16 122.75 146.71 145.33 153.02 165.66 158.98
Sr 380.09 292.72 313.53 280.41 337.39 303.5 72.69 121.18
Y 32.27 36.58 38.99 41.98 32.37 28.58 38.04 31.21
Zr 248.13 359.2 545.62 339.97 244.87 245.04 239.64 359.31
Nb 6.6 20.62 20.18 17.45 13.03 22.1 24.45 12.36
La 94.42 73.39 110.98 67.49 73.03 71.73 66.12 68.15
Ba 1187.88 838.01 1980.01 791.4 938.75 951.97 281.2 823.67
Instrumental Neutron Activation Analyses (ppm)
As 2.76 5.09 3.93 3.46 1.9
La 98.47 64.68 66.54 51.37 123.17
Lu 0.46 0.52 0.37 0.52 0.43
Mo 3.74 3.54 3.32 6 3.41
Sm 11.61 9.61 8.16 8.97 13.03
U 2.5 3.37 3.08 3.89 2.48
Yb 3.82 3.19 2.63 3.4 3.02
Ba 1767.74 720.6 1062.51 460.63 1199.04
Ce 208.43 124.54 122.13 99.61 229.4
Cs 4.03 5.09 4.33 4.46 3.8
Cr 6.63 13.36 7.42 2.21 4.01
Eu 2.4 1.23 1.33 0.82 2.56
Hf 13.93 9.82 7.39 8.39 10.31
Nd 63.31 49.76 38.18 34.66 132.42
Sc 6.7 5.75 4.48 2.42 6.07
Ta 1.18 1.38 1.2 1.6 1.24
Tb 1.26 1.33 0.92 1.19 1.3
Th 21.82 20.55 19.09 23.2 20.92
Zn 78.26 87.39 62.32 81.73 88.13

APPENDIX 2. (Continued)

144

 

Pre-rainier Mesa tephra secgencejbulk tephra)

 

 

 

 

 

 

 

178:7 178:5.5 17B;4.5 17B;3.5 17B;2 17B;0 16;2 15:0
Weight Percent Oxide (wt %)
Si02 70.64 72.13 72.36 71.88 70.35 67.09 74.98 73.21
Ti02 0.45 0.35 0.32 0.37 0.45 0.59 0.24 0.3
A1203 15.74 15.29 15.28 15.2 15.7 16.87 13.51 14.35
Fe0 1.88 1.55 1.61 2.01 2.2 3.24 1.25 1.58
Mn0 0.11 0.1 0.1 0.09 0.1 0.09 0.07 0.07
M90 0.45 0.49 0.52 0.44 0.64 1 .05 0.33 0.53
Ca0 0.98 0.81 0.82 1.08 1.36 2.52 0.74 0.9
Na20 3.57 3.29 3.24 3.34 3.54 3.52 3.2 3.55
K20 6.11 5.95 5.71 5.55 5.59 4.9 5.65 5.51
P205 0.05 0.02 0.03 0.04 0.05 0.12 0.01 0.01
X-ray Fluorescence (ppm)
Cr 0 0 4.85 0 0 11.87 0 0
Ni 0 4.42 7.35 0 0 1.72 0 0
Cu 4.06 0 0 3.1 0 16.04 0 0
Zn 69.47 54.25 67.52 73.57 72.27 65.88 1247.1 47.14
Rb 167.43 177.46 177.33 164.18 157.19 145.3 176.4 174.6
Sr 121.22 96.19 101.26 143.94 168.38 395.08 72.9 105.89
Y 36.1 34.18 32.31 37.5 32.99 34.19 33.88 39.51
Zr 394.55 323.96 269.87 295.59 356.91 293 188.99 207.19
Nb 7.24 24.29 23.44 18.07 17.88 16.84 25.91 2.71
La 90.17 92.57 53.74 60.61 88.41 60.64 60.24 75.97
Ba 857.44 677.04 444.23 427.27 529.5 768.04 254.61 358.32
Instrumental Neutron Activation Analyses (ppm)
As 2.27 4.16 5.15 4.49
La 113.4 102.05 99.19 66.02
Lu 0.47 0.51 0.42 0.52
M0 3.6 4.74 4.77 4.4
Sm 11.53 12.01 11.14 9.48
U 3.03 3.96 3.67 4.13
Yb 2.77 3.22 2.76 3.57
Ba 887.16 709.33 1104.34 454.15
Ce 218.6 185.1 189.39 121.33
Cs 4.09 4.57 5.11 4.91
Cr 4.17 6.25 17.19 5.02
Eu 1.89 1.56 2.04 1.14
Hf 9.82 8.58 9.09 6.83
Nd 83.79 119.81 104.97 49.56
Sc 4.73 4.23 7.59 3.62
Ta 1.33 1.46 1.36 1.68
Tb 1.16 1.4 1.15 1.05
Th 22.49 22.96 23.45 22.52
Zn 83.77 56.44 110.14 45.13

APPENDIX 2. (Continued)

145

 

Pre-rainier Mesa tephra seguence (bulk tephra)

 

 

 

 

 

 

 

14:7 14:5.5 14;4 14:1.5 13:18.5 13:9 13:3.5 13:0.5
Weight Percent Oxide (wt %)
Si02 72.24 71.93 72.41 72.01 71.62 71.86 72.33 72.27
Ti02 0.4 0.39 0.36 0.38 0.29 0.3 0.28 0.29
AI203 14.38 14.57 14.74 14.6 15.94 15.97 15.65 15.4
FeO 2.15 2.05 1.92 2.18 1.5 1.5 1.44 1.51
MnO 0.08 0.09 0.08 0.09 0.08 0.08 0.08 0.09
M90 0.78 0.71 0.69 0.81 0.32 0.56 0.44 0.48
CaO 1.4 1.4 1.33 1.5 1.1 0.97 0.96 1.01
Na20 3.51 3.68 3.25 3.34 3.29 3.15 3.29 3.41
K20 5.03 5.13 5.17 5.03 5.81 5.57 5.51 5.51
P205 0.03 0.04 0.03 0.05 0.04 0.04 0.02 0.03
X-ray Fluorescence (ppm)
Cr 3.34 0 9.08 0 0 4.58 19.41 7
Ni 0 0 1.54 0.92 0 0 0 0
Cu 21.44 7.31 5.84 0 5.86 0 2.9 4.31
Zn 139.65 85.87 58.81 57.04 56.37 59.71 53.96 57.4
Rb 160.12 160.59 158.28 149.97 173.54 174.4 176.71 163.49
Sr 193.83 187.47 168.99 207.48 148.52 123.17 111.7 126.55
Y 38.09 41.06 36.34 40.32 36.66 39.45 31.8 35.8
Zr 216.47 210.87 187.56 197.36 222.38 229.57 223.96 228.03
Nb 22.35 20.9 26.55 12.18 29.22 26.26 23.61 7.41
La 57.13 88.89 61.01 70.52 53.69 85.14 55.3 58.33
Ba 453.88 292.34 68.85 342.12 327.27 292.3 5.81 157.67
Instrumental Neutron Activation Analyses (ppm)
As 4.45 4.83 3.85 3.33
La 58.34 60.8 70.12 63.93
Lu 0.47 0.46 0.5 0.46
Mo 4.48 6.76 3.62 3.88
Sm 9.19 8.99 10.19 8.93
U 4.05 3.76 3.68 3.95
Yb 3.22 3.12 3.23 3.04
Ba 397.907 504.92 468.99 512.16
Ce 110.69 118.69 145.29 129.9
Cs 5.19 4.85 5.64 5.56
Cr 12.62 8.86 4.44 2.65
Eu 1.1 1.18 1.22 1.21
Hf 7.15 6.46 7.62 7.73
Nd 91.82 41.89 46.88 39.58
Sc 4.71 4.55 3.51 3.42
Ta 1.46 1.43 1.8 1.45
Tb 1.29 1.18 1.14 1.12
Th 21 .31 20.54 23.64 23.76
Zn 144.79 78.93 48.35 70.8

APPENDIX 2. (Continued)

I46

 

Pre-rainier Mesa tephra sequence (bulk tephra)

 

 

 

 

 

 

 

12:26 12:22 12:18 12;15 12:13 12:75 12:1.75 12:0
Weight Percent Oxide (wt %)
Si02 72.51 72.26 72.17 71.24 70.4 71.44 69.9 70.46
Ti02 0.34 0.35 0.34 0.36 0.43 0.37 0.46 0.43
Al203 14.92 14.94 15.12 15.57 16.12 15.43 16.07 15.76
FeO 1.72 1.88 1.75 1.96 2.37 2.14 2.76 2.64
MnO 0.09 0.08 0.09 0.09 0.1 0.1 0.1 0.1
M90 0.59 0.72 0.7 0.82 1.14 0.88 0.8 0.68
CaO 1.14 1.34 1.28 1.31 1.28 1.3 1.55 1.42
Na20 3.4 3.43 3.46 3.63 3.51 3.47 3.63 3.64
K20 5.26 4.97 5.06 4.98 4.62 4.82 4.69 4.85
P205 0.03 0.02 0.03 0.03 0.03 0.03 0.03 0.03
X-ray Fluorescence (ppm)
Cr 11.12 7.22 9.73 10.02 23.38 5.11 1.18 0
Ni 0 0 0 0 20.03 4.25 4.96 7.02
Cu 2.03 3.64 0 12.23 9.46 1.57 0 0
Zn 57.52 53.9 53.65 63.48 69.07 56.88 91.18 77.43
Rb 150.42 155.34 147.18 155.41 156.81 151.47 133.95 132.94
Sr 148.01 173.28 188.42 175.48 189.53 171.18 201.81 177.25
Y 35.75 34.47 39.81 38.47 48.44 48.58 53.6 56.9
Zr 223.8 214.43 217.68 233.28 271.78 210.71 257.91 243.5
Nb 28.36 34.82 29.97 24.99 18.89 19.08 22.78 17.06
La 44.7 57.86 83.3 68.2 84.9 14.82 43.51 35.44
Ba 138.57 162.83 278.79 207.79 165.81 25.83 135.72 177.86
Instrumental Neutron Activation Analyses (ppm)
As 3.39 3.79 4.07 3.89 4.67
La 62 82.39 70.22 75.27 59.4
Lu 0.5 0.5 0.47 0.58 0.54
Mo 5.07 5.17 4.07 3.86 4.39
Sm 9.88 9.87 9.52 12.5 10.2
U 4.66 4.15 3.3 3.52 3.89
Yb 3.31 3.18 3.09 3.94 3.68
Ba 411.04 510.96 429.12 468.51 396.33
Ce 127.81 128.85 132.25 180.58 128.82
Cs 4.88 4.98 4.84 5.81 5.76
Cr 9.97 9.35 8.98 10.72 11.09
Eu 1.28 1.34 1.22 1.51 1.05
Hf 7.31 7.58 7.84 9.38 7.3
Nd 108.31 44.74 50.54 58.51 47.05
Sc 4.46 4.4 4.44 5.46 4.8
Ta 1.52 1.31 1.55 1.63 1.7
Tb 1.07 1.4 1.08 1.41 1.18
Th 21.3 22.18 21.29 24.74 24.21
Zn 70.17 79.08 58.95 71.23 81.57

APPENDIX 2. (Continued)

I47

 

Pre-rainier Mesa tephra sequence (bulk tephra)

 

 

 

 

 

 

 

3:200 3:188 3:156 3:120 3:58 3:3 2:30 1:129
Wetht Percent Oxide (wt %)
Si02 77.01 77.24 76.11 76.7 76.88 76.39 77.52 78.24
Ti02 0.1 0.11 0.15 0.21 0.19 0.22 0.1 0.11
Al203 13.09 12.79 13.42 12.88 12.74 12.95 12.66 12.13
FeO 0.72 0.73 0.95 1.3 1.16 1.37 0.71 0.71
MnO 0.07 0.07 0.07 0.06 0.07 0.07 0.06 0.07
M90 0.64 0.71 0.96 0.78 0.44 0.88 0.2 0.15
CaO 0.48 0.5 0.62 0.74 0.68 0.7 0.44 0.49
N320 3 2.94 3.09 3.02 3.16 3.17 3.42 3.23
K20 4.88 4.89 4.63 4.29 4.87 4.43 4.88 4.86
P205 0.01 0.01 0.01 0.02 0.02 0.02 0.01 0.01
X-ray Fluorescence (ppm)
Cr 0 0 0 0 0 0 0 0
Ni 0 0 0 0 0 6.73 1.29 0
Cu 0 0 0 0 0 0 0 0
Zn 39.15 30.72 32.33 43 45.9 52.17 20.59 22.39
Rb 238.96 224.6 212.13 198.82 228.11 213.74 246.52 235.4
Sr 17.71 22.37 51.18 83.2 88.72 78.33 14.69 15.99
Y 23.77 21.13 21.97 25.03 30.81 32.93 28.32 32.28
Zr 88.81 78.99 102.05 115.25 107.49 133.08 80.72 80.61
Nb 28.11 28.63 28.79 27.5 26.32 24.19 23.55 21.84
La 15.51 31.46 31.28 34.51 18.59 51.76 9.69 40.53
Ba 33.99 117.07 185.87 219.36 267.5 195.87 0 80.68
Instrumental Neutron Activation Analyses (ppm)
As 4.35 5.18 6.23
La 24.52 30.83 21 .3
Lu 0.41 0.47 0.49
Mo 8.78 6.8 9.22
Sm 4.09 4.85 3.97
U 4.82 5.74 6.4
Yb 2.41 2.84 2.9
Ba 219.25 334.96 175.78
Ce 51.98 81.1 47.31
Cs 4.83 11.01 9.95
Cr 2.2 6.47 2.51
Eu 0.17 0.34 0.13
Hf 3.34 4.87 3.87
Nd 13.76 41.83 13
Sc 2.89 4.28 3.85
Ta 1.95 2.29 2.43
Tb 0.73 1.1 0.88
Th 20.24 19.96 17.71
Zn 39.03 85 34.77

APPENDIX 2. (Continued)

148

 

Pre-rainier Mesa tephra sequence (bulk tephra)

 

 

 

 

 

 

 

1:117 1:94 1:78 1:54 1:31 1:18 1:14 1:1
Weight Percent Oxide (wt %)
Si02 77.7 77.27 77.07 76.77 77.91 77.15 77.89 77.26
Ti02 0.11 0.12 0.14 0.15 0.12 0.17 0.16 0.18
Al203 12.49 12.77 12.86 12.84 12.33 12.69 12.26 12.51
FeO 0.74 0.86 0.81 1 0.93 1.1 1.09 1.26
MnO 0.06 0.06 0.06 0.07 0.06 0.07 0.04 0.05
M90 0.27 0.24 0.29 0.32 0.24 0.54 0.36 0.43
CaO 0.47 0.52 0.53 0.6 0.56 0.67 0.64 0.65
Na20 3.33 3.32 3.41 3.54 3.19 3.1 3.01 3.15
K20. 4.81 4.84 4.81 4.7 4.64 4.48 4.53 4.49
P205 0.01 0.01 0.01 0.01 0.01 0.03 0.02 0.02
X—ray Fluorescence (ppm)
Cr 0 0 0 0 0 16.05 0 0
Ni 6.14 29.4 6.8 8.4 8.58 0 5.73 8.99
Cu 0 1.66 0 0 0.21 0 0 12.96
Zn 28.75 74.71 283.55 1134.93 26.36 35.77 22.97 892.77
Rb 224.88 237.02 233.02 224.24 211.82 185.87 178.78 179.59
Sr 15.15 30.42 31.6 46.63 36.49 85.05 68.69 85.59
Y 29.64 30.18 28.49 31.27 22.56 30.82 20.05 21.03
Zr 78.78 91.76 88.81 82.4 78.57 100.85 86.8 99.38
Nb 22.52 20.94 19.48 18.55 20.03 15.11 16.98 15.06
La 51.74 40.7 73.84 45.2 35.21 8.31 47.42 42.92
Ba 82.84 94.83 0 34.14 19.87 37.8 118.68 192.38
Instrumental Neutron Activation Analysflpm)
As 5.44 5.83 5.3 8.02 3.54
La 22.7 21.21 22.89 20.75 23.34
Lu 0.51 0.48 0.47 0.48 0.38
Mo 9.71 9.26 9.18 8.44 8.15
Sm 4.18 3.98 4.17 3.57 3.81
U 8.04 8.49 8.31 8.23 4.59
Yb 2.98 3.28 2.98 2.71 2.53
Ba 144.43 199.55 271.7 65.65 288.81
Ce 52.45 49.83 51 .34 48.23 50.43
Cs 9.84 11.34 11.32 9.88 12.99
Cr 2.41 1.52 2.5 3.84 8.59
Eu 0.19 0.19 0.2 0.19 0.31
Hf 3.58 3.82 3.77 3.55 3.47
Nd 17.04 11.42 9.11 9.01 14.45
Sc 3.71 3.88 3.98 3.51 3.85
Ta 2.51 2.45 2.51 2.25 1.9
Tb 1.04 0.92 0.81 0.53 0.88
Th 19.85 18.12 19.28 18.89 18.58
Zn 38.53 31 .52 25.49 35.65 58.53

149

APPENDIX 2. (Continued)

 

Pre-rainier Mesa tephra sequence (bulk tephra)
592-802-3 592-802-23 592-802-40 592—802-33 292-802-70 592-802-13
Weight Percent Oxide (wt %)

 

 

 

 

 

 

 

Si02 73.3 72.3 72.95 73.36 76.03 72.48

Ti02 0.16 0.19 0.14 0.15 0.13 0.19

AI203 13.08 13.06 12.79 12.7 12.42 13.05

FeO 1.02 1.28 0.9 1.01 0.84 1.26

MnO 0.07 0.07 0.06 0.07 0.07 0.07

M90 0.73 0.84 0.56 0.68 0.21 0.83

CaO 0.65 0.71 0.58 0.6 0.57 0.69

Na20 2.82 2.99 3.04 3.13 3.15 2.83

K20 _ 4.71 4.62 4.91 4.69 4.89 4.61

P205 ' 0.04 0.02 0.02 0.02 0.02 0.02

X-ray Fluorescence (ppm)

Cr 48.03 48.43 44.58 48.88 21.74 36.55

Ni 0 0 0 0 0 0

Cu 0 0 0 0 0 0

Zn 26.04 27.21 24.64 22.15 26.46 33.1

Rb 232.42 237.22 250.33 231 .89 248.64 229.58

Sr 64.2 85.52 51.99 46.65 26 89.01

Y 22.64 19.3 21.74 18.81 28.93 24.3

Zr 92.15 113.51 90.67 82.87 72.74 104.66

Nb 15.75 16.07 14.76 11 .34 16.49 15.32

La 33.12 25.28 18.81 13.47 34.5 38.43

Ba 154.97 232.53 170.5 111.88 191.59 203.28
lnstmmental Neutron Activation Anal:

As

La

Lu

Mo

Sm

U

Yb

Ba

Ce

Cs

Cr

Eu

Hf

Nd

Sc

Ta

Tb

Th

Zn

150

APPENDIX 2. (Continued)

 

 

Pre-rainier Mesa tephra sequence (glassy pumice)

 

 

 

 

 

 

28 20 3A 60 7E 7F 8A 1 1A
Weight Percent Oxide (wt %)
Si02 77.39 77.23 77.54 74.08 69.99 75.15 77.84 75.68
Ti02 0.09 0.09 0.09 0.23 0.34 0.2 0.19 0.16
Al203 12.8 12.88 12.76 13.99 16.31 13.55 12.3 13.57
FeO 0.56 0.6 0.48 1.63 1.44 1.13 0.91 0.89
MnO 0.07 0.07 0.07 0.13 0.14 0.09 0.08 0.12
M90 0.07 0.17 0.11 1.15 2.73 0.7 0.65 0.31
CaO 0.41 0.42 0.41 0.57 1.33 0.54 0.44 0.3
N320 3.4 3.42 3.46 3.3 3.79 3.32 2.81 3.13
K20 5.19 5.09 5.05 4.9 3.49 5.3 4.76 5.85
P205 0.01 0.02 0.02 0.02 0.43 0.01 0.01 0.01
X-ray Fluorescence (ppm)
Cr 5.9 11.2 9.2 10.6 23.5 18.1 17.1 0
Ni 0 8.9 1 .3 4.2 0 0 0 0
Cu 58.7 58.5 57.5 61.8 59.7 58.1 56.9 57.2
Zn 55.8 34.2 44.3 74.6 76 61.8 51.5 63.5
Rb 290.5 282 283.5 207.5 126.7 204 184 191.8
Sr 19.7 23 15.5 75.4 131.7 61 38.4 24.6
Y 41.1 37.2 38.9 41.1 55.4 35.5 39.8 48.9
Zr 68.1 65.8 62.4 218.2 292.4 195.4 150.9 204.6
Nb 30.4 26.3 30.5 24.5 18.5 23.9 26.6 25.1
La 29.2 0.2 14 71.5 91.5 40.5 34.9 16.3
Ba 202.3 240.5 82.3 280 1108.1 385.6 159.4 138.5
Instrumental Neutron Activation Analyses (ppm)
As 8.23 7.11 655 4.37 10.22 2.72 2.66 3.45
La 18.81 17.4 18.85 45.97 102.1 45.51 36.14 32.71
Lu 0.49 0.55 0.58 0.53 0.81 0.45 0.49 0.5
Mo 9.83 11.93 10.57 4.95 3.27 5.91 6.95 5.26
Sm 3.85 3.77 3.74 8.34 12.34 7.25 7.44 6.56
U 8.81 8.08 7.39 3.96 2.5 3.92 4.7 4.31
Yb 3.57 3.81 3.73 3.33 4.72 2.87 3.22 3.3
Ba 170.27 180.39 185.72 157.8 997.95 353.53 129.58 104.84
Ce 37.94 38.62 41.67 96.98 158.82 84.77 82.52 88.33
Cs 10.88 11.4 82 5.2 2.72 4.4 5 5
Cr 2.34 2.46 2.38 4.78 2.88 2.34 2.49 1.01
Eu 0.09 0.12 0.12 0.81 1.86 0.79 0.49 0.34
Hf 3.22 3.13 3.3 7.69 8 6.85 5.78 8.03
Nd 8.09 8.5 8.88 28.44 58.81 28.09 25.44 23.37
Sc 3.73 3.9 3.97 2.76 4.29 2.58 2.37 1.78
Ta 2.59 2.58 2.88 1.58 1.3 1.31 1.82 1.74
Tb 0.76 0.98 0.88 1.11 1.27 1.02 1.18 0.98
Th 17.75 17.49 18.63 26.89 21 .4 23.22 23.52 26.39
Zn 24.87 14.57 39.53 72.75 58.18 64.68 47.21 78.97

APPENDIX 2. (Continued)

15]

 

Pre-rainier Mesa tephra sequence (glassy pumice)

 

 

 

 

 

 

 

11F 12F 14B 15A 16A 17G 17H
Weight Percent Oxide (wt °/g)
8102 75.88 73.51 73.81 74.67 73.87 72.49 75.84
Ti02 0.16 0.26 0.28 0.23 0.25 0.27 0.17
AI203 13.28 14.46 14.44 13.86 14.48 14.87 13.08
FeO 0.79 1.37 1.24 1.04 1.26 1.28 1
MnO 0.12 0.08 0.09 0.08 0.09 0.13 0.11
M90 0.16 0.55 0.48 0.31 0.37 0.39 0.13
CaO 0.3 0.82 0.75 0.65 0.68 0.86 0.32
Na20 3.48 3.16 3.02 3.06 2.97 3.96 3.58
K20 5.83 5.78 5.86 6.07 5.99 5.92 5.75
P205 ‘ 0.01 0.01 0.02 0.02 0.03 0.03 0.01
X-ray Fluorescence (ppm)
Cr 0.3 0 0 0 2.1 0 2.6
Ni 0 0 0 0 0 0 0
Cu 57 58.1 58.1 57.4 57.9 56.4 57.2
Zn 63.9 50.8 44.4 55.5 42 84.9 74.1
Rb 187.3 207.8 192 203.2 192.3 167.2 187.6
Sr 21.5 102 79.8 63 80.8 74.8 25.4
Y 44.3 48.1 41.5 43.5 47.7 55 47.1
Zr 213.1 222.5 209.2 193.8 187.2 318.9 202.5
Nb 26.7 26.5 22.3 24.2 21.7 22.3 24.8
La 0 29.3 29.7 19.3 31.2 12.5 35.1
Ba 83 438.5 344.5 189.8 205.3 282.3 115.8
Instrumental Neutron Activation Analyses (ppm)
As 3.34 3.98 3.97 3.81 3.54 3.76 3.74
La 34.4 67.86 52.67 81.87 80.88 50.34 40.91
Lu 0.5 0.5 0.49 0.48 0.49 0.52 0.54
Mo 6.98 4.1 4.87 6.92 5.27 4.89 7.74
Sm 6.57 9.59 7.48 8.72 8.99 13.25 7.54
U 4.18 3.89 4.19 4.22 4.19 3.45 4.48
Yb 3.37 3.17 3.35 3.78 3.27 3.57 3.82
Ba 116.45 389.83 303.14 335.9 321.39 387.84 184.28
Ce 78.8 119.71 101.11 117.26 114.71 119.34 73.43
Cs 4.88 5.02 4.8 4.85 4.47 4.18 4.93
Cr 1.54 2.84 2.39 2.62 3.32 4.8 2.29
Eu 0.39 1.17 0.88 1.02 1 0.81 0.44
Hf 8 7.15 6.24 6.21 6.29 9.29 7.85
Nd 18.41 36.27 35.85 32.92 73.8 49.22 22.9
Sc 1.77 3.31 3.02 2.98 2.88 2.25 1.82
Ta 1.51 1.43 1.33 1.5 1.53 1.42 1.49
Tb 1.25 1.12 0.85 1.08 1.25 1.83 1.17
Th 25.12 23.55 21.39 23.57 23.18 20.82 24.79
Zn 68.55 72.77 80.58 40.45 45.89 80.41 85.25

152

APPENDIX 2. (Continued)

 

 

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~93. 9N... no.3 no. .Nvm and... 0.8.. . ..amou :N
: ..~.~: : n ...«: : : 2.63 so
: : : : : : : .2
:N.:. ::.m. vwd. mi: 9.3 and. 3.8. .0
8:88.02". >8...“
8.: 3.: 3.: 3.: 8.: 8.: 8.: 88m
0...... ~..m mod aa.v mu... 8:... and 08x
3...: mad 3.: mo... :8 mm... .:.v 0882
on: 3.: 2.: no: 3.: 8.: : ... 080
.«..: : ..: mm: mm: on: «N: m...: 092
8.: 3.: 8.: 8.: ::.: 3.: 8.: 0::
wt. 8.. mm. ..~.. ~:.. 8.. 0.. Co“.
.a.... 9.8. ..~..... .:.v. 3.... v.8. «...... 8082
.a.: 98.: .a.: mu: 3.: mu: 3.: «0:.
no... 3.... .....o. «.... 2am. no.2. :o. ... 89w

 

.8. .E. 2.86 ...88. .382.
88-8.94. ~mm-:....< 388...... 888...? 888.-.... ..«.:....< 388.-....
8.8.2 ...88. 8:808 Esme. 9E8... 8.:oEEaéa

 

APPENDIX 3

162

 

APPENDIX 3. Mineral Analyses

8.3 8+ 8.8 8.8 8.8 8.3 8.3 8.3 3+ 8.3 8.8 8.8 8.3 2.8 5.8.8.888
8.3 8+ 8.8 8.8 8.5 8.3 8.3 8.3 8+ 2.3 8.8 8.8 8.3 8.8 3.8.8.888
8.3 8.8 8.8 8.. 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.8 8.3 8+ 3.8.8888
8.3 8.8 8.8 8.: 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.8 8.3 8+ 388.8888
8.3 8.8 8.8. 8.: 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.8 8.3 8+ 3.8.8888
8.3 8.8 8.8 8.8 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.8 8.3 8.8 3878.888
8.3 8.8 8.8 8.8 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.8 8.3 3.8 3-578-888
8.3 8.8. 8.8 8.8 8.8 8.3 :3 8.3 8.3 8.8 8.8 2.8 8.3 8.8 3.078-888
8.3 8.8 88+ 8.. 8.8: 8.3 8.3 :3 8.3 8.8 8.8 8.8 8.3 8.8 3.8.8888
8.3 8.8 8.8 8.: 8.8 8.3 8.3 8.3 8.3 :8 :8 8.8 8.3 8.8 3.8.8888
8.3 8.8 8.8 N: 8.8 8.3 8.3 8.3 8.3 8.3 8.8 8.8 8.3 8.8 3.8.8888
8.3 :3. 8.8 8.8 8.8 8.3 8.3 8.3 8.8 8.. 8.8 8.8 8.3 8.8 8.378.888
8.3 8.8 8.: 3.8 :8 8.3 8.3 8.3 8.8 8.: 8.8 8.8 8.3 8.8 ...-8-8.888
8.3 :8 8.8 8.. 8.8: 8.3 5.3 8.3 8.3 8.8 8.8 8.8 8.3 8+ 3.88.7888
8.3 8.8 8.9. 8.: 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.: 8.3 8+ 3.887888
8.3 8.8 8.3 8.: 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.: 8.3 8+ 388.7888
8... 8.8 8.8 8.: 8.38 8.3 8.3 8.3 8.8 8.: 8.8 8.8 8.3 8.8 8-88.7888
8.3 8.8 8.8 8.: 8.38 8.3 8.3 8.3 «+8 8.: 8.8 3.8 8.3 8.8 ...-578.7888
8.3 8.8 8.8 8.8 8.8 8.3 8.3 8.3 8.8 8.: 8.8 8.8 8.3 8.8 .98-87888
8.3 :8 8.8 8.: 8.8 8.3 8.3 8.3 8.8 2... 8.8 8.8 8.3 8.8 .98-8.7888
8.3 8.8 8.8 8.8 8.8 8.3 8.3 8.3 3; 8+ 8.8 8.8 8.3 +2. .388.:-888
8.3 8.8 8.; 8.: 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.: 8.3 8+ 388.7888
8.3 8.8 8.8 8.8 8.5 8.3 8.3 8.3 8.3 8.8 3.8 8.8 8.3 8.8 3.88.7888
8... 8.: 8.8 8.8 8.8 8.3 8.3 8.3 8.3 8.8 8.8 8.8 5.3 88 3-88.7888
8.3 8.8 8.8 8.8 8.8 8.3 8.3 8.3 8.. 8+ 8.8 8.8 8.3 88 3.88.7888
8.3 8.8 8.8 8.8 8.8 8.3 8.3 5.3 8.8 8.: 8.8 8.8 8.3 8.3 ...-888.7888
8.3 8.8 :8 8.: 8.8 8.3 8.3 8.3 8.8 8.: 8.8 8.8 8.3 8.8 .98-8.7888
8.3 8.8 8.8 8.: 8.8 8.3 8.3 8.3 8.8 8.: 8.8 8.8 8.3 8.8 888.7888
8.3 8.8 8.8. 8.: 8.8 8.3 8.3 5.3 8.3 8.8 8.8 8.8 8.3 8+ 3.88.7888
8... 8.8 8.8 8.. 8.8 8.3 8.3 8.3 8.3 8.3 8.8 8.8 8.3 8+ 3-5.8.7888
8.3 8.8 8.8+ 8.: 8.8 8.3 8.3 3.3 8.3 8.8 8.8 8.8 8.3 8+ 3.888888
8 5 2 E .22 0.8 08 08 08 03. 8.8 882 082 082

 

3:858 88.8 .8380 om=8u8om
wmw>-_<z< «(awe-fin.

163

APPENDIX 3. (Continued)

 

88 8: 8.8 8.8 8.8 88 88 8.: 8.8 8.8 88 .98-8.85
88 88 8.8 8.8 8.8 88 8.:. 8.: 8.8 8.8 888 .98-8-85
88 8.8 8.8 8.8 :8 88 88 8.: 8.8 8.8 88 ...-8.8.85
88 8.8 8.8 8.8 8.8 :8 88 8.. 8.8 8.8 88 .93-8-85
88 88 8.8 8.8 8.8 88 88 8.: 8.8 8.8 88 ...-3.8.8.:
88 8.8 8.8 8.: 8.8 88 88 8.8 8.8 8.8 88 8.8-8-85
88 8.8 8.8 8.: 8.8 88 88 8.8 8.8 8.8 88 8.8-8-82
88 8.8 8.8 8.: 8.8 88 88 88 8.8 8.8 8... 8.8.8.85
8.8 8.8 8.8 8.: 8.8 88 88 8.8 8.8 8.8 88 3.8.8.8.:
88 8.8 8.8 8.8 88.8 88 88 88 8.8 8.8 8.. 38.8.85
88 8.8 .888 8.8 8.8 88 88 88 8.8 8.8 88 88 8.8-8-82
88 88 8.8 8.8 8.8 88 88 88 88 88 8.8 8.8 88 8.8 ...-88.38.88
88 88 8.8 8.8 8.8 88 88 88 88 88 888 8.8 88 88 8.58-8.8.88
88 8.8. 8.8 8.8 8.8 88 58 88 8.8 88 8.8 8.8 88 88 ...-58.38.888
88 88 8.8 8.8 8.8 88 88 88 88 8.: 8.8 8.8 88 88 .88-8.8.88
88 88 8.8 8.8 8.8 88 S8 88 88 88 8.8 8.8 88 88 .a.-58.8.8888
88 8.8 8.8 8.8 8.8 88.8 88 88 88 88 8.8 8.8 88 88 .88-8.8.88
88 88 8.8 8.: 888 88 88 88 8.8 8..: 8.8 8.8 88 88 ...-83.8.88
88 8.8 8.8 :8 8.8 88 88 88 8.8 8.: 8.8 8.8 88 88 .a.-518.88
88 88 8.8 8.8 8.8 88 88 88 8.8 8.: 8.8 8.8 888 88 8.81-.888
88.8 88 8.8 8..: 8.8 88 88 88 8.8 8.: 8.8 8.8 88 88 .98-8.8.88
88 8.8 8.8 8.: 8.8 88 88 88 8.8 8.: 8.8 8.8 888 88 .98-8.8.88
88 88 8.8 8.: 8.8 88 88.8 88 8.8 8.: 8.8 8.8 38 88 ...-58.58.88
88 88 8.8 8.: 8.8 88 88 88 8.8 t... 8.8 8.8 88 :8 .88-38.88
88 8.8 8.8 8.: 8.8 88 88 88 88 88 8.8 8.8 88 88 .88-8.8.88
88 8.8 8.:. 8.: 8.8 88 88 888 88 88 8.8 8.8 88 8.8. 3.8-8.8.88
88 8.8 8.8 8.: 8.8 88 888 88 88 8.8 8.8 8.8 888 8.:. 8.8-8.888
88 8.8 8.8 8.: 888 88 88 888 88 88 8.8 8.8 88 8+ .88-8.8.88
88 8.8. 8.8. 8.8 8.8 88 88 88 88 88 8.8 8.8 88 88 8.8-3888
88 8.8 8.8. 8.: 8.88 88 S8 88 88 :8 8.8 8.8 88 88 8.8-8.888
88 88 8.8 8.: 8.8 88 88 :8 8.8 8.: 8.8 8.8 88 88 .88-8.8.88
88 N: 8.8 8.8 8.8 88 88 88 8.8 8.: 8.8 8.8 88 8.8 .88-8.8.88
co 5 2 :< .22 08 0.8 08“. 08 09. «08 882 082 082

 

8:8.de 8&8 coxcao 839.38.“.

164

APPENDIX 3. (Continued)

 

28 8.8 8.:. 8.: 8.8 88.8 88 88 88 8.. 8.8 8.8 88 88 8.58.8-8
88 8.8 8.:. 8.: 8.82 88 88 28 88 8.. 8.8 8.8 88 88 8.8.8-88
88 88 8.8 8.2 8.8 88.8 88 88 888 8.. 8.8 8.8 88 88 82.8.8-8
88 8.8 2.8 8.2 8.8 88.8 88.8 88 88 8.. 8.8 8.8 88.8 88 82-88-88
888 88 8.8 8.8 8.8 88.8 88.8 28 8+ 8.. 2.8 8.8 88 88 8-88.8-8
88 88 8.8 8.8 8.8 88.8 88 88 88 8.. 8.8 8.8 88 88 8.58.8-8
88.8 28 8.8 8.8 8.8 88 88 28 8.8 8.. 8.8 8.8 88 88 8.888.848
28 8.8 2.8 88 8.8 88.8 888 88 28 8.: 8.8 8.8 88.8 88 8.8.8.888
28 8.8 8.8 88 8.8 88.8 88 88 :8 8.: 8.8 8.2 88 88 3-588.848
28 8.8 8.8 88 8.8 88 B8 88 :8 8.: 8.8 2.2 88.8 88 8.8.88.8
88.8 888 2.8 88 8.8 88.8 88.8 888 28 8.: 8.8 2.8 88.8 88 8.8.8.8.8
88.8 8.8 8.8 8.8 8.8 88.8 88.8 :8 88 8.2 8.8 2.8 88 88 8888.8
88.8 88 8.8 8.8 8.8 88 88.8 28 88 88 8.8 8.8 88.8 8.8 3.88.8-8
88 8.8 8.8 8.8 8.8 88 88.8 88 88 8.. 8.8 8.8 88 88 8.88.848
88.8 8.8 8.8 8.8 8.8 88.8 88.8 88 88 8.. 8.8 8.8 88 88 8888.8
88 888 2.8 88 8.8 88.8 88 88 :8 8.8 8.8 8.8 88 88 8-8.8.888
88 8.8 8.8 88 8.8 88.8 88.8 88 28 8.: 2.8 8.2 88 888 8-5.8.828
88 8.8 8.8 88 8.8 88.8 88 88 8.8 8.: 8.8 8.2 88 88 88.8.8-8
88 8.8 8.8 88 8.8 88.8 88 88 88 8.: 8.8 8.2 88 88 8.88.8-8
88.8 8.8 8.8 88 8.8 88.8 88 88 28 8.: 8.8 8.2 88 888 3.888.848
88 8.8 8888 88 8.8 88.8 88 88 :8 8.: 8.8 8.2 88.8 88 3.8.8.888
88 8.8 8888 88 8.8 88.8 88 88 28 8.: 8.8 8.2 88 88 8.8.8.8.8
88 8.8 8.8 88.8 8.8. 88 88 88 8.8 8.2 88 8.88.885
88 8.8 8.8 8.. 8.8 88 88 88 8.8 8.2 88 888-885
8.. 88.8 8.8 8.8 3.8 88 88 88 8.8 8.2 28 3.8.8-885
88 88 8.8 8.8 2.22 88 88 8.. 8.8 8.8 88 8.8.8-885
88 88 2.8 8.8 8.8 88 88 8.. 8.8 8.8 8.8 88.8-88...
88 8.8 8888 8.2. 8882 88 88 8.. 8.8 8.8 88 3.8.825
88.8 8.8 8.8 8.2 8.8 88.8 28 8.. 8.8 8.8 88 8.8.8.25
8.. 2.8 8.8 2.2 8.8 88 88 8.2 8.8 8.2 88 8.8.8.25
8.2 8.8 8.8 8.. 8.8 88 88 8.2 8.8 8.2 88 3.88.825
28 8.8 8.8 2.2 8.8 :8 88 88.2 8.8 2.8 88 82.88::
.0 2 8< .22 08 0.8 08“. 08 8x 88 882 082 082

 

8:268 Esme. .8880 88:833.“.

165

 

APPENDIX 3. (Continued)

86 8.8 8.8 8.. 8.8 86 86 86 .~6 8... 866 6.8 86 86 8.8-8.8-8
86 8.8 8.8 86 .68 86 86 86 86 8.8 866 .66. 86 ...6 ...-8.8.8.8
86 8.8 66.8 86 8.8 86 86 86 86 8... 866 86. 86 .66 3.8-8-8-8
86 .88 8.8 86 8.8 86 86 86 86 .6... 8.8 8.8 86 86 8.8-«.«......
86 86 .668 8.8 8.8 86 66.6 86 6.6 8.. 8.8 8.8 86 86 8.8-«.888
86 666 2.:- 86. 8.8 86 86 86 86 8.. 8.8 8.8 86 86 8.5.88-8
86 86 8.8 88 8.8 86 86 86 86 8.. .68 8.8 86 86 86.8-«-86
86 86 8.8 8.6. 6666 86 86 86 8.6 8.. 8.8 8.8 86 86 8.38.8-8
86 86 8.8 ts. 8.8 86 86 6.6 86 8.. 8.8 8.8 86 86 8-5-8.8-8
86 86 8.:. 86. 8.8 86 86 ...6 666 8.. 8.8 8.8 .66 6.6 8.58-«.8
86 86 8.8 86. 8.8 86 86 86 86 8.. 8.8 8.8 .66 86 8.28.8.8
6.6 86 8.8 8.8 8.8 86 86 86 86 .a.. 8.8 8.8 86 666 8.88-«-8
86 86 86.. 8.8 8.8 86 86 ..6 86 8.. 8.8 8.8 86 86 8-56-~.-.8
6.6 86 .8: 8.8 8.8 86 86 6.6 86 8.. 8.8 ...8 .66 86 88.8.8.8.
86 8.8 8.8 86 66.8 86 86 86 86 8.8 .68 666. 86 666 8.8-8.8.8
86 8.8 8.8 86 8.8 86 86 86 86 8... .666 8..... 86 86 8.8.88.8
:6 8.8 8.8 .6. .68 86 666 86 86 8... .666 6.6. 86 86 8.88-48
.66 8.8 8.8 86 8.8 86 86 6.6 ...6 86. 8.8 8.: 86 86 8.88.8.8
.66 66.8 8.8 86 8.8 86 86 86 86 8... 866 86. 86 86 8.88-86
86 8.8 8.8 86 8.8.86 86 ..6 86 8.8 8.8 8.8 86 86 8.8-8-8-8
86 86 8.8 .6... 8.8 86 86 6.6 86 8.. 66.8 66.8 .66 86 8.36.6-8
86 .66 8.8 8.8 66.8 86 86 6.6 86 8.. 8.8 8.8 86 666 8.3.66-8
86 86 86. 8.8 8.8 86 86 86 86 8.. 8.8 8.8 86 86 8.86-6.9.8
86 86 8.:. 86. 8.8 86 86 86 86 8... 8.8 8.8 .66 8.6 8.8-8.6-8
86 8.8 8.8 86 8.8.86 86 6.6 86 8.... 8.8 8.8 .66 86 8.6-6.6.8
86 8.8 8.8 86 8.8.86 86 86 t6 8... 8.: 8.8 .66 86 8.8-6.6-88
666 8.8 8.8 86 8.6. 86 86 86 ...6 8... ..-.8 86. 86 86 8.8-6.6.8
86 66.8 8.8 86 8.8. 86 86 86 86 8... 66.8 86. 86 86 8.8-6.6-8
86 8.8 8.8 86 8.8 86 86 86 86 8... 8.8 8.: 86 86 8.8-6.6-8
666 8.8 8.8 86 8.6. 86 86 6.6 86 8... 8.8 86. .66 86 8-8-6.6-8
86 8.8 8.8 6... 66.8. 86 86 86 86 8... 8.8 6.6. 86 86 8.8-6.9.8
86 8.8 .66» .66. 8.8 86 86 ..6 86 8.. 8.8 8.8 86 666 8.8.6.6-.8
co 5 3 2 88. 9m 08 08 08 09. ~08 682 092 082

 

35:88 2.88 cox—.80 $68338

166

APPENDIX 3. (Continued)

 

5.5 8.8 55.5 8.5 8.8 55.5 8.5 55.5 ...5 8... 8.8 .55. 8.5 .55 53.5.7.8
8.5 8.8 :55 .55 8.8 55.5 5.5 55.5 5.5 58... 8.8 5.... 8.5 85 8.8.8-.-.55
55.5 .555 8.5 8.5 8.8 55.5 8.5 55.5 ...5 ...... 8.8 .55. 8.5 85 3.588-.-.8
58.5 52.5 8.8 8.5 8.8 55.5 5.5 8.5 5.5 .5... 8.8 8... 8.5 555 38.58....55
8.5 8.8 8.5 .55 8.8 55.5 5.5 ..55 8.5 8... 55.8 5.5. 8.5 85 3.5.8-...55
8.5 5.55 8.5 55.5 8.8 55.5 85 8.5 ...5 5.... 8.8 85. .55 $5 595.58..-.8
.55 5.55 8.5 8.5 855 55.5 8.5 8.5 5.5 8... 8.8 85. 85 555 3.5.8..-.8
85 8.5 8.8 85. 855 85 85 5.5 5.5 8.. 8.8 8.8 85 85 5.8.8..-.55
85 5... 8.8 .5... 55.8 85 85 5.5 555 8.. 8.8 55.8 85 555 3-558-.-.8
85 8.5 555. 8.58 58.8 85 85 5.5 ...... 55.. 8.8 55.8 85 .55 3.8.8....8
85 8.8 55.55 85 8.8 85 5.5 5.5 5.5 8... 55.8 5.5. 85 85 3.3.8..-.8
..55 5.8 8.55 8.5 8.8. 85 8.5 85 5.5 .5... 8.8 85. 85 555 3.3.8-.-.55
5.5 .555 .555 8.5 8.8 85 85 85 ...5 .5... 55.8 585. 85 85 3-5.5.8-.-.55
8.5 8.8 .558 8.5 855 85 85 5.5 5.5 8... v5.8 585. .55 85 38.58-.-.55
8.5 .5... 8.8 555. 55.8. 85 .55 5.5 5.5 8.. 8.8 8.8 85 555 38.8-.-.55
85 8.. 8.8 .55. .555 85 85 5.5 85 8.. 55.8 8.8 8.5 85 8-5.5.8-.-.55
85 8.5 8.8 .45. 8.55 85 85 5.5 55.5 58.. 8.8 8..... 85 .55 8.8.8-.-.8
...5 5.55 .58 85 8.8. 85 85 85 ...5 8... 8.8 85. 85 85 38.8..-.55
8.5 8.8 .555 ..55 8.8 85 ...5 55.5 ...5 55... 3.8 85. 85 85 3.5.8.8....55
85 .555 .558 85 8.8 85 85 85 8.5 8... 8.8 8... 85 85 3.8.8-.-.8
8.5 8.8 5.5.. 55.. 8.8. 85 5.5 5.5 8.5 85 55.55 5.5. .55 8.5 3-5.8-...55
5.5 8.8 5.8 55.. 5.8 85 85 85 8.5 85 5.55 85. 85 8.5 3.5.8-.-.55
85 8.8 58.8 85 8.55. 85 85 8.5 .55 85 8.8 5.5. 85 8... 3.5.8....55
5.5 85 8.8 85. 855 85 55.5 85 85 5... 8.5 8.8 85 .55 3.88-8-8
5.5 .55 5.8 85. 8.55. 85 85 85 8... 8.5 8.8 8.8 85 85 8.558885.
85 8.5 8.5. 85. 8.8. 85 85 85 ...« .... 8.8 8.8 85 85 3.8-88-8
...5 855 8.8 5.5 5555 85 85 .55 5.5 8... 5.55 85. 85 555 8.88.5.8
8.5 555. .58 55.5 855 85 8.5 85 ....5 555. 8.8 8... 85 85 8-28-8.8
...5 8.5. ..58 85 5.55. 85 85 55.5 5.5 5.5. 8.8 8... 85 85 3.3.8885
85 8... 8.8 85 8.8 85 85 85 5.5 555. 855 5.5. 85 5.5 8.5.88-8
85 855 358 .55 8.8 85 55.5 85 5.5 8... 55.8 3.5. .55 85 3.8-88.8
55.5 5.55 8.8 8.5 8.8 55.5 55.5 55.5 5.5 8... 8.8 85. 8.5 585 595588-85
5 2 E. .22 95 055 05“. 08 09. 88 83 G5: 082

 

85268 8:52 cacao 05523555.

167

APPENDIX 3. (Continued)

 

 

 

 

.55 55.5 ...8 5...... 855 55.5 55.5 .55 5.5 .55 855 55.55 ...5 .55 ...-555.5..5.
85 55.55 5.... 55.. 8.5 55.5 8.5 8.5 8.5 5.5 ....8 55... 55.5 8... .5.5...5.5...5.
...5 8.55 8.... 55.. 855. 55.5 55.5 8.5 555 5.5 8.8 .55. 55.5 55... ...-535......
5.5 55.55 8.8 5... 8.8. 55.5 .55 555 8.5 85 8.8 5.5. .55 8.5 ...-5.5.5.75.
8.5 8.8 5.55 555 8.8 55.5 .... 85 55.5 555 5...5 8... 55.5 8.. .58-5.5.75.
5.5 55.8 .555 85 .555 55.5 8.. 8.5 ...5 85 55.8 555. 55.5 8.5 ...-55.5.8.5.
85 55.8 ..55 .55 8.8 55.5 55.. 8.5 55.5 85 ....8 85. 8.5 55.. .58-5.5.75.
5... 855 8.8 .5. 8.8 55.5 55.5 55.5 8.5 555 .555 85. .55 8.5 .55555...5.
8.5 5555 5.55 55.. 8.5 55.5 8.5 ...5 .55 85 5.8 55... 8.5 85 .5-...5.5.5...5.
.....5 5.55 5555 8.5 8.8 55.5 8.5 85 ...5 .55 .555 5.... 8.5 8.5 .5.55.5.5...5.
.55 85 .55. 8... .555 55.5 8.5 55.5 555 8.. 8.5 8.55 8.5 .55 ...-5.5.8.5.
5.5 8.8 8.8 8.. 8.55 55.5 55.5 85 55.5 55.5 .55. 5.... 8.5 8.5 ...-5.5.5.75.
8.5 555 555. ...5. 8.8 55.5 .55 .55 555 5... 55.8 8.55 .55 85 ...-5.5.8.5.
8.. .55 5.... 555. 8.8 55.5 55.5 .55 5.5 .5. 5...5 8.8 8.5 85 ...-5.5.5.75.
555 8.5 5.5. 5.8 55.5 55.5 55.5 .55 5.5 8.5 ....55 5.55 55.5 5.5 ...-5.8-5.5.
5.5 5.5 55.5 5.8 8.8 55.5 8.5 5.5 5.... 8.5 8.5 8.8 55.5 55.. ...-55.8-5.5.
5.5 55.5 55.... 5.5. 5555 55.5 5.5 .55 555 5... ...55 8.5 55.5 85 ...-55555.5.
...5 5.5 ...5. 55.5. 8.8 55.5 85 5.5 85 .5. 8.8 .555 55.5 85 ...-55.8-5.5.
.55 8.5 .58 5... 8.5 55.5 55.5 ...5 8.5 .55 55.8 5.... 8.5 5... ...-8555.5.
55.5 .5..5 ...55 .5. .5... 55.5 5.5 ...5 8.5 85 8.55 55... 55.5 .5... ...-55555.5.
...5 55.5 5.55 8.., 55.8 55.5 8.5 ...5 8.5 ...5 ....55 5.5. 8.5 .55 .58-85.5.
.5.5 5555 5.55 5.... 55.8 55.5 55.5 ...5 55.5 85 5.55 555. 85 5.... ...-55555.5.
85 :55 5.55 5.... .555 55.5 8.5 5.5 55.5 85 55.55 55.5. 55.5 ...... ...-5.5.85.5.
5.5 855 855 8.. 8.8 55.5 55.5 ...5 .55 85 55.55 5.5. 8.5 53 ...-55555.5.
5.5 8.5 .555 8.. 5.55 55.5 55.5 5.5 55.5 85 55.8 8... 55.5 5.... ...-5.55-5.5.
55.. 855 55.5 55.. 8.5 55.5 55.5 5.5 55.5 55.5. .58 8... 8.5 5.... ...-5.55-5.5.
.... 855 8.8 8.. 5.55 55.5 5.5 5.5 55.5 85 .555 8... 8.5 55... ...-5.55.5.5.
.o 2 5< .22 0.5 055 05". 08 05.. 50.5 5052 05.2 0552

3:263. 8552 5522 8.58.85.
85 8.5. 8.8 5.5 8.8 .55 5.5 55.5 5.5 8... 5.55 55... 8.5 85 ...-5.5.7.8
55.5 8.8 5.55 5.5 5.55 55.5 ...5 55.5 5.5 8... 55.8 555. .55 555 ...-5.5.7.55

.o 2 5< .22 0.5 055 05.. 08 00. 50.5 5052 08,. 082

 

52.859. 8.32 55.50 3:83.555.

168

APPENDIX 3. (Continued)

 

8.5 855 8.8 8.. 55.8 55.5 5.5 5.5 8.5 85 8.8 8... 85 55.5 ...-55.5.5.5.
55.5 5555 8.8 5... 8.8 55.5 5.5 55.5 55.5 555 .58 .5... .55 55... ...-25.5.5.5.
5.5 5... 5.... 5...... 55.5 55.5 .55 55.5 .55 8.. 55.8 8.55 .55 85 ...-55.5.5.5.
555 8.5 55.5.. .5. 8.5 55.5 55.5 55.5 8.5 .55 855 5.... 8.5 5.... ...-5.55.5.5.
55.5 5.8 8.55 55... 55.5 5.5 55.5 .55 8.5 55.. 5.8 555. .55 5.5 ...-55.5.5.5.
.55 8.8 855 85 55.5 55.5 .55 55.5 .55 555 .555 8.5. 55.5 55.5 .555-5.5.5.
5.5 55.55 8.8 8.5 5.55 55.5 55.5 .55 8.5 85 8.8 35. 55.5 85 ...-5.5.5.5.
.55 8.55 ...5 8.. 5.... 8.5 55.5 55.5 8.5 85 5....5 85. .55 55... ...-5.5.5.5.
.55 55.55 55.... .5. 55.8 55.5 55.5 5.5 85 85 55.8 85. .55 555 ...-5.5.5.5.
...5 8.5 855 55.5 55.8 55.5 8.5 8.5 ...5 8... 8.55 5.5. .55 85 8.8.8.55.
55.5 55.5 5...... 55.5 8.5 55.5 55.5 .55 5.5 8... 855 55... 55.5 85 ...-5.5.8.55.
8.5 .5.... 55.5 55.5 5.8 55.5 5.5 8.5 5.5 8... 8.8 8... .55 85 3.8.8.55.
...5 8.5 8.5 8.5 8.5 8.5 8.5 5.5 5.5 55... 8.8 .55... .55 85 ...-88.55.
55.5 55.5 .555 ...5 .555 55.5 55.5 85 55.5 .5... .5... 55... .55 55.. ...-8.8.55.
85 ...5.. 5555 55.5 55.8 55.5 55.5 55.5 55.5 55.5 ...55 555. 55.5 ...5 .555-8.55.
55.5 8.8 55.5 55.5 5.55 55.5 55.5 .55 55.5 .55 8.8 5.... 55.5 55.5 3.55-8.55.
55.5 855 .555 55.5 8.8 55.5 55.5 8.5 55.5 ...5 55.8 8... 55.5 .55 .555-8.55.
8.5 8.8 5.55 8.. .555 55.5 5.5 ...5 .55 55... 8.8 55... 55.5 .55 ...-2.8.5.5.
55.5 55.55 5.55 .... 55.8 55.5 8.5 55.5 8.5 .55. 8.8 5.... 8.5 555 85.-8.55.
5.5 85 5.5. 555. 8.5 55.5 55.5 «.5 5.5 8.. 8.8 5.5. .55 85 .555-8.5.
8.5 5.5 555. .55. 55.8 55.5 55.5 ...5 5.5 55.. .58 55.5. 8.5 85 ...-55.8.5.
55.5 8.. 555. 8.5. 8.5. 55.5 .55 8.5 .55 8.. 855 8.5 8.5 85 .555-8.5.
55.5 .... .55. 55.... 8.8 55.5 8.5 ...5 8.5 8.. 8.8 8.8 55.5 .55 ...-5.5.8.5.
8.5 .... .55. 555. 855 55.5 55.5 5.5 8.5 8.. .555 8.5 8.5 85 .555-8.5.
55.5 8.55 8.8 8.5 5.8 55.5 ...5 8.5 ...5 55.5. ...55 .55. 8.5 55... .555-8.5.
8.5 .555 8.8 55.5 8.8 55.5 5.5 8.5 ...5 8.5. 8.55 .55. 8.5 85 3.5.18.8.
5.5 8.5 .....5 8.5 5555 55.5 55.5 5.5 5.5 555. 55.55 85. .55 5.5 .5.5.....5...5.
.5. 855 5.55 55.5 .5... 55.5 55.5 55.5 55.5 8.5 8.55 8.5 8.5 55.5 .5.5.....5-..5.
...5 .555 5.8 85 8.8 55.5 5.5 55.5 .55 8.. ...5. .55. 55.5 5... .555-8.5.
55.5 8.. .55. 5.5. ....55. 55.5 .55 ...5 85 .5. 55.8 8.8 .55 85 .555-8.5.
55.5 555 85. 555. .555. 55.5 55.5 55.5 5.5 3.. 5.55 2.8 85 .55 ...-5.5.8-75.
55.5 55.. 55.5. 5.5. .555 55.5 55.5 5.5 8.5 .5. 55.8 8.8 55.5 85 .555-8.5.
.o 2 5< 8.2 0.5 055 05.. 0.8 09. 50.5 5052 om: 082

 

85863 8.32 5522 8:52.85.

169

APPENDIX 3. (Continued)

 

.55 .5. 555. 55.5. 55.55. .55 5.5 55.5 5.5 5... 8.55 ....5 55.5 555 3-855.-..5.
.55 55.. 555. 55.5. 8.5 .55 ..55 .55 55.5 55.. 8.55 5..5 55.5 5.5 38.55....5.
..55 .555 5.55 5.5 55.55 5.5 ...5 55.5 55.5 55.5 55.5. 55... ..55 55.. 355.55.-..5.
3.5 55.55 .5..5 5.5 55.55 55.5 ...5 55.5 55.5 .5... 55.5. 55.5. 55.5 55.. 35.55.-..5.
5.5 .5..5 .5..5 55.5 55.55 55.5 ..55 .55 55.5 55.5 55.5. 55.5. 55.5 .5. 3-555.-..5.
.55 55.55 55.55 55.5 ...55 .55 55.5 5.5 5.5 55.5. 55.55 .55. 55.5 55.5 355.55.-..5.
55.5 5.55 ...55 55.5 55.55 55.5 55.5 5.5 5.5 5.5. 55.55 55.5. .55 55. 3-5555775.
85 5.55 .555 55.5 ...55 55.5 55.5 55.5 5.5 ...5. ...55 85. .55 55.5 355.55.-.-5.
8.5 5.... 55.55 55.5 55.55 55.5 .55 55.5 55.5 55.5 .55. 55.5. 55.5 5.5 555.55.-.-5.
.55 55.55 55.5 55.5 5555 55.5 .55 5.5 8.5 5.5 ...55 55.5. 55.5 55... 3.55.555.
55.5 55.5 .55. 55.5. 55.55 .55 55.5 .55 55.5 .5. 55.55 55.55 55.5 .55 3-55.5575.
55.5 55.8 55.8 55.5 55.55 55.5 55.5 ...5 55.. 55.5 ...5 .55. 55.5 .55 3.8.55.5.
55.5 55.55 5.55 55.. 55.5 55.5 ...5 ...5 55.5 55.5. 55.5 55... 55.5 55... 3-55.5.5..5.
55.5 55.5 8.5.. .5. ...55 55.5 5.5 5.5 .55 3.5. 55.55 55.5. 55.5 55... 3.8.55.5.
5.5 55.5 55.5.. 55.. 55.55 55.5 55.5 5.5 55.5 5.5 55.55 55... 55.5 55. 3.8.55.5.
5.5 55.5 .58 .5. ...55 55.5 55.5 5.5 55.5 555 .555 5.5. 55.5 55.. 3.555.575.
...5 55.5 55.55 5...5 .555 55.5 55.5 55.5 55.5 .55 5555 55.55 55.5 55.. 5.555-575.
8.5 55.5 .555 .555 .555 55.5 55.5 .55 55.5 .55 ..555 55.55 .55 8.. 3.555-575.
8.5 55.. .555 .5..5 55.55 55.5 .55 55.5 55.5 5.5 ....5 .555 .55 55.. 5-55.5575.
5.5 55.. 55.5 .555 .555 5.5 55.5 55.5 5.5 5.5 .5..5 5.55 55.5 55.. 5.55.5575.
55.5 55... .5... 55.5 55.55 55.5 55.5 5.5 55.5 5.5 55.55 5.55 55.5 5.5 3855.5.
55.5 55... 8.5. .5..5 55.55 55.5 55.5 55.5 8.5 55.5 55.55 5.55 .55 55.5 3-8.55.5.
55.5 55.5 55.5. 55.5. 855 55.5 55.5 55.5 55.5 55.. .5..5 8.55 55.5 55.5 5.8.55.5.
55.5 55.5 .555 .555 .555 ...5 55.5 55.5 55.. 8.5 5555 .555 55.5 .... 3.55.5575.
55.5 .55 .555 55.55 55.55 ...5 55.5 ...5 55.5 5.5 55.55 8.55 55.5 55.. 3.5.5-5575.
..55 .55 .555 55.55 5555 55.5 55.5 5.5 .5.. 55.5 55.55 .5..5 .55 55.. 3.55.555.
55.5 ...5 ...... 55.55 55.55. 55.5 55.5 5.5 5... 55.. 5.55 8.55 55.5 85 3-55.5575.
.55 .5. 8.5. ..55. .5..5 55.5 ..55 ..55 ...5 ..5. 55.55 55.5 55.5 55.. 5.555.575.
55.5 .5. 8.5. 55.5. ...55 55.5 55.5 ..55 55.5 55.. 55.55 ...55 .55 55.5 3-55.5575.
...5 55.5 8... 55.5. .555 55.5 55.5 5.5 5.5 5.. 55.55 55.55 55.5 55.5 5.55.5575.
55.5 55.5. 55... .55. 55.5 55.5 .55 ..55 5.5 55.. 55.55 .55. 55.5 55.5 3.555.575.
55.5 5.5 8.5 55.. .555 .55 55.5 55.5 55.5 55.. 5.55 .555 55.5 55.5. 35555.5.
.o 2 5< 3.2 95 055 05“. 08 09. 50.5 5052 058 0552

 

8:55.58 5255. 5522 3.58.25.

170

APPENDIX 3. (Continued)

 

00.0 00.00 0150 00.5 00.50 #00 00.0 05.0 00.0 00.05 3.3 00.0.. 00.0 51* 3.00.05. 5.070.
50.0 00.00 50.00 50... 00.00 ..0.0 00.0 00.0 00.0 3.05 .100 00.0.. ..0.0 V0.0 50.....005. 5.070..
00.0 00.00 0.00 00.5 05.00 00.0 00.0 00.0 50.0 00.05 5100 00.0.. 00.0 010 3.00.05. 5.050.
00.0 00.0 05.00 00.00 00.00 00.0 0 _..0 05.0 010 00.0 00.00 50.10 00.0 #15 3.0005. 5.070.
00.0 50.0 #100 .100 00.00 00.0 010 510 00.0 00.0 50.00 50.00 00.0 015 .0.—:005. 5-0 5.0..
50.0 00 .0 00.00 00. .0 00.00 00.0 3.0 55.0 00.0 00.0 0100 50.00 00.0 00.5 3.0005. ..-0—30.
00.0 00.00 #000 00. 5 05.50 00.0 00.0 00.0 00.0 50.5 0.10 05.05 00.0 00.0 5.0705. 5.070..
00.0 50.00 0100 .0... 00.00 00.0 00.0 00.0 50.0 05.0 00.00 010.. 00.0 05.0 50-515.705.05
5.0 00.00 00.00 01 F 00.00 ..0.0 00.0 v0.0 .00 00.0 00.00 00.05 00.0 05.0 3.0705. 5-0.30.
00.0 50.00 55.0v 50.0 0100 50.0 00.0 50.0 010 05.0 0100 00.05 00.0 50.0 3.00.05. ..-0 ...0.
05.0 05.00 50.00 «0... 50.00 00.0 3.0 010 00.0 00.0 110 50.0.. 00.0 00.0 ..0-E0.05. 5.050.
50.0 00.00 0105. 00. 5 00.00 00.0 50.0 00.0 00.0 00.0 00.10 00.05 ..0.0 0.20 3.0005. 5-0 70..
00.0 00.50 00.00 00.0 0060 00.0 00.0 00.0 00.0 010 00.15 05. .... 00.0 01—. ..0-E0..0_.-_...0..
00.0 00.05 00.00 006 05.00 00.0 00.0 00.0 00.0 00.0 00.05 05. E. 3.0 0..... 50.00-0770.
000 0150 0.. ..v 00. _. 50.00 V0.0 0.0 05.0 00.0 00.0 05.00 ..0.0. 00.0 00.0 ..0.0 ..-0—x70.
00.0 05.50 V0. 3 0 5.5 05.00 00.0 00.0 0 ...0 10.0 00.05 0100 00.0.. 00.0 55.0 .5.... 5-05-5-0.
00.0 00.50 00. 3 00. 5 05.00 00.0 00.0 00.0 00.0 05.0 00.00 00.05 00.0 00.0 .5...: 5.07—.0..
00.0 00.00 00.00 01v 00.00 00.0 00.0 010 00.0 00.0 05.05 50.05 00.0 00. _. 3.00.00 5.70.
00.0 00.5 ..0.05 00.0.. 00.00. 010 05.0 50.0 05.0 015 00.0 01 ..0 00.0 00.0 3.0700770.
b0 n< :< .32 0.0 000 00... 000 09. 00.0 0003 092 0002

 

352.08 8.52 5522 .2589...

171

 

 

 

00.00 ..0.0 00.0 ..0.0. 010 00.0 00.0 50.0.. 3.0 00.01 00.0 013 00.0 0040-...
00.00 0 ...0 00.0 05.0 510 55.0 0..... 00.0.. 0...0 50.05. 05.0 55.05 510 00.00-...
00.005 3.0 00.0 00.0 010 05.0 05.0 05.00 2.0 0.200 ..0... 00.05 00.0 0040-...
50.00.. 00.0 50.0 00.0 00.0 00.0 v0.5 00.00 2.0 00. ..0 00... 00.3. #10 0..-V0-2
00005 05.0 50.0 510 00.0 00.0 00.5 v0.00 00.0 00.50 50.0 00.0.. 010 ...-00-....
00.00.. «0... 50.0 00. 5.. v0... 0...0 v0.5 00.0.. 15.0 00.00 05.0 00.0.. 00.0 05. ..-0...
0100 015 00.0 00. .... 00. F 00.0 00. .. 00.05 00.0 00.00 ..0.0 0.0.. 00.0 05. ..-0...
.0.00 0 ...0 00.0 00.0 010 05.0 00.0 00.0.. 00.0 0150 05.5 05.3. 50.0 05. 5-05..

00:ng 8..—ace 0002 .0_c_w..u0..l
50. ..0_. 05.0 .. 00. .. 00.0 05.0 00.0.. 50.0 ..0.00 05.0 00.05 010 5-5-50E
00.005 00.05 00. 5 00.0 5...0 00.05 ..0.0 00.00 00.0 010 00.0 5-5-..0E
00.00 00.05 #1.. 00.0 5 ...0 05.05 00.0 50.00 00.0 55.0.. #10 75.0.:
0100 00.0.. ..0... 00.0 05.0 #00— ..0.0 ..0.00 05.0 51 .... 50.0 75-..0E
00.00 00.0 00.0 00.0 50.0 5 5.00 50.0 50. ..0 015 50.0.. 010 70.02.:
5150 ..0.0 00.0 00.0 00.0 00.00 50.0 50. 50 00. _. 010.. 010 70.0 ..E
00.00 3.0 00.0 00.0 00.0 00.00 00.0 00. ..0 00.5 50.05 ..0.0 70.05....
.1005 00.0.. 00... 00.0 3.0 05.00 00.0 ..0.00 55.0 00. 5.. 510 0-000-E
00.00 v0.0. 00. .. 50.0 00.0 00.00 00.0 00. ..0 00.0 0 ... ... ..10 0-000....

.29 n. .0 00". 0:5. 000.0 50.... 000 00v. 00.0 0002 00.2 0002

 

APPENDIX 3 (Continued)

8555055 5.505. 50.550 55580-355.
0m0>4<z< 029.00.602.40

172

APPENDIX 3 (Continued)

 

55.55 5.55 55.5 55.5 55.5 .5. 55.5 55.5 .555 55.. 5.5. 55.5 55.5 x55é5....5e
.555 ....5 55.5 55.5 .55 .5. 55.5 55.5 55.5 .5. 55.5. .55 55.5 x5555..-.5e
55.55 ...55 55.5 55.5 .55 .5. 55.5 55.5 55.55 55.. 55... 55.5 55.5 x55.o5..-.5e
55.55 55.55 5.5 .55 5.5 55. 55.5 .55 55.5 5.5 55.5 55.5 55.5 55.5.5555...
5555 55.5 .55 55.5 ...5 55.. 55.5 .55 5.... .55 .5.. 55.5 55.5 5.5.5.5555...
.555 55.5 55.5 55.5 ...5 .5. 55.5 55.5 55.8 55.5 .55 .55 55.5 vac-5.5-555...
5555 55.55 55.5 55.5 ...5 55.. 55.5 55.5 55.8 55.5 555 55.5 55.5 65.5.5555...
55.55 55.5 5.5 55.5 5.5 55.. 55.5 55.5 55.55 ...5 55.. 55.5 55.5 35.55.5555,:
.555 55.55 55.5 .55 55.5 55.5 55.5 55.5 .555 5.5 55.. 55.5 .55 65.55.5555...
:22 05“. 05.2 5055 5o: 050 055. 595 505.5. 05.2 0552 n.

 

3:55.65 8205. 5550 05520-355.
wmw>..<z< mzmegQOIPZO

173

APPENDIX 3 (Continued)

 

55.50 510 00.0 00.05 00.5 010 05.0.. ..0.0 00.0 00... 00.0 ...0-000.000....
0...00 010 00.0 05.05 00... 00.0 ..0.0. 0...0 00. 0 05.5 00.0 30.00.0000...
00.00 010 00.0 00.55 00. 0 ..10 00.0.. 010 v0. 0 00... 00.0 .0.-00.0-0000.
55.50 010 ..0.0 55.55 00. .. 010 00.3. 00.0 00. 0 00. .. 00.0 .a.-000-000....
00.00 ..10 ..0.0 0105 00. _. 010 ..0.0.. 0...0 00.0 3. p 00.0 ...0-070-000...
05.00 510 00. 0 05.05 50... 00.0 013. 010 00.0 '0... 00.0 .0.-070-000....
v0.3 010 00. 0 50.00 00.0 50. 0 #1.. 510 00.0 00.0 ..0.0 «...-0070......
v0.00 01 0 00. 0 05.00 00.0 00. 0 0. 0 0...0 50. 0 ....0 05... «...-0070......
0...00 01 0 51 0 00.00 05.0 00. 0 00. 0 0...0 00. 0 55.0 00.« «...-E. ...0—....
010 51 0 V0. 0 00.00 «5.0 00. 0 05.0 5...0 50. 0 00... 50.0 .80.. 70......
00.00 ..0.0 00. 0 00.00 05.0 00. 0 00.0 0...0 00. 0 00.0 #0... «...-0.. 70...:
00.00 ..10 00.0 0...00 00. 0 010 50.00 50. 0 00. 0 00.0 50.5 ...0070......
0.00 00.0 010 v0. 50 v0. 0 00.0 00.05. 00. 0 00. 0 0...0 510 ...00705...
32.50 00.0 010 05. ..0 00. 0 00. 0 05.00 50. 0 3.0 00.0 00... ...0070..E
00.0 00.0 00.0 00.00 00.. 00. 0 00.05 00. 0 00.0 00.5 00.0 2:00.05...
00.00 00.0 55.0 00.00 05.5 00. 0 50.0.. 00. 0 00.0 00.0 00.0 .a.-00.0.....
00.00 00.0 .....0 00.05 00... 00.0 55.0 50. 0 00.0 3... 50.0 .0.-00-0.....
50.00 00.0 0...0 00. .0 #0.. 00.0 00.0 00. 0 50. 0 50.0 50.0 ...0-00.0.....
05.00 00. 0 3.0 00.00 00.. 50.0 00.0 00. 0 00. 0 ..0... 10.0 ~.0-05.05....
..0.0 00. 0 . 0...0 00.00 ..0... 00. 0 5...0.. 00. 0 00. 0 00.0 00.0 3.65.05...
0100 00. 0 00.0 00.55 00.5 3. 0 00.0.. 00. 0 0...0 *0... ..0.0 .....0700-000...
00.00 00.0 00.0 50.00 50... 50. 0 0.0.. 00. 0 010 00.0 ..0.0 .a.-0700-000...
00.00 00.0 50.0 55.05 05.5 00. 0 00. 0.. 00. 0 05.0 00.0 00.0 «...-0.00-000...
00.00 00.0 05.0 0...05 510 00.0 #15 0... 0 3.. 0 015 0...0 .0.-0000-000...
00.00 00.0 00.0 ..0.00 00.5 50. 0 ..5. 0 00.0 00. 0 ..0... 05.0 .a.-0000-000...
0100 00.0 00. 0 00.55 00.. v0. 0 00.0.. 00. 0 3. 0 50.0 00.0 «...-0700-000...
00.00 50.0 00. 0 00.55 00... 50. 0 0105 00. 0 5...0 50.0 50.0 .0.-0700-000...
00.00 50.0 50. 0 010* 0...0 00.0 00.0v 00. 0 00.0 00.0 00.0 ...0. 70.000...
55.00 00.0 00.0 5 .00 ..0.0 00.0 500* .50. 0 00.0 00.0 50.0 ...0. 70-000...
00.00 00.0 50.0 00.00 50.0 50.0 00.00 00. 0 00.0 00.0 00.0 ...00-0-000...
05.50 00.0 05.0 00.35 00.0 00.0 00.0v 3.0 00.0 00.0 00.0 ...00-0-000...
.08. 0.2 005> 00“. 0...... 000.0 00.... 000 00.0 00~.< 0d:

 

52.55055 5205. 59.550 8550-65..
www>4<z< mtzmfi... DZ< m....._.m20<.2

174

APPENDIX 3 (Continued)

0060 010 0.0 .100 00.0 05.0 50 60 .0.0 ..0.0 00. . 00.0 ...0-00.60- ..

 

0060 00.0 .1 0 00.05 00.0 00. 0 00.0. 00.0 00.0 00.0 50.0 .a.-00.60-...
.060 010 00. 0 00.00 00.0 .50. 0 615. 00. 0 00.0 010 00.0 .0.-060-...
0060 .0.0 00. . 05.00 010 51 0 .060 00. 0 00.0 00.0 00.0 ....-5-60...
6100 00. 0 65.0 00.06 00.0 50.. 00.06 00.0 00.0 00.. 010 ...0-60....
0060 00. 0 00.0 50.00 010 00. . 00..0 610 010 010 00.0 .....0-60...
01.0 .1 0 05.0 5100 .0.0 .1 . 05. 6. 010 00.0 05.0 60.0 .....760...
00.50 00.0 510 6100 00.0 00. . 60. 0 .0.0 00.0 00. .. 00.0 ...0-00.60....
0150 50.0 010 00.00 610 00. 0 51 0 010 00.0 00.. 00.0 .a.-00.60-...
00.00 00.0 60.0 5060 00.. 00. 0 010 010 60.0 01. 00.0 .a.-00.60-...
0100 00.0 010 00.00 00.0 01 0 00.0 010 00.0 00.. ..0.0 .....00-60-..
0100 00.0 00.0 60.05 0 .6 00.0 00.0 50. 0 00.0 60.0 010 .0.-0.60-...
05.00. 010 010 05.00 010 05.0 0066 00. 0 .0.0 00.0 60.. ...0005. 70....
00.00. 00 .0 51 0 0106 010 510 .066 010 010 00.0 60.. ...0005. ..-0 ..
0100 00.0 60. 0 00.06 510 00.. 05.06 55. 0 00.0 ..0.0 00... ...0-05. 70....
00.00 00.0 00. 0 0106 010 05.0 .006 60. 0 60.0 00.0 50.. ...0705. 70....
0100 .10 60. 0 .005 51. 010 05.0. 01 0 60.0 05.0 00.0 .a.-00.05. 70...
00.00 .10 .510 50.05 00. . 01 0 0.... 010 00.0 50.0 .10 .0.-00.05. ..0...
00.00 00.0 00.0 .000 .00 V0. 0 00.0 510 00.0 00.0 60.0 22-0.05. ..0 ..
.060 00.0 00.0 05.00 00.0 01 0 05.0 010 00.0 .0.0 05.0 .....00-05. ..0 ...
00.00 010 00.0 00.00 00.0 01 0 00.0 .0.0 00. 0 00.0 65.0 5.60.0170...
5060 00.0 50.0 00.00 .5.0 610 00.0 .0.0 00. 0 00.0 00.0 ......0 705. 70....
0100 50.0 00.0 00.00 00.0 010 00.0 010 60. 0 00.0 ..0.0 .0.-0705. .-0 ..
.06. 0.2 005> 00“. 0:2 000.0 00.... 000 00.0 000.41 002

 

8:55.58 5.552 555.2 8.58-25.

 

0060 00.0 00.0 00.55 60.. 00.0 05.0. 60.0 00.0 00.0 50.0 .E-oo..-5-.0...
.060 00.0 50.0 50.05 05.. 00. 0 00.0. 0 ...0 010 .0.0 00.0 5.5075- .0...
00.00 00.0 010 0105 51 . 00. 0 00.0. 010 00.0 01. 010 .0.-0.5- .0...
0060 00.0 610 .5.55 00. . 00. 0 60.0. 50.0 010 00.0 50.0 .0.-00.5.0...
6 . 60 .0.0 ...0 05.55 65. . 00.0 50.0 . 00.0 00.0 00.0 00.0 .0.-00.5.0...
.30. 0.2 000> Own. 0...: 000.0 00.... 000 00.0 000_< 0d:

 

852.005 8.32 5.50 $523555.

175

APPENDIX 3 (Continued)

 

00.00 00.0 .500 00.0 00.0 00.0 00.0 00.0 00.0 55.0 .:-U00070-070..
00.00 010 00.00 50.0 00.0 010 00.0 60.0 60.0 05.0 :-a0.... 70.070.
0100 00. 0 60.00 01 0 00.0 00.0 00. 0 00.0 00.0 55.0 8.00 .0.-0.070.
00.00 60. 0 50.56 60. 6 00.0 0166 00. 0 .0.0 00.0 00.. ..-0000700 70.
00.50 00. 0 60.56 .0. 6 00.0 06.06 00.0 00. 0 50.0 00.. ..-0000 70.070.
0000 00.0 .0.56 00.6 00.0 60.06 00.0 00. 0 010 00.. 700370-070.
00.00 00.0 0106 00.6 00.0 0166 00.0 ..0. 0 610 60.. ..-—.0 .0.-0.0 70..
60.00 010 .000 00.0 50.0 60. 0 00. 0 60. 0 00.0 00.0 ...:- .00-0.070.
.000 00.0 50.06 010 00.0 00. 06 00. 0 00.0 0...0 00... ..- .00-0.070..
0150 00.0 50.56 00.6 00. 0 00.06 00.0 00.0 00.0 60.. ..- .00-0.070.
65.00 010 00.00 01 0 00. 0 60.0 00.0 00. 0 .0.0 05.0 ..:-..0 ..-0-070.
0150 00.0 05.56 00. 0 00. 0 06.06 .0.0 00. 0 50.0 00.. ..- .0 70.070.
00.50 010 0106 00. 0 00. 0 01 06 00.0 .0. 0 00.0 50.. ... .0 70.070.
05.00 00.0 00.05 00. . 60. 0 06.6 00.0 00.0 0... 06.0 .5. .00-00 70.0..
50.00 50.0 00.05 00. . 00. 0 00. 6 00.0 60.0 01.. 00.0 2:-.0000 70.0..
00.60 00.0 00.05 00.. 00. 0 00. 0 ..0.0 010 60.. .0.0 .E- .06-0070.0.

. .060 50.0 00.05 00.. 60. 0 00. 6 00.0 00. 0 00.. 06.0 2:. .00-00 ...0-0..
00.00 00.0 .005 00.. 00. 0 60.6 00. 0 00.0 00.. 06.0 .8. .00-00 70.0..
00.60 00.0 00.05 .1. .0.0 00.6 60. 0 01 0 .... 06.0 .E-.00.00 70.0..
50.00 00.0 0105 00. . 00.0 60.6 00.0 60. 0 00.0 00.0 .5. .00-00 70.0..
01.0 ..0.0 50.66 00.0 00.0 05.06 00.0 00. 0 00.0 .1. ..- .00-00 70.0..
65. .0 00.0 06.66 00.0 00. 0 00.06 00. 0 ..0.0 00.0 .... ..- .00-00 ..0.0.
00.00 50.0 00. ..6 00.6 00. 0 05.06 00. 0 00. 0 00.0 50.0 ..-.0700 70.0..
0...00 00.0 00.06 06.0 00. 0 00.06 00. 0 60. 0 0...0 .0.. ..-.0000...0.0.
00. .0 .0.0 06.06 00.0 00.0 .5.06 00. 0 00. 0 00.0 00.. ..-.0000 70.0..
60.00 00.0 00.06 016 00.0 00.06 00. 0 00.0 ...0 00.0 ..-.0000 ...0-0..
00.00 60.0 06.06 00. 6 .0.0 06.06 00. 0 00.0 00.0 010 ..-.00-00...0-0..
06.00 00.0 00.06 60. 0 .0.0 0... .6 00. 0 00.0 010 00.0 ..- .0700 ...0-0..
66.00 50.0 50.06 .0. 0 .0.0 05. .6 ..0.0 00.0 00.0 00.0 ..- .0700 ..0.0.
_0.o. 0.2 005> 00”. 0:5. 000.0 00.... 000 00.0 000.< 0%

 

02.2605 5.502 555.2 3.58.0.5.

176

APPENDIX 3 (Continued)

 

 

 

.0.55. 50. .. 55.. 0.5 5... 50.5. .05 05.5 55.56 00.0 0...0. 05.0 60.. 2.50.05. .0..
50.55 .0. .. o... 5 ..5 55.. 50.5. 00.5 5 ...5 50.06 55.0 05.0. .50 50.. 2.50.0... .0..
55.5 .5. .. 55.5 .05 5.0 .5. .. 05.5 05.5 .5. .6 50.0. 50.0. 00.0 60.5 2.5.0.. ..0 E
05.55 05.0. 05.5 6.5 00.0 05... 00.5 5.5 05.06 56.0. 50.0. 00.0 .65 2.0.. ..0..

55555555 5.25. 555.2 3.5.5.5...
05.55 00.50 56.. 55.5 .0.. 56.5 6.5 55.5 65.06 50.0 00.5 55.0 06.5 2.50-5.0...
50.65 65.50 50.. 55.5 0... 00.0 05.5 55.5 6.. .6 00.0 60.5 .50 .65 2.50.5.0...
0...0 00.00 06.. 55.5 60... ...5. 05.5 5.5 55.66 50.0 06.. 5.0 00.5 2-500-550...
50.05 5.60 56. . 55.5 .6. . 06.5 0.5 0 ...5 .506 0 ... 60.0 60.0 00.5 2.550-500...
5650 66.50 55.. 55.5 00.. 0.5 05.5 05.5 05.66 00.0 66.5 .50 00.5 2.8.0-500...
05.50 55.60 50... 55.5 00.. 5 ..5 55.5 6.5 .506 .60 00.0 60.. 06.5 2.5.0-500...
00.00 05.60 56. . 55.5 50. . 00.5 5.5 05.5 6.06 05.5 00.0 60.. 56.5 2.5.0-500...
60.60 50.00 00.. 55.5 06.. .50 00.5 5.5 00.06 55.0 05.0 .0.. .05 2.505-500...
55.00 65.00 50.. 55.5 50.. 50.5 05.5 0.5 00.06 .00 05.0 .5... 05.5 2.500.500...
05.05 .550 50.. 65.5 .0.. 6 ..5. 05.5 5 ..5 05.66 06.0 0 ..0 55.. 00.5 2.50.0.0 ....
5655 55.50 56... 65.5 .5. 0.5. .05 ...5 50.66 .50 65.0 5... 50.5 2.80.0.5 ....
.505 00.50 00.. 05.5 50.. 65.5.. 50.5 ...5 60.66 .00 00.0 5... 0.5 2.55.0.0 ....
5...0 55.50 55.. .55 .5. 05.5 5.5 5.5 5.56 55.0 0 ..5 0... 05.5 2.50.0.0 .8
.650 06.50 06.. .55 5 ... 55.5 6.5 ...5 06.06 00.0 00.5 5... 65.5 2.50.0.5 ....
55.55 .550 00.. .55 05.. 55.5. 0.5 ...5 05.06 60.0 66.0 50.. 00.5 2.50.0.0 :5
05.5 . ..50 .6. . 55.5 .5. . 60.5 5.5 0 ..5 0.66 50.. 00.0 05.. 0.... 25 ..0-5 ....
05.0 00.5.. 56.. 55.5 06... 05.5. 55.5 55.5 05.66 05.0 06.0 .... .05 2.5 ...0-5 .5
05.60 50.5. ...5 55.5 05.0 00.5. .05 65.5 ...00 50. .. .0. .. .60 06.5 2.5.0.500...
55.50 0.5. 50.5 55.5 .55 5.5. 05.5 55.5 50.50 .0. .. 05. .. 50.0 .65 25.-0-500...
5.55 05.50 65.. 65.5 55... 56.0 .05 05.5 05.66 00.0 50.. .0.. 05.5 2.55 ..5-500...
6.05 ...50 00.. 05.5 .5. 6.0 .05 55.5 05.06 ...0 06.. 00.. 05.5 2.53-0-550...
05.00 55.00 60.. 05.5 55.. 00.0 55.5 ...5 05.06 00.0 00.0 00.. 00.5 2.5.0.5500...
55.05 50.50 56. . 55.5 50. . .05 5.5 ...5 00.06 06.0 50.0 05.. 55.5 2.500-550...
05.3 55.00 05.. 55.5 .0.. 0.5 6.5 55.5 5 ..06 60.0 60.0 60.. 0.5 2.500-500...
00.55 55.50 05.. 55.5 .5. 56.0 50.5 55.5 05.06 5 ... 5 ... ...0 60.5 2.550-500...
.58. 055. 05.2 500.0 00.... O50 00x _0 00.0 5002 00.2 0052 u.

 

55555555 5205. 55.550 55.50-.555.
wmm>-.<z< 00230sz...

177

APPENDIX 3 (Continued)

 

55.05 ...0. 55.5 5.5 65.5 60.5 55.. 55.5 5.55 66.0. .0.6. 56.5 6... 5-.-5...-0t
05.05 ...0. 05.5 0.5 50.5 55.5 55.. 0.5 0.50 50.0. 55.6. 56.5 55.0 5.56-5....0:
50.55 55.6. 5.5 6.5 .50 .65 55.. ...5 00.55 56.0. 05.5. 55.5 55.0 5.56-5..-?

55555555 5255. 5555. 8.52.5.5
.5.55 50.50 56.5 55.5 55.6 5.5 55.. 55.5 56.50 05... 5.5 50.5 55.5 5.55-.-.0...
.005 .0.0. 00.0 00.0 00.6 .00 00.. 00.0 00.06 00.0. 00.. 60.0 06.0 5.50-.-.0...
55. .5 .... 50.5 55.5 65.5 05.5 55.. 55.5 5 ..06 56.5. 55.. . ..5 05.5 55.-.-.0...
.0.55 50.5. 55.5 .55 55.6 65.5 .55 .55 55.56 05.5. 65.. 5.5 55.5 55.-.-.0...
55. .5 50.50 55.5 55.5 50. . 6.5 65.6 55.5 00.60 06.0 55.5 50.5 55.0 ...-550.500...
55.55 .50. 55.5 55.5 .5.. 55.5 55.6 55.5 05.65 55.5 55.5 5.5 65.0 ...-5.5.0-550...
5.. .5. ...5. 56.5 55.5 .50 55.5 5.0 55.5 0.05 55.5 55.5 55.5 .50 5-55-5-500...
55.55 50.0. 5.5 55.5 55.. 55.5 50.5 6.5 .0.56 55.5. .55 55.5 05.5 ...-5.5.0-550...
66.55 55.5. 0 ... 55.5 50. . . ..5 55.5 5 ..5 56.66 05... 06.0. 50.5 65.5 3-55-0-500...
.000 0.00 65.0 00.0 00.0 50.0 00.0 5.0 00.06 00.0 55.0 0.0 06.0 5.500-550...
0.55 .05. 56.5 55.5 .50 .05 .65 6.5 .550 00.5. 56.5 55.. 55.5 ...-6.55....
05.65 55.5. 55.5 05.5 .5.6 5.5 .55 0.5 5.50 0.0. 5.5 55.5 .... 55.-5.5....
5.55 00.5. 65.5 .55 .5.6 60.5 .0.. 0.5 55.50 .65. 05.5 50.5 .5.. ...-5.5.5....
.050 60. .. 00.0 00.0 50.0 .00 0.0 0.0 00.00 00.0 00.0 00.0 00.0 5.55.0.0 ....
55.55 55.5. 05.5 05.5 5..6 00.5 55.. 6.5 05.0 05... 6.5 55.5 .5.. ...-5.5.5.5....
05.05 00.5. 56.5 55.5 6..6 6.5 55.. 0.5 5.55 ...0. 55.5 60.5 .... 555-55....
05.05 55.5. 05.5 05.5 .55 5.5 55.. ...5 56.55 56.5. .55 ...5 05.5 .555-5.5....
.32 05... 052 80.5 8.. 055 09. _5 8.5 802 05.2 0.52 .

 

55555555 5.505. 05.550 5555.0..555
wmw>-_<Z< NFC-9m

APPENDIX 4

BIBLIOGRAPHY

178

APPENDIX 4. 40Ar/39Ar Analyses

Key:

Pre-rainier Mesa tephra sequence
12R8 series
AFB series
*RM92 series

Post-grouse Canyon tephra sequence
M* series
P6293 series

Pre-ammonia Tanks tephra sequence
AT92 series
2AT92 series
AT 19 series
3EAT92 series
7 -30-92
2EAT92 series

I79

APPENDIX 4. (Continued)

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APPENDIX 4. (Continued)

.555
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181

APPENDIX 4. (Continued)

.555 55.... ..55 555.5. ...55 555.5 .55.. .555 555.5 5.5.5. 555.. .5
5.5.5 .55... .85 555.5. 58.5 ..55 .55.. .555 ..55 5.5.5. 555.5 55
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182

APPENDIX 4. (Continued)

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APPENDIX 4 (Continued)

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APPENDIX 4 (Continued)

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APPENDIX 4 (Continued)

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APPENDIX 4 (Continued)

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