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THESis

This is to certify that the
thesis entitled

Petrologic Characterization of the Paleogene Arc, Costa Rica,
Central America

presented by

Beth Apple

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has been accepted towards fulfillment
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PETROLOGIC CHARACTERIZATION OF THE PALEOGENE ARC,
COSTA RICA, CENTRAL AMERICA

BY

Beth Apple

A THESIS

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

MASTER OF SCIENCE

Department of Geological Sciences

2005

ABSTRACT
Petrologic Characterization of Paleogene Arc, Costa Rica, Central America
By

Beth Apple

Lava clasts in Late Cretaceous to Eocence debris flows are manifestations of the earliest
Costa Rican volcanic arc. This study characterizes the petrography and geochemistry of
volcaniclasts from four sample locations (Punta Samara, lsla Paloma, Quebrada
Buenaventura, and Plays Soley) to determine the subduction parameters for the
primitive arc. Geochemical compositions of the primitive arc lavas are compared to
samples from the volcanic front from the modem arc, concentrating on the Costa Rican
and Nicaraguan segments. Selected trace element ratios of these clasts are used to
infer subduction parameters including mantle source (Zr/Nb), degree of mantle melting
(La/Yb), slab input (Ba/La), and sediment addition (BaIT h, UIT h). The majority of
primitive arc lava clasts originated from a depleted, mantle source (MORB-like), and
have higher Zrle ratios than those from modern central Costa Rica (CCR). However,
two lava clasts have Zrle ratios similar to CCR volcanics, indicating a contribution from
a more enriched mantle source. The degree of mantle melting increases (low LaIYb)
with an increase in slab signal (high BalLa) in the primitive arc lava clasts. and these
values are more similar to what is observed in modern western Nicaragua than in
modem Costa Rica. This may be a reflection of a steeper angle of subduction offshore
Costa Rica during the Late Cretaceous to the Eocene, possibly because the Farallon
Plate was older, colder, and denser than the modern day Cocos Plate. The primitive arc
lava clasts have high Ba/Th and low U/T h ratios indicating that the sediment subducted
in the primitive are likely had a carbonate component (high BaIT h), but lacked the

hemipelagic sediment component (high UIT h) that is subducted in the modern arc.

ACKNOWLEDGEMENTS

I have learned a great deal and had many invaluable experiences while
working towards my M.S. There are many people to thank for this...

The Geological Society of America and Michigan State University College
of Natural Science funded the travel to Costa Rica. Michigan State University
Graduate School also supplied additional funding. I greatly appreciated that my
advisor, Lina Patino, offered guidance throughout this process, along with
interest and faith in my project. My committee members, Tom Vogel and Duncan
Sibley, provided input and assistance in meeting deadlines. Thanks also to the
department office and library staff for your help. Sampling in Costa Rica, would
not have went smoothly without Guillermo Alvarado. Kennet Flores also
provided samples, maps, and information on my sample locations. Thanks to
Chad Deering and Dave Szymanski for the laughs and the muscle behind the
sledgehammer in Costa Rica. Thanks also to Ela Vrray for passing on data from
Nicaragua.

I also appreciate the support of my fellow graduate students, friends. and
family. Thanks to Hillary and Ryan, Karen Tefend, Amy Lansdale, Leah Piwinski,
Mike Kramer, Michelle Vrt, Brent & Miranda Lucyk, Tim Place, Robin Rumpf and

my Mom.

iii

TABLE OF CONTENTS

LIST OF TABLES .............................................................................................................. v
LIST OF FIGURES .......................................................................................................... vi
INTRODUCTION ............................................................................................................... 1
Background ...................................................................................................... 1
Sample Locations ............................................................................................. 6
METHODS ......................................................................................................................... 9
Sampling Methods ........................................................................................... 9
Sample Preparation ........................................................................................ 10
Analytical Techniques .................................................................................... 10
RESULTS ......................................................................................................................... 12
Petrography ................................................................................................... 12
Alteration ........................................................................................................ 29
Geochemistry ................................................................................................. 33
DISCUSSION .................................................................................................................. 55
Mantle Source .............................................................................................................. 55
Slab Signal ..................................................................................................... 59
Speculations on Subducted Sediment ............................................................ 64
CONCLUSIONS .............................................................................................................. 69
APPENDICES .................................................................................................................. 72
Appendix A: Petrographic descriptions of altered samples ............................. 73
Appendix B: Minerals present in thin sections ................................................ 82
Appendix C: Anomalous slab signal (Ce/Pb) .................................................. 85
REFERENCES ................................................................................................................ 95

iv

Table 1.

Table 2.

Table 3.

Table 4.

LIST OF TABLES

Petrographic descriptions of lava samples. .......................................... 13

Major and trace element geochemistry from the primitive arc
volcaniclastic lavas. Samples not collected in the 2004 field season
originated from (#) Kennet Flores and location is in the Costa Rican
coordinate system, (+) Patino and others (2004), and (=) TA Vogel. .34

Range of select trace element ratios for primitive arc lavas. ................ 56

Minerals present in thin section ............................................................ 83

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

LIST OF FIGURES

From the Late Oligocene-Early Miocene to present, the modern
Central American volcanic arc results from the Cocos Plate beneath
the Caribbean Plate. Figure modified from Feingenson et al., 2004. ....2

A reconstruction of the Central American arc during the Paleocene
and Eocene. The Farallon Plate is subducting beneath the Caribbean
Plate. Figure modified from Hauff et al., 2000 .......................................... 3

A map of the primitive are sample locations in Costa Rica. Sample
locations are as follows: 1. Punta Samara, 2. Isle Paloma, 3.
Quebrada Buenaventura, and 4. Play Soley. ............................................ 7

Photos of volcaniclasts from the primitive arc located at ....... 8

Photomicrograph of a lava sample, 040709-15, from Quebrada
Buenaventura, in plane polarized light (top) and crossed polarized light
(bottom). Plagioclase, pyroxene, and opaques form the phenocryst
assemblage in lava samples from Punta Samara, lsla Paloma, and
Quebrada Buenaventura, and 2 lava samples from Playa Soley
(040710-16 and 040709-19).Plag is plagioclase, Pyx is pyroxene,

Opq is opaque, and Cal is calcite. ............................................................. 25

Photomicrograph of lava sample, 040709-2, from Quebrada
Buenaventura, in plane polarized light (top). and cross polarized light
(bottom). Reaction rims are present on some pyroxene grains. .......... 26

Photomicrograph of a lava sample, PA10, from Punta Samara, in
plane polarized light (top) and crossed polarized light (bottom).
Complete or partial replacement of grains by secondary products was
present in some samples. Plag is plagioclase, Opq is opaque, Cal is
calcite, Chi is chlorite, and repl is completely replaced grain. .............. 27

Photomicrograph of a fiamme sample, 031024-4a, from Playa Soley,
in plane polarized light (top) and crossed polarized light (bottom).
Anhedral feldspar and quartz form the phenocryst assemblage in

fiamme samples from Playa Soley, and opaques appear to be
secondary. Plag is plagioclase, Repl is completely replaced grain. ..28

Figure 9. The majority of the modern arc volcanic samples have Weathering
Index of Parker (WIP) values > 60. Primitive arc samples with WIP
values < 60 (open primitive arc symbols) were not included in the
geochemical portion of this study. ........................................................... 31

Figure 10. Pumice 031024-4a, 4b, 4a, and 4h from Playa Soley have'unusual
REE patterns. These samples were not included in the geochemical
portion of this study. .................................................................................... 32

Figure 11. Total alkalis versus SIC; classification diagram of Le Bas et al.
(1986) of the primitive arc volcaniclasts. ................................................. 49

Figure 12. Major element variation versus SiOz (wt%) amongst the primitive arc
lava samples .............................................................................................. 50

Figure 13. Trace element concentrations (ppm) versus SiOz (wt%) for the
primitive arc lava samples. ........................................................................ 51

Figure 14. The spider diagram, normalized to the primitive mantle after Sun and
McDonaugh (1989), illustrates that the primitive arc lavas samples are
enriched in LILE relative to HFSE, characteristic of volcanics
originated from subduction zone magmatism. ........................................ 53

Figure 15.REE trend for the lava samples from the primitive arc are relatively
similar to each other. Normalized to chondrite after Sun and
McDonaugh (1989). .................................................................................... 54

Flgure 18. Volcanics with higher Zrle ratios indicate a more depleted mantle
source. Increasing BalLa ratios indicate greater slab contribution to
the lavas. DM and EM compositions are N-MORB and OIB,
respectively, from Sun and McDonaugh, 1989. ..................................... 58

Figure 17. The Zr/Nb (A) and BalLa ratios (8) of the lava samples are not
correlated with SIOz (wt%). ........................................................................ 6O

vii

Figure 18. The BalLa ratios from the volcanics from the primitive arc (A) show a
correlation with the Lale ratio, similar to the modern arc volcanics
(B). ................................................................................................................ 62

Figure 19 . The BaIT h variation for the primitive are also decreases with an
increase in LaIY b ratio (A). The primitive arc lava samples have
BaIT h ratios most similar to Miocene and modern western Nicaraguan
volcanics (B). ............................................................................................... 66

Figure 20. The primitive arc lava samples have UfT h ratios similar to those from
Miocence Nicaragua and modern Costa Rica. ....................................... 67

Figure 21. Comparison of the primitive arc in Costa Rica (A) to the modern arc in
Costa Rica (B) and western Nicaragua (C). Diagram after Carr et al.,
1990. ............................................................................................................. 71

Figure 22. Some lava samples from the primitive arc have high Cele ratios and
high BalLa ratios (>50). Samples from the primitive arc with Cele
ratios >14 are shown as open symbols. OlB and MORB are from Sun
and McDonaugh, 1989. ............................................................................. 87

Figure 23. The lava samples with high Cele ratios from the primitive arc (open
symbols) are due to low Pb concentrations in the samples. MORB
and OIB are from Sun and McDonaugh, 1989. ...................................... 88

Figure 24. The low-Pb lava samples have the characteristics of are related
magmatism. OIB and MORB are from Sun and McDonaugh,
1989. ............................................................................................................ 91

Figure 25. Selected elements versus Pb. Notice that the low-Pb samples (open
symbols) also have low K20 (wt%), Rb. Th, and U (ppm) compared to
other primitive arc lava samples. However, the low-Pb samples do not
have the lowest concentrations of Ce and Ba (ppm). ........................... 94

viii

Introduction

Background

The modern Central American volcanic arc is located along the western
portion of the Caribbean Plate (Figure 1). From the Late Oligocene-Early
Miocene to present, the arc is the product of the subduction of the oceanic Cocos
Plate underneath the Caribbean Plate (Hay, 1977). However, there is evidence of
subduction related volcanism prior to the Oligocene along the western portion of
the Caribbean Plate (Bourgois et al., 1984; Meschede et al., 1988; Maury et al.,
1995; Hauff et al., 2000; Lissinna et al., 2002). The earlier arc was the result of
the Farallon Plate subducting beneath the Caribbean Plate (Pindell and
Barrett,1990; Hauff et al., 2000; Hoernle et al., 2002) (Figure 2). The Farallon
Plate broke into the southern Cocos and Nazca plates and the northern Juan de
Fuca Plate, at approximately 22.7 Ma (Hey, 1977; Barckhausen et al., 2001).

The earliest appearance of subduction related volcanism in southern
Central America is found in formations from the Aptian to the Eocene (Bourgois
et al., 1984; Tournon, 1984; Calvo and Boltz, 1994; Hauff et al., 2000; Hoernle et
al., 2002; Lissinna et al., 2002). In the Santa Elena peninsula in Costa Rica,
Hauff et al. (2000) reported arc related rocks dated between 124 and 109 Ma.
Along the Pacific coast in Costa Rica, andesitic cobbles are found in Campanian
to Paleocene turbidite deposits (Kuijpers, 1979; Lew, 1983). Manifestations of
the ancient are are also found in other areas of Central America. Cenomian to
Eocene volcaniclastic sediments are found on the Nicaraguan Trough (Weyl,

1980). Weyl (1980) suggests that these originated from Cretaceous to Tertiary

 

   

 

1 l l
Caribbean
15 Sea ~
Costa Rica
10 _ .—
Middle America Trench

Pacific Ocean

 

 

 

Figure 1. From the Late Oligocene-Early Miocene to present, the modern Central
American volcanic arc results from the subduction of the Cocos Plate beneath
the Caribbean Plate. Figure modified from Feigenson et al., 2004.

 

20

 

      

Galapagos
hotspol

Farallon Plate

   

 

   

  

 

Figure 2. A reconstruction of the Central American arc during the Paleocene
and Eocene. The Farallon Plate is subducting beneath the Caribbean Plate.
Figure modified from Hauff et al., 2000. CH is the Chortis Block, CLIP is
Caribbean Large Igneous Province.

subduction, which produced volcanism in southeastern Central America. In
western Panama, on Coiba Island, Lissinna et al. (2002) indicate that subduction-
related calk-alkaline lavas (48.8 to 60.89 Ma) are present. Maury et al. (1995)
utilized petrography and geochemistry to determine that Late Eocene
volcaniclasts located in streambeds from the Turia-Chucunaque Basin in eastern
Panama were related to an early volcanic arc.

ln Costa Rica, the most common manifestations from the earlier arc occur
as igneous clasts in turbidite deposits or debris flows. Other workers have
studied volcaniclasts from sedimentary deposits to determine provenance,
magma genesis, and tectonic histories in a range of geologic settings. For
example, Dickinson (1985) found that the compositions of sandstones are
primarily the result of the tectonic settings of provenance and devised a primary
classification scheme based on point counts of thin sections. Dickinson (1985)
concluded that quartzofeldspathic sands and feldspatholithic sands originated
from arc plutons or basement uplifts, and arc volcanics, respectively. In Costa
Fiica, Calvo (2003) investigated the provenance of plutonic rock fragments in
sandstones from El Viejo and Rivas Formations. Oalvo (2003) used point counts
of thin sections to conclude that the sandstones, underlain by the Nicoya
Complex, originated partially from the erosion of this complex.

In addition, studies based on the geochemistry of volcaniclasts have also
been used to make geologic interpretations. For example, Saito (1998) used
major and trace element geochemistry of cuttings from drill cores, which included

volcaniclastic sediments and turbidites, to determine provenance and

paleoenvironmental changes in the East Greenland Margin. Using Nd-Pb
isotopic compositions of volcaniclastic sandstones preserved in turbidite
deposits, Gill et al. (1994) inferred the temporal variation in sources of volcanics
from the lzu-Bonin arc. Gill et al. (1994) also resolved changes in the arc, such
as back arc basin formation, from variations in major and trace element
geochemistry of volcaniclastic material. Schott and Johnson (1998) utilized the
geochemistry of igneous conglomerate clasts from the Cretaceous Gualala basin
in California to infer an arc related origin and paleotectonics of the region.
Lytwyn et al. (2001) investigated the geochemistry of the Late Cretaceous
turbidite deposits to infer the origin of the Sumba region in Indonesia, and found
a close proximity to an intra-oceanic island arc. In a modern example, Ballance
(1991) describes that debris and gravity flows are responsible delivering
volcaniclastic debris and turbidites to different areas of the Tonga Trench slope.
The author states that several stages of transport may be possible before
volcaniclasts arrive at the deep trench slope.

Similar to the above studies, this study characterizes the petrography and
geochemistry of volcaniclasts from three sample locations, along with more
detailed information on an area described by Patino et al. (2003) in Costa Rica to
infer subduction parameters. The volcaniclasts range in age from Late
Cretaceous to Eocene. In this thesis, these volcanics are referred to as
expressions of the “primitive arc", based on the temporal difference with the
modern arc in Central America. Geochemical compositions of the primitive arc

lavas are then compared to samples from the volcanic front from the modern arc,

concentrating on the Costa Rican and Nicaraguan segments, to determine the

subduction parameters for the primitive arc.

Sample Locations

Sample locations include Isla Paloma, Quebrada Buenaventura, and
Playa Soley which are located in Costa Rica. The outcrop at Punta Samara,
Costa Rica, studied by Patino et al. (2004), will also be included. Figure 3 shows
a map of sample locations in Costa Rica.

Patino et al. (2004) collected volcanic cobbles (Figure 4) from a turbidite
deposit located in the northern portion of Punta Samara (coordinates not
recorded). Twelve lava clasts are included in this study from Punta Samara.
According to Baumgartner et al. (1984), the volcaniclasts from Punta Samara
were deposited during the Upper Maastrichtian and the Lower-Middle Paleocene.
The mechanical weathering of lava flows most likely created the volcaniclasts.
The volcanic cobbles were part of high density submarine flows from an unstable
shallow marine platform (Baumgartner et al., 1984).

Isla Paloma is located to the west of Isla Chira in the Gulf of Nicoya, at
approximately 10.119 N and 85.209 W. According to Flores (2003), the outcrop
at Isla Paloma is part of the Cerco de Pierda Member of the Descartes
Formation. The Descartes Formation (Middle to Upper Paleocene) is a
conglomerate that contains angular to rounded clasts, and it is associated with a
carbonate platform (Rivier, 1985; Flores, 2003). Volcaniclasts within the
Formation range from basaltic to andesitic (Flores, 2003). The volcaniclasts in

the Cerco de Pierda Member are mostly andesitic, but basaltic compositions are

Costa Rica

  

 

 

Gulfo :'
de Papagayo :'
,2, :'
10 —— 5
1 .-'
Pacific Ocean G°"° 5
I de Nlcoya '-
-85

 

 

-86

Figure 3. A map of the primitive arc sample locations in Costa Rica. Sample
locations are as follows: 1. Punta Samara, 2. lsla Paloma, 3. Quebrada

Buenaventura, and 4. Play Soley.

! 4,, .--»-- "its

"Uh

. - . .‘ I -- ‘ ’I' It ‘
‘ votcamclast'.‘
. 4r. - ' ‘

 

Figure 4. Photos of volcaniclasts from the primitive arc located at Punta Samara.

also present (Flores, 2003). Thirteen lava clasts were included in this study from
Isla Paloma.

Samples were taken along a creek, Quebrada Buenaventura, which is
located at approximately 10.659 N and 85.429 W. Samples from Quebrada
Buenaventura are also from the Cerco de Pierda Member of the Descartes
Formation (Flores, 2003). The igneous clasts collected at this location included
12 lavas samples.

Playa Soley is located 11.049 N and 85.679 W. At this location both lava
and ignimbrite clasts occurred in a turbidite deposit or debris flow. The samples
from Playa Soley are inferred to be Eocene in age (Alvarado, personal
communication, 2003). Eleven ignimbrite and six lava samples were collected
from this location. Two lava clasts were collected by Kennet Flores just east of
Playa Soley at Playa Rajada. The samples from Playa Rajada are from the

Descartes Formation.

Methods

Sampling Methods

This study includes sixty samples, which consists of 14 ignimbrite and 46
lava samples. The majority of these hand samples were collected during the
2004 field season. Nine ignimbrite samples were previously collected during
October 2003 by TA. Vogel (Michigan State University). Five lava samples were
provided by Kennet Flores (Universidad de Costa Rica). Twelve of the samples

were collected during the 1999 field season by Patino et al.,(2004).

Geochemical analyses of ignimbrites were performed on fiamme
fragments from the collected hand samples. When whole fiamme fragments
could not be obtained from the ignimbrite hand samples, only thin sections were
prepared. Geochemical analyses were performed on all lava samples, except for

one, which was obviously altered and collected solely for petrographic analysis.

Sample Preparation

Hand samples were cut to extract whole fiamme fragments or the center
and, presumably the freshest portion, of the ignimbrite and lava samples,
respectively. The extracted portions were submerged in a sonic bath for
approximately 20 minutes and dried. The samples were then disaggregated
using a chipmunk and ground into a fine rock powder using a ceramic flat plate
gnnden

Fused glass disks were made for each sample using a mixture of 3.0 g of
rock powder, 9.0 g of a low temperature flux, lithium tetraborate (LigB407). and
0.5 g of an oxidizer, ammonium nitrate (NH4N03). The mixture was melted in
platinum crucibles at 1,000‘2 C over an oxidizing flame for 20 to 30 minutes while
stirred on an orbital mixing stage. The melted samples were then poured into

platinum molds to form glass disks.
Analytical Techniques

. Major element chemistry and select trace element chemistry were

obtained by X-ray fluorescence (XRF), and additional trace elements were

10

obtained through laser ablation inductively coupled plasma mass spectrometry
(LA lCP-MS). For XRF, analyses were performed on the glass disks using a
Rigaku S/Max or a Bruker S4Pioneer X-Ray fluorescent spectrograph at
Michigan State University. Major element XRF data was processed using a
fundamental parameter data reduction method (Criss, 1980), using XRFWIN or
SPECTRAPLUS software for the Rigaku S/Max and Bruker S4Pioneer,
respectively. XRF trace element data were calculated using standard linear
regression.

Trace element analyses were performed at Michigan State University
using a Cetac LSX200+ laser ablation system coupled with a Micromass Platform
lCP Mass Spectrometer. A 266 nm UV laser is used in this system. For most
samples, the laser was fired at a frequency of 10 Hz. A preablation scan rate of
150 ,um/sec was used with a spot size of 250 pm. The ablation parameters were
a scan rate of 10 pm/sec, a spot size of 200 pm, and a defocus of 50 pm. For
samples that had a high SiOz content, the laser was fired at a frequency of 20
Hz. Preablation conditions for high SI02 samples were a scan rate of 50
pm/sec, a spot size of 250 pm, and a defocus of 100 pm. The ablation
conditions for these samples were a scan rate of 10 urn/sec, a spot size of 200
pm, and a defocus of 200 pm. Strontium from the XRF analyses was used as the
internal standard to calculate LA lCP-MS results. A linear regression method
using USGS and Japanese rock standards determined the trace element

concentrations in the samples using MassLynx Software. The background

11

values are subtracted from the samples and standards prior to calculation of

concentrations.

RESULTS

Petrograph y and Mineralogy

Only the least altered samples were chosen for the study. Detailed
petrographic descriptions are included in Table 1 for samples that were included
in the geochemical portion of the study. Appendix A contains petrographic
descriptions of samples that were not included in the geochemical portion of the
study. Appendix B contains a list of the minerals present in each sample.

Petrographic analyses were perfOrmed on sixty thin sections. Grains 0.5
mm and greater were considered phenocrysts. The lava clasts from the primitive
arc at Punta Samara, Isla Paloma, Quebrada Buenaventura, and two from Playa
Soley do not vary in mineralogy among sample locations. These lavas are
comprised of 35 to 65% phenocrysts euhedral to subhedral phenocrysts of
plagioclase and pyroxene, and subhedral to anhedral opaques (Figure 5).
Sericite was present on the majority of the plagioclase phenocrysts. Many
samples contained reaction rims on the pyroxene phenocrysts (Figure 6). The
partial or complete replacement of phenocrysts by secondary minerals, including
calcite and chlorite occurred in most sections (Figure 7). The groundmass
consists of microphenocrysts and alteration products.

The remaining lava samples from Playa Soley have a small percentage of

phenocrysts of anhedral feldspar, opaques, and quartz. The

12

Table 1. Petrographic description of lava samples.

Sample
PAS

PA6

PA9

Descrlptlon

Sample PA5 is a dacite from Punta Samara. Phenocrysts
comprise 55% of the sample. Forty percent of the phenocrysts
are subhedral plagioclase and are up to 2.4 mm in length.
Phenocrysts of plagioclase exhibit sericite and some are altered to
chlorite. Subhedral pyroxene phenocrysts are up to in 0.75 mm in
size and comprise 5% of the sample. Reaction rims are present
on some of the pyroxene phenocrysts. Opaques are included
within the pyroxene grains. Less than 1% of the grains are
replaced by opaques and alteration products. Anhedral opaques
are up to 0.5 mm in size and comprise 5% of the sample. Larger
opaques appear to be secondary. The groundmass comprises
45% of the sample and consists of the phenocrysts assemblage,
cryptocrystalline material, opaques, and veins of alteration
products and possibly quartz. Cryptocrystalline. The quartz veins
may explain the difference between the chemistry and mineral
assemblage.

Sample PA6 is a basaltic andesite from Punta Samara. The
phenocrysts comprise 45% of the sample. Subhedral plagioclase
phenocrysts comprise 35% of the sample and are up to 2 mm in
size. Plagioclase phenocrysts exhibit sericite and partial alteration
to chlorite. Five percent of the sample is comprised subhedral
pyroxene grains are up to 4 mm in size and most are broken.
Anhedral opaques comprise 5% of the sample and are up to 1
mm in size. Some opaques may be secondary. The groundmass
comprises 55% of the sample. The groundmass consists of the
phenocryst assemblage and previously mentioned alteration
products. Calcite is also present in 5% of the groundmass.

Sample PA9 is a basaltic andesite from Punta Samara.
Phenocrysts comprise 55% of the section. Forty percent of the
phenocrysts are subhedral plagioclase that are up to 2 mm in
size. Plagioclase phenocrysts display sericite and are partially
altered to chlorite. Five percent of the phenocrysts consist of
subhedral pyroxene, and are up to 3.5 mm in size. The majority
of the phenocrysts are broken and reddish brown alteration
crosses the crystals. Approximately 5% of the phenocrysts are
completely altered to calcite and less than 1% to chlorite.
Anhedral opaques comprise 5% of the phenocrysts and are up to
1 mm in size. Some opaques may be secondary. The
groundmass comprises 45% of the sample. The groundmass
consists of the phenocryst assemblage and alteration products in
approximately the same proportion.

13

Table 1.

Sample
PA1 1

PA1 2

PA1 3

(confinued)

Descrlptlon

Sample PA11 is an andesite from Punta Samara. The sample
consists of 50% phenocrysts. Forty percent of the phenocrysts are
subhedral plagioclase that are up to 3.5 mm in size. Sericite is
present on the plagioclase. Ten percent of the phenocrysts are
subhedral pyroxene that are up to 3.5 mm in size. The majority of
the pyroxene phenocrysts are broken. Opaques are included in
the phenocrysts. Secondary opaques that have reaction rims of
reddish brown material are also present near phenocrysts and are
up to 1 mm in size. Less than 1% of the phenocrysts have been
completely replaced by calcite. The sample contains 50%
groundmass that is approximately 35% plagioclase microlites and
less than 1% microphenocrysts of pyroxene. Opaques comprise
approximately 15% of the groundmass. Fissures of calcite are
present in less than 1% of the section.

Sample PA12 is an andesite from Punta Samara. The section is
composed of 45% phenocrysts. Thirty-five percent of the
phenocrysts are subhedral plagioclase that are up to 2.25 mm in
length. Subhedral pyroxene are present in 10% of the sample,
and the grains are up to 3 mm in size. Alteration to chlorite is
present on a small percentage of phenocrysts. Opaques are
included in pyroxene phenocrysts. Anhedral opaques are up to 1
mm in size and display reaction rims. Less than 1% of the
phenocrysts are completely replaced by calcite. The groundmass
comprises 55% of the section. The groundmass consists of 40%
microphenocrysts is the same proportion as the phenocryst
assemblage and included 15% opaques.

Sample PA13 is a basaltic andesite from Punta Samara. The
section is comprised of 65% phenocrysts. Fifty-five percent of the
phenocrysts are subhedral plagioclase that are up to 2 mm in
size. Sericite is present on plagioclase grains. Five percent of the
sample is subhedral pyroxene phenocrysts. Altered phenocrysts
are present in 5% of the sample including phenocrysts that have
reaction rims of opaques and those that have been completely
replaced by calcite. Partial alteration to chlorite also occurs on the
phenocrysts. The groundmass comprises 35% of the section.
The groundmass is comprised of microphenocrysts and alteration
products in the same percentage as previously described.

14

Table 1.

Sample
PA1 4

PA1 5

PA1 8

(conflnued)

Descrlptlon

Sample PA14 is a basaltic andesite from Punta Samara. The
sample consists of 45% phenocrysts. Subhedral plagioclase
phenocrysts comprise 35% of the sample. Sericite is present on
the plagioclase. Alteration to chlorite is also present on the
phenocrysts. Ten percent of the phenocrysts are subhedral
pyroxene that are up to 1 mm in size. Anhedral opaques are
included in the pyroxene, and are up to 0.5 mm in size. Calcite
replaced less than 1% of the phenocrysts. The groundmass is .
55% of the section. The groundmass consists of
microphenocrysts in the same proportion as the phenocrysts and
opaques. The groundmass also contains less than 1% calcite.

Sample PA15 is a basaltic andesite from Punta Samara. The
sample consists of 35% phenocrysts. Twenty-five percent of the
phenocrysts are subhedral plagioclase that are up to 2 mm in
size. Plagioclase phenocrysts display little sericite. Subhedral
pyroxene phenocrysts are up to 2 mm in size. Reaction rims are
present on most pyroxene and secondary opaques are also
present near the grains. A small amount of alteration to chlorite is
present on some phenocrysts. The groundmass is comprised of a
similar mineralogy as the phenocryst assemblage.

Sample PA18 is a basaltic andesite from Punta Samara. The
phenocrysts comprise approximately 35% of the sample.
Subhedral plagioclase phenocrysts contribute to 35% of the
section, and are up to 1.5 mm in length. Subhedral pyroxene are
up to 1 mm in length. Approximately 1% of the phenocrysts have
been completely altered to calcite. Alteration to chlorite is also
present on phenocrysts. The groundmass consists of 30%
microphenocrysts, 20% opaques, 10% calcite and 5% chlorite.

15

Table 1.

Sample
040709-1 5

040709-1 6

(confinued)

Descrlptlon

Sample 040709-15 is an andesite from Isla Paloma. Fifty-five
percent of the sample is composed of phenocrysts. Forty-five
percent of the sample is comprised of subhedral plagioclase
phenocrysts are up to 1.6 mm in length. Approximately 1% of the
plagioclase grains have been completely replaced with calcite.
Sericite is present on the plagioclase grains as well as, partial
alteration to calcite and chlorite. The subhedral pyroxene
phenocrysts comprise 10% of the section and are up to 1.5 mm in
size. Reaction rims of opaques are present around the majority of
the pyroxene phenocrysts. Less than 1% of what appears to have
been pyroxene phenocrysts are replaced with opaques and a
reddish brown material. Clots of smaller plagioclase, pyroxene,
and replaced grains occur in the section. Five percent of the
section is anhedral opaques that are up to 1.25 mm in size. The
groundmass comprises 45% of the sample and contains 35%
cryptocrystalline material and 10% opaques.

Sample 040709-16 is a dacite from Isla Paloma. Fifty-five percent
of the sample is composed of phenocrysts. Subhedral plagioclase
phenocrysts are up to 3.5 mm in length and display sericite,
alteration to calcite and chlorite. Plagioclase phenocrysts
comprise 40% of the sample. Less than 1% of the plagioclase
phenocrysts have been completely replaced by calcite. Fifteen
percent of the sample is comprised of subhedral pyroxene
phenocrysts that are up to 2.5 mm in size. Less than 1% of what
may have been pyroxene phenocrysts has reaction rims of
opaques and appears to be replaced with a reddish brown
material. Anhedral opaques comprise 5% of the phenocrysts and
are up to 1 mm in size. Clots of smaller plagioclase, pyroxene,
opaques, and replaced grains occur in the section. The
cryptocrystalline groundmass, which composes 40% of the
sample, appears to be 30% cryptocrystalline material and 10%
opaques. Lines of reddish brown alteration cross the section in
PPL.

l6

Table 1.

Sample
040709-1 7

040709-1 8

040709-20

(confinued)

Descrlptlon

Sample 040709-17 is a basaltic andesite from Isla Paloma. Sixty-
five percent of the sample is composed of phenocrysts. Fifty
percent of the sample is subhedral plagioclase phenocrysts that
are up to 2.6 mm in length. Sericite is present on the plagioclase.
Fifteen percent of the sample is composed of pyroxene
phenocrysts and are up to 1.3 mm. The majority of pyroxene
phenocrysts have reaction rims of opaques and are almost
completely altered to chlorite. The groundmass comprises 30% of
the sample consisting of 20% cryptocrystalline material, 10%
opaques, and 5% chlorite.

Sample 040709-18 is a basaltic andesite from lsla Paloma.
Phenocrysts comprise 65% of the sample. Forty-five percent are
plagioclase phenocrysts up to 2.5 mm in size. Sericite is present
on the plagioclase, along with alteration to chlorite. Approximately
5% of the grains are completely altered to chlorite. Subhedral
pyroxene phenocrysts are up to 1.5 mm in size and comprise 10%
of the sample. Opaques. are included within the pyroxene.
Approximately 1% of the phenocrysts are anhedral opaques that
are up to 1 mm. Forty percent of the sample is comprised of the
groundmass which includes 30% cryptocrystalline material and
10% opaques.

Sample 040709-20 is an andesite from Isla Paloma. The sample
is comprised of 40% phenocrysts. Thirty percent are subhedral
plagioclase phenocrysts that are up to 1.5 mm in length. Sericite
and alteration to chlorite occurs on the plagioclase grains. Ten
percent of the sample is comprised of subhedral pyroxene
phenocrysts are up to 1 mm. Opaques are included with pyroxene
grains. The groundmass comprises 60% of the sample and
includes 25% cryptocrystalline material, 10% chlorite, 15%
plagioclase microlites, and 10% opaques.

l7

Table 1.

Sample
040709-21

040709-22

040709-24

(confinued)

Descrlptlon

Sample 040709-21 is a basaltic trachyandesite from Isla Paloma.
The phenocrysts comprise 60% of the section. Forty percent of
the phenocrysts are comprised of subhedral plagioclase
phenocrysts are up to 2 mm in size. Plagioclase grains display
sericite and alteration to chlorite. Euhedral pyroxene phenocrysts
comprise 20% of the section and are up to 2.5 mm in size.
Opaques are included in pyroxene grains. The groundmass
comprises 40% of the section. The groundmass is comprised of
15% phenocryst assemblage, 10% opaques, 10%
cryptocrystalline material, and 5% chlorite. A vein runs through the
section calcite.

Sample 040709-22 is a basaltic andesite from Isla Paloma. The
phenocrysts comprise 55% of the sample. Subhedral plagioclase
phenocrysts compose 55% of the sample and are up to 2 mm in
size. Sericite and partial alteration to chlorite and calcite is
present on the plagioclase phenocrysts. Approximately 2% of the
phenocrysts have been completely replaced by calcite. Subhedral
pyroxene phenocrysts up to 0.6 mm in size comprise less than 1%
of the sample. Secondary opaques are included in broken and
replaced phenocrysts. Forty-five percent of the sample is the
groundmass. Twenty percent of the groundmass is comprised of
the phenocryst assemblage and alteration products, 15%
cryptocrystalline material, and 10% opaques are also present.

Sample 040709-24 is an andesite from Isla Paloma. The sample
is comprised of 40% phenocrysts. Subhedral plagioclase
phenocrysts compose 20% of the section and are up to 2.8 mm in
size. Sericite is present on plagioclase grains. Chlorite appears
along mineral fissures. Subhedral pyroxene phenocrysts are up to
1.5 mm in size and are present in 10% of the section. Ten
percent of the phenocrysts appear to be secondary amphibole.
Secondary opaques are up to 1 mm in size and occur near
pyroxene grains and grains that appear to have altered to
amphibole. The groundmass is comprised of 35%
cryptocrystalline material possibly from the phenocryst
assemblage and alteration products and 25% opaques.

l8

Table 1.

Sample
040709-25

14-1 1-1201

13-11-1201

(confinued)

Descrlptlon

Sample 040709-25 is a basaltic andesite from Isla Paloma. The
sample contains 50% phenocrysts. Forty percent of the
phenocrysts are comprised of subhedral plagioclase are up to 2.3
mm in size. The plagioclase phenocrysts exhibit sericite
alteration. Ten percent of the phenocrysts are composed of
subhedral pyroxene are up to 2.5 mm in size and account for 10%
of the section. Both plagioclase and pyroxene contain inclusions
of opaques. Reaction rims are present along a few pyroxene
grains and less than 1% of the pyroxene are almost completely
replaced by secondary opaques. Alteration to chlorite is present
on phenocrysts. The groundmass consists of the 40%
microphenocrysts as in the same proportion as the phenocryst
assemblage and 10% opaques.

Sample 14-11-1201 is an andesite from Isla Paloma. The sample
consists of 40% phenocrysts. Subhedral plagioclase phenocrysts
are up to 2 mm in length. Plagioclase phenocrysts display sericite
and alteration to calcite and chlorite. Subhedral pyroxene
phenocrysts are up to 1.5 mm in size and have reaction rims of a
reddish brown material and alteration to chlorite. Opaques are up
to 0.5 mm in size and comprise 15 % of the sample. The
groundmass is 60% of the sample. The groundmass consists of
microphenocrysts and alteration products.

Sample 13-11-1201 is an andesite from Isla Paloma. The
phenocrysts comprise 35% of the sample. Twenty-five percent of
the phenocrysts are subhedral feldspar phenocrysts that are up to
2.8 mm. Plagioclases exhibit sericite and alteration to chlorite.
Subhedral pyroxene phenocrysts are up to 2.8 mm in size.
Opaques are included in pyroxene grains. Reaction rims are
present around several phenocrysts. The groundmass comprises
65% of the sample. The groundmass consists of 35% of the
phenocryst assemblage. Secondary opaques comprise 15% of
the section and are up to 0.8 mm in size. The remaining portion of
the section appears to be alteration products including calcite and
chlorite. A calcite vein runs across the section.

19

Table 1.

Sample
040709-1

040709-2

(confinued)

Descrlptlon

Sample 040709-1 is a basaltic trachyandesite from Quebrada
Buenaventura. The sample is composed of 40% phenocrysts.
Phenocrysts consist of plagioclase, pyroxene, and replaced
phenocrysts. Thirty-five percent of the phenocrysts are
plagioclase that range up to 2.5 mm. Plagioclase phenocrysts
occasionally have inclusions of pyroxene. Plagioclase
phenocrysts display sericite. Some plagioclase phenocrysts have
been partially altered to secondary products, mostly calcite and
chlorite. Other phenocrysts, approximately 5% of the sample,
have been completed replaced by a combination of secondary
products including opaques, chlorite, and calcite. Some of these
may have been olivine phenocrysts. Less than 1% of the sample
is composed of euhedral pyroxene phenocrysts that display
exsolution lamellae and are up to 0.8 mm. Opaque minerals
constitute <1 % of the phenocrysts and are up to 0.5 mm. The
sample is composed of 60% groundmass that contains 25%
opaques (probably secondary), 15% cryptocrystalline material,
10% plagioclase microlites, 5% chlorite, and 5% calcite. Fissures
of alteration products out across areas of the section.

Sample 040709-2 is an andesite from Quebrada Buenaventura.
This sample contains 65% of generally fine grained phenocrysts.
Forty-five percent are subhedral plagioclase phenocrysts that are
up to 1.25 mm long, with the majority near 1 mm long, and exhibit
sericite. The plagioclase phenocrysts appear to have reaction
rims along some edges (looks like resorbing). Ten percent of the
sample is subhedral pyroxene up to 1 mm in length. There are
reaction rims around pyroxene grains. Opaques are included in
the plagioclase and pyroxene grains. Five percent of the
phenocrysts are completely replaced with calcite and/or chlorite
and also have reddish brown alteration. The groundmass
comprises 35% of the sample and includes 10% of the phenocryst
assemblage, 5% cryptocrystalline material, 5% opaques, 5%
chlorite, 5% calcite, and <1%reddish brown alteration.

20

Table 1.

Sample
040709-3

040709-5

(conflnued)

Descrlptlon

Sample 040709-3 is a basaltic trachyandesite from Quebrada
Buenaventura. This sample is composed of 65% phenocrysts.
This sample is composed of 60 % subhedral plagioclase up to 2.3
mm in length, along with fragments of grains that may have been
larger. Sericite is present in the plagioclase grains. Five percent
of the sample is composed of subhedral pyroxene phenocrysts
that are up to 1.3 mm long. Opaques that are probably secondary
are up to 0.5 mm. Reaction rims are present around pyroxene
grains and an abundance of opaques. Calcite and chlorite occur
on grains and calcite has replaced <1 % of the grains. The sample
is composed of 35% groundmass, which includes 20% smaller
crystals of the phenocryst assemblage, 5% secondary opaques,
5% calcite, and 5% chlorite.

Sample 040709-5 is an andesite from Quebrada Buenaventura.
Forty percent of the sample is composed of phenocrysts.
Subhedral plagioclase phenocrysts are up to 1.25 mm in length,
and comprise 30% of the sample. Sericite is present on the
plagioclase grains. Fifteen percent of the sample is composed of
subhedral pyroxene phenocrysts that are up to 0.625 mm in size.
Reaction rims of opaques and reddish brown material surround
the pyroxene phenocrysts. The opaques appear to be secondary
and comprise. The groundmass is composed of
microphenocrysts of 40% plagioclase, 15% opaques, 5%
cryptocrystalline material and <1 % calcite and chlorite.

21

Table 1.

Sample
040709-6

040709-9

(confinued)

Descrlptlon

This sample is a trachyandesite from Quebrada Buenaventura
and is composed of 35% phenocrysts. Twenty-five percent of the
phenocrysts are subhedral plagioclase that range from 0.75 to 5
mm in length. Sericite, along with alteration to calcite and chlorite
is present in the plagioclase phenocrysts. Euhedral pyroxene
phenocrysts range from 0.375 to 1 mm in size and comprise 5 %
of the section. Approximately 5% of the phenocrysts are
completely replaced calcite and chlorite replaced grains.
Phenocrysts appear to be partially and completely replaced by
calcite and chlorite. Less than 1% of the sample is composed of
anhedral opaque phenocrysts, which appear to be secondary and
occur near pyroxene phenocrysts. These are up to 0.5 mm in
size. The groundmass comprises 65% of the sample. Veins of
predominately calcite traverse the section. Secondary opaques
are present. The groundmass is composed of 20%
cryptocrystalline material, 20% is possibly remnants of plagioclase
microlites, 10% opaques, and 10% calcite and 5% chlorite.
Reddish brown alteration is less than 1% of the sample.

This is a basaltic trachyandesite from Quebrada Buenaventura.
Fifty-five percent of the sample is comprised of phenocrysts.
Subhedral plagioclase phenocrysts comprise 40% of the sample
and are up to 2.25 mm in length. Sericite along with alteration to
calcite and chlorite is present on the plagioclase phenocrysts.
Less than 1% of the plagioclase grains are completely replaced by
calcite. Subhedral pyroxene phenocrysts comprise 10% of the
sample and are up to 2 mm in length. Opaques are included in
the both the plagioclase and pyroxene phenocrysts. Five percent
of the sample is opaque anhedral phenocrysts that are up to 0.25
mm in size. The groundmass comprises 60 % of the sample and
includes microphenocrysts in approximately the same proportion
as the phenocrysts and also includes chlorite and calcite.

22

Table 1.

Sample
040709-1 0

040709-1 3

040709-1 4

(confinued)

Descrlptlon

Sample 040709-10 is a basaltic trachyandesite from Quebrada
Buenaventura. Sixty-five percent of the sample is comprised of
phenocrysts. Subhedral plagioclase phenocrysts are up to 2.8 mm
and compose 50 % of the sample. Sericite is present on the
plagioclase phenocrysts. Plagioclase phenocrysts are partially
replaced by calcite and chlorite. Fifteen percent of the sample is
comprised of anhedral pyroxene phenocrysts that are up to 2.9
mm in size. Reaction rims of opaques occur along the pyroxene
phenocrysts. Plagioclase are included in pyroxene phenocrysts.
Anhedral opaque phenocrysts are up to 0.5 mm in size. Some
grains are replaced by calcite. The groundmass is composed of
plagioclase, pyroxene, opaques, and less than 1% calcite and
chlorite.

Sample 040709-13 is an andesite from Quebrada Buenaventura.
The sample contains 54% subhedral plagioclase phenocrysts that
are up to 2 mm in size. Chlorite and sericite is present on
plagioclase grains. Calcite replacement of grains occurs in 1% of
the sample. Less than 1% of sample may be subhedral pyroxene
phenocrysts up to 1.5 mm in size. The sample contains 45%
groundmass, which is comprised of the same minerals and
alteration products near the same proportion as in the
phenocrysts, and includes a small amount of opaques.

Sample 040709-14 is an andesite from Quebrada Buenaventura.
Approximately 15% of the sample is comprised of phenocrysts.
The phenocrysts display several intramineral fissures. Ten
percent of the sample is subhedral feldspar phenocrysts are up to
2 mm in size. The plagioclase display sericite. Five percent of
the phenocrysts are composed of what appears to have been
euhedral pyroxene phenocrysts, up to 0.75 mm in size, that are
altered to a reddish brown material and reaction rims of opaques.
Less than 1% of the phenocrysts are subhedral pyroxene up to 2
mm in size. Less than 1% of the phenocrysts are anhedral quartz
up to 1 mm in size. The groundmass is comprised of
cryptocrystalline material and opaques. The groundmass
comprises 85% of the sample. In areas the groundmass appears
to be the remnants of grains.

23

Table 1.

Sample
040710-16

04071 0-1 9

(confinued)

Descrlptlon

Sample 040710-16 is a dacite from Playa Soley. The phenocrysts
comprise 40% of the section and are subhedral plagioclase
phenocrysts that are up to 1.25 mm in length. The plagioclase
phenocrysts are partially altered to chlorite. There appears to be
reaction rims near the boundaries of some plagioclase
phenocrysts. Less than 1% of the phenocrysts have been
completely altered to calcite. The groundmass comprises 65% of
the sample and consists of 40% cryptocrystalline material, 15%
secondary opaques, 5% chlorite, and 5% areas of reddish brown
alteration. Secondary opaques are up to 0.35 mm in size.
Fissures cut across the section and may be filled with quartz,
which may account for the difference between mineralogy and
chemistry.

Sample 040709-19 is defined by chemistry as a trachyandesite
from Playa Soley. Approximately 50% of the sample consists of
phenocrysts. Thirty-five percent of the section is altered
subhedral plagioclase phenocrysts that are up to 1.75 mm in
length. Approximately 1% of the phenocrysts have been
completely replaced by calcite. Partial replacement of calcite is
present on several grains. The majority of plagioclase
phenocrysts have reddish brown alteration and inclusions of
several alteration products. What may have been pyroxene
phenocrysts, inferred from the grain boundaries, have been
completed replaced alteration products and display reaction rims
of opaques and reddish brown material. Other areas of the section
consist of clusters of secondary opaques. The groundmass is
approximately 40% of the section. This consists of secondary
opaques, chlorite, and various alteration products. Fissures of
calcite cross the section.

24

 

Figure 5. Photomicrograph of a lava sample, 040709-15, from
Quebrada Buenaventura, in plane polarized light (top) and crossed
polarized light (bottom). Plagioclase, pyroxene, and opaques form the
phenocryst assemblage in lava samples from Punta Samara, Isla
Paloma, and Quebrada Buenaventura, and 2 lava samples from Playa
Soley (040710-16 and 040709-19). Plag is plagioclase, Pyx is
pyroxene, Opq is opaque, and Cal is calcite.

25

 

Figure 6. Photomicrograph of lava sample, 040709-2, from
Quebrada Buenaventura, in plane polarized light (top), and
cross polarized light (bottom). Reaction rims are present on
some pyroxene grains. -

26

 

Figure 7. Photomicrograph of a lava sample, PA10, from Punta
Samara, in plane polarized light (top) and crossed polarized light
(bottom). Complete or partial replacement of grains by secondary
products was present in some samples. Plag is plagioclase, Opq is
opaque, Cal is calcite, Chl is chlorite, and repl is completely replaced

grain.

27

 

Figure 8. Photomicrograph of a fiamme sample, 031024-4a, from
Playa Soley, in plane polarized light (top) and crossed polarized light
(bottom). Anhedral feldspar and quartz form the phenocryst
assemblage in fiamme samples from Playa Soley, and opaques
appear to be secondary. Plag is plagioclase, Repl is completely
replaced grain.

28

groundmass is comprised of microcrystalline material. The fiamme samples from
Playa Soley generally contain a small percentage of anhedral phenocrysts of
feldspar and quartz (Figure 9). The groundmass consists of microcrystalline

material, which is most likely altered glass.

Alteration

When using geochemical data, particularly samples that have undergone
mechanical weathering, the degree of chemical alteration must be considered.
Low totals from major element analyses, the Weathering Index of Parker (W IP),
and an evaluation of the rare earth element (REE) pattern were used to identify
altered samples.

Low totals from major element analyses may have occurred due improper
sample preparation or due to alteration of the samples. The presence of clay
minerals in altered samples results in lower sample totals as volatiles from these
minerals escape during the fusing process. Samples that had totals below 96%
were discarded from the rest of the geochemical analysis.

The Weathering Index of Parker (W IP) is based on the fraction of
potassium, sodium, calcium, and magnesium present in the sample (Parker,
1970)

WIP = [(2 x Na20/0.35) + (MgO/O.9) + (2K20/O.25) + (CaO/O.7)] x 100
These elements are generally the most mobile of the major elements. The
mobility of the individual alkali and alkaline earth elements is accounted for using

the bond strength of the element to oxygen (Parker, 1970). Following Patino et

29

al. (2004), samples that had a WIP< 60 were discarded from the study. This
value was chosen because samples with a WIP above 60 are comparable to the
majority of the WIP values for the modern arc samples that are assumed to
represent unweathered samples (Figure 9). Fifteen samples were excluded from
the geochemical portion of the study using the WlP, including 7 ignimbrites and 8
lavas. The majority of the samples discarded using WIP were from Playa Soley.
These samples have 78 to 84 wt% $102, which also suggested alteration had
occurred.

Even when alteration is not revealed by major element geochemistry,
trace-element geochemistry might. Patino et al. (2003) studied weathering of
corestone-shell complexes of basalt and andesites from Hawaii and Guatemala.
Although the major element chemistry and petrographic analysis did not show
signs of alteration, trace element patterns from the core and shell complexes
indicated alteration when compared to other lavas from the same locations.
Unusual REE patterns were observed in the remaining four ignimbrite samples
from the primitive arc (Figure 10). For example, samples 031024-4a and
031024-4h, showed a slight negative Ce anomaly and an elevated REE pattern
when compared to the rest of the sample set, and when compared to REE
patterns from samples from the modern arc in Costa Rica. Patino et al. (2003)
observed similar patterns in corestones, when compared to fresh lavas from their
study area, and determined that this was due to alteration. The four ignimbrite

samples with unusual REE patterns were excluded in the analysis.

30

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32

Geochemistry

Whole rock major and trace element data for the lava samples from the
primitive arc are listed in Table 2. All major element values discussed and in
plots have been normalized to 100%. The total iron is presented as F9203. The
samples from the primitive arc in Costa Rica range from basaltic to dacitic in
composition (50. 67 to 66.30 wt% SIOz); some samples have higher alkali
contents and are basaltic trachyandesite to trachyandesite in composition (Figure
11). The majority of the alkaline lava samples are from Quebrada Buenaventura.

The major element variations for samples from various locations are
presented in Figure 12. M90 in is relatively low, and ranges between 0.45 to
3.65 wt% for the samples from the primitive arc. The lava samples from the
primitive arc have Al203 and NaZO values between 15.45 and 21.17 wt% and
2.23 and 6.29 wt%, respectively. The samples from Punta Samara have the
most similar AI203 (18.19 to 19.75 wt%) and Na20 (2.70 to 4.26 wt%)
compositions from the primitive arc. The values for K20 vary between 0.50 to
2.54 wt% for the primitive arc volcaniclasts, and are more similar in Punta
Samara (0.53 to 1.60 wt%), second only to Playa Soley (1.68 to 2.16 wt%), for
which there are only two samples.

Trace-element concentrations for the samples are shown in Table 2.
Figure 13 displays trace elements versus SiOz. Lavas from Punta Samara have
a smaller range of values than the other sample locations for Nb, Ta, and Zr.
The samples from the primitive arc are enriched in large ion lithophile elements

(LILE) and depleted in high field strength elements (HFSE), which is

33

Table 2. Major (wt%) and trace element (ppm) concentrations for primitive arc
volcaniclasts. Samples not collected in the 2004 field season originated from (It)
Kennet Flores (and location is in the Costa Rican coordinate system), (+) Patino
and others (2004), and (=) T.A. Vogel.

 

Sample PA 040709-1 PA 040709-2 PA 040709-3 PA 040709-4
Latitude (N) 10.65 10.65 10.65 10.65
Longitude (W) 85.42 85.42 85.42 85.42
Location name Quebrada Quebrada Quebrada Quebrada
Buenaventura Buenaventura Buenaventura Buenaventura
Sample type lava lava lava lava
sro, 53.30 57.41 52.66 32.43
TlO, 0.93 0.84 0.97 0.68
N203 19.75 16.54 18.21 13.62
F9203 7.28 8.15 8.28 7.51
MnO 0.15 0.11 0.15 0.34
M90 2.39 2.1 1 2.94 2.97
CaO 7.36 8.12 8.50 22.67
Nazo 4.87 3.28 4.38 2.05
K20 1 .60 1 .44 0.99 0.13
P205 0.26 0.20 0.24 0.16
Totals 97.89 98.20 97.32 82.56
Cu (XRF) 35.00 42.00 41.00 64.00
Zn (XRF) 53.00 36.00 71.00 65.00
Rb (XRF) 23.00 27.00 13.00 0.00
Sr (XRF) 461.00 342.00 357.00 300.00
Zr (XRF) 88.00 75.00 86.00 33.00
V 194.99 199.53 262.55 197.67
Cr 9.20 8.20 7.18 20.37
V 23.88 22.03 31.54 17.58
Nb 3.27 1.88 2.08 1.32
Ba 573.55 454.08 535.30 129.99
La ‘ 9.18 5.99 7.32 5.99
Ce 19.20 12.97 16.66 9.82
Pr 3.07 2.09 2.77 1.60
Nd 14.52 10.25 13.68 8.00
Sm 4.05 3.15 4.04 2.44
Eu 1.25 1.08 1.32 0.80
Gd 4.16 3.46 4.65 2.69
Tb 0.65 0.57 0.76 0.42
Dy 3.81 3.54 4.86 2.72
Ho 0.85 0.81 1 .1 1 0.60
Er 2.38 2.33 3.16 1.81
Yb 2.16 2.19 2.97 1.63
Lu 0.33 0.36 0.46 0.27
Hf 2.70 2.34 2.70 1 .28
Ta 0.29 0.21 0.23 0.18
Pb 2.69 1.38 1.22 1.00
Th 1 .21 0.52 0.59 0.30
U 0.59 0.16 0.20 0.18

34

Table 2. continued

 

Sample PA 040709-5 PA 040709-6 PA 040709-9 PA 040709-10
Latitude (N) 10.65 10.65 10.65 10.65
Longitude (W) 85.42 85.42 85.42 85.42
Location name Quebrada Quebrada Quebrada Quebrada
Buenaventura Buenaventura Buenaventura Buenaventura
Sample type lava lava lava lava
SID, 59.62 56.76 51 .47 52.04
TlOz 0.62 0.75 0.82 0.89
Arzo, 17.33 18.52 18.69 20.67
F4520, 7.78 5.1 1 7.32 6.46
MnO 0.10 0.08 0.12 0.17
M90 1.71 2.27 2.89 1.74
CaO 6.30 6.96 8.38 9.82
Naao 3.81 4.69 4.91 4.49
K20 1.03 2.34 1.64 1.16
P205 0.14 0.22 0.25 0.22
Totals 98.44 97.70 96.49 97.66
Cu (XRF) 97.00 103.00 56.00 87.00
Zn (XRF) 33.00 34.00 67. 00 49.00
Rb (XRF) 20.00 26.00 15. 00 17.00
Sr (XRF) 402.00 362.00 399. 00 401.00
Zr (XRF) 53.00 131.00 86. 00 62.00
V 571.46 220.93 180. 20 258.99
Cr 8. 84 59.75 8.18 5.13
Y 9. 74 19. 06 18.58 23.07
Nb 1.38 4. 05 2. 84 1.54
Ba 489.33 762. 20 623. 44 581.21
La 4.13 11.57 8. 50 5.50
Ce 8.38 25.42 18.02 12.55
Pr 1.20 3. 58 2. 82 2.09
Nd 5.66 15.39 12.96 10.33
Sm 1.97 3. 83 3.30 3.38
Eu 0.76 1.02 1.08 1.13
Gd 1.89 3.59 3.38 3. 63
Tb 0.30 0.52 0.51 0.60
Dy 1 .88 3.19 3.06 3.80
Ho 0.41 0. 63 0.65 0.83
Er 1.29 1 7.6 1.86 2. 35
Yb 1.23 1.58 1.72 2.17
Lu 0.21 0.26 0.27 0.34
Hf 1.80 2.98 2.37 2.01
Ta 0.19 0.36 0.22 0.21
Pb 1.33 2. 31 1.93 1.04
Th 0. 51 1.94 1.20 0. 46
U 0. 24 0.88 0.38 0. 20

35

Table 2. continued

 

Sample PA 040709-12 PA 040709-13 PA 040709-14 17-5-1201
#
Latitude (N) 10.65 10.65 10.65 242.75
Longitude (W) 85.42 85.42 85.42 410.35
Location name Quebrada Quebrada Quebrada Quebrada
Buenaventura Buenaventura Buenaventura Buenaventura
Sample type lava lava lava lava
8.02 49.56 56.30 61 .88 48.96
1102 0.78 0.75 0.69 0.78
Aleg 17.74 18.92 16.34 17.28
F0203 5.60 5.51 5.75 5.73
MnO 0.21 0.17 0.08 0.22
M90 1 .66 1 .80 2.60 1 .76
CaO 12.33 8.56 4.42 12.51
Naao 5.40 4.37 5.00 5.28
K20 0.90 0.75 1 .38 0.90
P205 0.22 0.20 0.18 0.22
Totals 94.40 97.33 98.32 93.64
Cu (XRF) 28.00 46.00 89.00 26.00
Zn (XRF) 67.00 64.00 54.00 67.00
Rb (XRF) 1 1 .00 10.00 8.00 1 1.00
Sr (XRF) 366.00 381 .00 371 .00 357.00
Zr (XRF) 84.00 70.00 138.00 82.00
V 189.54 200.54 164.65 182.53
Cr 9.59 7.46 52.97 1 1.01
Y 19.74 20.01 19.18 20.59
Nb 1.95 1.13 4.48 2.1 1
Ba 479.84 575.23 91 1 .73 454.13
La 7.04 4.97 1 1.87 7.24
Ce 16.1 1 12.61 24.59 15.96
Pr 2.49 1 .97 3.63 2.53
Nd 11.76 9.41 15.71 12.05
Sm 3.16 2.86 4.25 3.54
Eu 1.04 0.99 1.17 1.14
Gd 3.18 2.88 3.95 3.51
Tb 0.49 0.47 0.60 0.54
Dy 3.20 3.38 3.72 3.49
Ho 0.66 0.68 0.76 0.73
Er 1.87 1.92 2.21 2.12
Yb 1.71 1.78 2.22 2.04
Lu 0.28 0.30 0.37 0.34
Hf 2.06 1.72 3.93 2.31
Ta 0.19 0.17 0.50 0.26
Pb 1 .80 0.80 4.23 1 .17
Th 0.74 0.41 2.96 0.78
U 0.35 0.21 1 .02 0.46

36

Table 2. continued

 

Sample PA 040709-15 PA 040709-16 PA 040709-17 PA 040709-18
Latltude (N) 10.11 10.11 10.11 10.11
Longitude (W) 85.20 85.20 85.20 85.20
Location name Isla Isla Isla Isla
Paloma Paloma Paloma Paloma
Sample type lava lava lava lava
SID, 59.77 62.29 55.82 55.39
TIC, 0.66 0.71 0.80 0.63
A120, 15.24 15.04 18.07 17.51
F920, 4.47 5.03 8.65 8.50
MnO 0.10 0.09 0.10 0.18
M90 0.87 1 .30 2.72 3.59
CaO 9.70 7.31 7.48 7.61
N820 3.56 3.75 3.82 2.20
K20 1.77 1.68 1.02 2.50
P305 0.17 0.17 0.19 0.33
Totals 96.31 97.37 98.67 98.44
Cu (XRF) 22.00 47. 00 59. 00 166.00
Zn (XRF) 24.00 50. 00 55. 00 71.00
Rb (XRF) 24.00 24. 00 19.00 35.00
Sr (XRF) 332. 00 316. 00 376. 00 586.00
Zr (XRF) 132. 00 140.00 62.00 55.00
V 169. 76 175.24 168. 37 173.75
Cr 14.65 11.97 5. 63 5.33
Y 24.91 27.18 22.76 20.31
Nb 2.43 3.02 2. 02 3.73
Ba 671.89 659.33 397. 52 849.53
La 9.48 9.44 6. 34 12.22
Co 22.97 22.14 13.37 21.81
Pr 3. 34 3. 34 2. 30 3. 03
Nd 14.48 14.96 11.14 13.11
Sm 4.06 4.29 3.04 3.14
Eu 1.12 1.13 1 .05 1.00
Gd 3.88 4.13 3. 45 3. 36
Tb 0.62 0.65 0.53 0. 49
Dy 4.33 4.45 3.69 3.26
Ho (1. 82 0.87 0.76 0.67
Er 2. 29 2.47 2.08 1.92
Yb 2.16 2.33 1.96 1.87
Lu 0.37 0.38 0.33 0.31
Hf 2.96 3.38 1.83 1.66
Ta 0. 34 0.33 0.18 0.28
Pb 2. 63 2. 89 1 .02 2.99
Th 1.55 1.70 0.71 2.14
U 0.82 0.84 0.25 0.54

37

Table 2. continued

 

Sample PA 040709-19 PA 040709-20 PA 040709-21 PA 040709-22
Latltude (N) 10.11 10.11 10.11 10.11
Longitude (W) 85.20 85.20 85.20 85.20
Location name Isla Isla Isla Isla
Paloma Paloma Paloma Paloma
Sample type lava lava lava lava
SID, 50.26 58.48 52.83 55.50
TIC, 0.85 0.71 0.84 0.80
N203 17.20 18.21 18.99 18.62
F6203 6.74 7.29 7.03 6.16
MnO 0.25 0.15 0.16 0.25
MgO 1.96 2.97 2.14 1 .78
0110 11.72 7.48 10.17 10.03
Nago 4.74 3.46 4.52 3.72
K20 0.84 0.53 0.80 0.63
P205 0.22 0.19 0.23 0.20
Totals 94.78 99.47 97.71 97.69
Cu (XRF) 34.00 39.00 25.00 46.00
Zn (XRF) 65.00 64.00 79.00 70.00
Rb (XRF) 11.00 7.00 9.00 9.00
Sr (XRF) 336.00 398.00 426.00 393.00
Zr (XRF) 79.00 63.00 76.00 68.00
V 219.43 225.33 202.87 201.06
Cr 8.63 21 .20 9.49 5.91
V 25.52 19.62 20.09 21 .83
Nb 1.80 1.44 2.16 1.38
Ba 456.62 486.87 459.56 489.90
La 6.09 4.70 6.67 5.32
Ga 13.91 10.92 15.09 12.03
Pr 2.28 1 .83 2.59 1 .95
Nd 11.17 9.04 12.05 9.56
Sm 3.17 2.87 3.78 2.70
Eu 1.06 0.97 1.09 0.96
Gd 3.46 3.06 3.35 3.02
Tb 0.56 0.50 0.53 0.48
Dy 3.98 3.17 3.37 3.42
Ho 0.81 0.69 0.70 0.73
Er 2.27 2.04 2.01 2.02
Yb 2.11 1.91 1.85 1.86
Lu 0.35 0.33 0.30 0.31
Ht 1 .95 2.03 2.20 1 .77
Ta 0.16 0.20 0.22 0.15
Pb 0.47 1 .35 1.28 0.76
Th 0.40 0.46 0.65 0.39
U 0.16 0.19 0.30 0.17

38

Table 2. continued

 

Sample PA 040709-24 PA 040709-25 PA 040709-26 14-11-1201
It

Latitude (N) 10.11 10.11 10.11 241.7
Longltude (W) 85.20 85.20 85.20 339.8
Location name Isla Isla Isla Isla

Paloma Paloma Paloma Paloma
Sample type lava lava lava lava
SID, 57.26 54.84 56.80 58.35
TIC, 0.86 0.75 0.59 0.73
AlgO; 16.42 19.00 19.14 18.48
F920, 8.73 8.75 6.77 5.99
MnO 0.21 0.21 0.16 0.18
MgO 3.58 3.50 2.09 1.88
CaO 6.94 8.79 9.17 8.25
Nazo 2.98 3.06 2.85 3.57
K20 0.96 0.50 0.57 0.55
9205

0.16 0.16 0.17 0.20

Totals 98.10 99.56 98.31 98.18
Cu (XRF) 73.00 36.00 46.00 22.00
Zn (XRF) 106.00 72.00 66.00 65.00
Rb (XRF) 12.00 6.00 8.00 9.00
Sr (XRF) 304.00 404.00 405.00 416.00
Zr (XRF) 96.00 45.00 33.00 74.00
V 226.30 277.38 185.19 177.18
Cr 5.78 8.04 4.64 7.30
Y 28.80 18.45 15.41 23.83
Nb 6.1 1 0.84 0.68 1.33
Ba 585.06 413.32 465.92 510.39
La 8.25 3.70 3.81 5.57
Ce 17.53 8.67 8.56 12.71
Pr 2.66 1.48 1 .39 2.07
Nd 12.31 7.44 6.92 10.17
Sm 3.37 2.18 2.09 2.86
Eu 0.97 0.81 0.74 0.98
Gd 3.82 2.50 2.25 3.25
Tb 0.62 0.41 0.34 0.51
Dy 4.12 2.82 2.42 3.65
Ho 0.92 0.61 0.50 0.75
Er 2.62 1.70 1.44 2.13
Yb 2.44 1.60 1.30 1.94
Lu 0.38 0.27 0.23 0.34
Hf 2.55 1.24 1 .03 1 .96
Ta 0.41 0.10 0.10 0.15
Pb 2.64 0.39 0.56 0.72
Th 1 .30 0.27 0.30 0.46
U 0.41 0.10 0.12 0.22

39

Table 2. continued

 

Sample 13-11-1201 PA 040710-12 PA 040710-14 PA 040710-15
It
Latltude (N) 241.7 11.04 11.04 11.04
Longitude (W) 339.8 85.67 85.67 85.67
Location name Isla Playa Playa Playa
Paloma Soley Soley Soley
Sample type lava lava lava lava
SID: 56.29 83.22 82.39 79.97
TIC, 0.79 0.26 0.33 0.33
Ar,o, 18.10 9.54 10.59 10.39
F0203 7.00 0.35 0.48 0.80
MnO 0.15 0.01 0.01 0.01
MgO 2.26 0.00 0.04 0.14
CaO 9.34 1.00 1.47 1 .35
Nazo 3.1 1 3.12 3.21 3.20
K20 0.56 1.82 1.97 2.08
P205 0.17 0.05 0. 08 0.06
Totals 97.77 99.37 100. 57 98.33
Cu (XRF) 14.00 bd 1 1.00 bd
Zn (XRF) 50.00 bd 32.00 bd
Rb (XRF) 10.00 23.90 30. 00 31.40
Sr (XRF) 327.00 90. 30 120. 00 116.00
Zr (XRF) 63.00 142. 50 140. 00 146.10
V 190.19 35.16 47. 49 74.64
Cr 6.05 28.18 13.44 18.40
Y 22.37 24.55 26. 90 26.71
Nb 1.26 4. 45 2. 32 4.05
Ba 386.50 560.36 595.64 668.53
La 4.54 8. 27 9.10 6.68
Ga 10.59 19.17 20. 21 13.92
Pr 1.75 2.83 3. 00 1.94
Nd 8.77 10.06 13.30 5.99
Sm 2.63 2. 91 3.68 1.99
Eu 0.98 0. 97 0. 98 1.02
Gd 3.10 2.93 3. 78 2.15
Tb 0.49 0.61 0. 62 0.54
Dy 3.59 3.75 5.98 3.36
Ho 0.72 0.91 0.94 0. 93
Er 2.02 2. 78 2. 70 2. 92
Yb 1 .80 2. 89 2. 72 2. 90
Lu 0. 31 0.57 0.54 0.59
Ht 1 .70 3. 68 3. 81 4.05
Ta 0.15 1.13 0. 69 1 .04
Pb 0. 27 8. 29 1 .72 10.08
Th 0. 34 2.19 2. 00 2. 22
U 0.13 3. 21 0. 98 3. 49

4O

Table 2. continued

 

Sample PA 040710-16 PA 040710-17 PA 040710-19 PA 040710-20
Latitude (N) 11.04 11.04 11.04 11.04
Longltude (W) 85.67 85.67 85.67 85.67
Location name Playa Playa Playa Playa
Soley Soley Soley Soley
Sample type lava lava lava ignimbrite
5'02 65.37 80.1 1 55.35 80.82
TIC, 0.72 0.32 0.63 0.27
Alto, 16.48 10.62 17.63 1 1 .08
F920, 4.48 0.49 7.80 0.50
MnO 0.03 0.01 0.13 0.01
MgO 0.44 0.00 0.74 0.08
CaO 4.58 1.38 6.55 1.54
N820 4.15 3.34 6.09 3.73
K20 2.13 1.97 1.63 1.82
P205 0. 21 0.06 0.22 0.06
Totals 98. 59 98.30 96.77 99.91
Cu (XRF) bd bd bd 10.00
Zn (XRF) bd bd bd 9.00
Rb (XRF) 44.50 22.10 21.90 22. 00
Sr (XRF) 469. 70 119.50 310.70 117. 00
Zr (XRF) 149. 70 140.20 58.90 108. 00
V 189.78 36.93 118.01 21.60
Cr 39.33 10.77 8.93 8.19
Y 17.59 28.60 16.64 21.55
Nb 4. 69 2. 95 1.19 1 .80
Ba 723.15 618. 59 552. 36 431.72
La 13.33 8. 98 4.01 5.14
Ga 28. 78 20. 32 8.89 12.37
Pr 4.02 2. 97 1.40 1 .80
Nd 17. 08 12.79 7.05 8.13
Sm 3.83 3.60 2.13 2. 63
Eu 1.16 0.95 0.80 0. 75
Gd 3. 46 3.58 2.14 2.80
Tb 0. 53 0. 66 0.38 0.50
Dy 3.10 4. 41 2.44 5.09
Ho 0.62 1.01 0.52 0.74
Er 1.78 3.12 1.54 2.26
Yb 1 .82 3.07 1 .40 2.17
Lu 0.29 0.59 0.23 0.48
Hf 3.94 4.10 1.69 2.98
Ta 0.47 0.80 0.12 0.51
Pb 5.44 8.05 1 .22 0.32
Th 2.30 2. 35 0.19 1 .60
U 1.17 1 .78 0. 20 1.02

41

Table 2. continued

 

Sample PA 040710-23 031024-4a 031024-4b 031024-4c
Latitude (N) 11.04 11.04 11.04 11.04
Longltude (W) 85.67 85.67 85.67 85.67
Locatlon name Playa Playa Playa Playa
Soley Soley Soley Soley
Sample type ignimbrite ignimbrite ignimbrite ignimbrite
SID: 81 .01 75.64 74.53 77.73
TlOz 0.24 0.55 0.61 0.35
Argo, 10.93 12.62 13.40 12.18
F0203 0.47 1 .74 1 .33 0.91
MnO 0.01 0.02 0.01 0.01
MgO 0.04 0.26 0.29 0.09
CaO 1 .42 1 .44 2.00 1 .86
who 3.73 4.71 5.13 4.08
K20 1.66 1.72 1.16 1.73
P205 0.06 0.08 0.14 0.07
Totals 99.57 98.78 98.60 99.01
Cu (XRF) 19.00
Zn (XRF) 13.00
Rb (XRF) 20.00 30.00 14.30 15.90
Sr (XRF) 1 18.00 170.90 183.70 149.20
Zr (XRF) 100.00 170.70 99.50 1 1 1 .60
V 23.36 48.12 67.45 37.49
Cr 7.78 9.77 14.81 14.77
Y 24.66 129.79 27.85 42.07
Nb 1 .31 3.80 2.78 2.36
Ba 423.97 503.57 389 48 545.84
La 5.18 25.93 6.76 10.62
Ce 11.78 51.33 16.29 17.92
Pr 1 89 9.05 2.74 3 35
Nd 8 61 43.18 12.97 14 66
Sm 2 75 10.87 4 03 4 02
Eu 0 73 3.38 1 24 1 25
Gd 3 15 12.97 3 96 4 48
Tb 0 54 2.35 0 70 0 84
Dy 5 30 15.96 4 47 5 82
Ho 0 79 3.68 0 94 1 35
Er 2 40 11.04 2 82 4 16
Yb 2 12 10.13 2 60 4 02
Lu 0 49 1.61 0 48 0 73
Ht 2 89 5.01 3 13 3 62
Ta 0 50 0.60 0.48 0 56
Pb 0 17 7.49 5.14 4 49
Th 1.57 1.80 1.35 1.99
U 0.91 1.37 1.37 1.63

42

Table 2. continued

 

Sample 031024-4d 031024-4e 031024-4f 031°24’4L
Latitude (N) 1 1 .04 1 1 .04 1 1 .04 1 1 .04
Longltude (W) 85.67 85.67 85.67 85.67
Locatlon name Playa Playa Playa Playa
Soley Soley Soley Soley
Sample type ignimbrite ignimbrite ignimbrite ignimbrite
IO, 79.73 78.12 80.70 77.1 1
TIC, 0.35 0.40 0.32 0.36
Al203 1 1.13 11.84 10.32 12.63
F1320, 0.52 0.71 0.47 0.57
MnO 0.01 0.01 0.01 0.01
M90 0.00 0.1 1 0.00 0.00
CaO 1.68 1.10 1.40 2.09
Nago 3.58 4.20 3.47 4.51
K20 1.81 2.36 1.77 1.39
P205 0.07 0.01 0.06 0.12
Totals 98.88 98.86 98.52 98.79
Cu (XRF)
Zn (XRF)
Rb (XRF) 26.70 35.40 24.50 11.00
Sr (XRF) 146.70 145.00 133. 60 156.40
Zr (XRF) 135.50 186.90 133.10 107.80
V 64.68 53.15 29. 55 35.24
Cr 15.61 13.34 9. 06 17.54
Y 30.68 23.84 30. 49 26.51
Nb 3.11 4. 22 2. 54 2.27
Ba 677.95 557.53 631.66 470.43
La 8.55 8. 83 9. 48 7.78
Ce 17.64 14.67 21.01 17.55
Pr 2. 54 2.08 3. 02 3. 06
Nd 10.19 7.27 13.14 14.07
Sm 2.82 2.05 3.65 4.09
Eu 1 .03 1 .50 0.95 1.22
Gd 3.00 1 .94 3.69 4.04
Tb 0.63 0.44 0.70 0.74
Dy 4.46 3.30 4.74 4.39
Ho 0.99 0.79 1 .02 0.96
Er 3.29 2.78 3.30 2.90
Yb 3.21 2.95 3.45 2.77
Lu 0. 59 0.59 0.60 0.51
Ht 4. 22 5.33 4.33 3.33
Ta 0. 92 0.81 0.75 0.53
Pb 7. 58 5. 79 7. 57 4. 65
Th 2.12 2. 02 2. 38 1 .81
U 2.33 1.71 1.61 2. 03

43

Table 2. continued

 

Sample 031024-4h 031024-4I PD3-21-0202 PD1-21-0202
= = # #
Latltude (N) 1 1 .04 1 1 .04 336.3 336.3
Longltude (W) 85.67 85.67 348.0 348.0
Locatlon name Playa Playa Playa Playa
Soley Soley Rajada Rajada
Sample type ignimbrite ignimbrite lava lava
5'02 76.56 80.1 6 84.28 79.99
T10, 0.40 0.29 0.22 0.58
A1203 1 1.84 10.43 9.20 1 1 .51
F620;, 1 .56 0.54 0.51 0.62
MnO 0.03 0.01 0.01 0.05
MgO 0.1 1 0.01 0.01 0.00
CaO 1 .22 1 .39 0.91 1 .48
"8:0 4.42 3.56 2.90 4.86
K20 2.06 1.77 2.16 0.73
P205 0.06 0.06 0.04 0.17
Totals 98.26 98.22 100.24 99.99
Cu (XRF) 0.00 7.00
Zn (XRF) 15.00 33.00
Rb (XRF) 36.30 . 19.20 36.00 10.00
Sr (XRF) 147.10 127.50 84.00 154.00
Zr (XRF) 182.00 129.40; 134. 00 119.00
V 27. 27 18.90 29. 20 31.99
Cr 8. 98 19.53 7.54 9. 06
Y 55.73 25.64 19.05 64.23
Nb 2. 96 2.68 2.31 1.29
Ba 554.45 600.55 562.68 370. 01
La 15.18 6. 71 7. 22 9. 32
Ce 26.93 14.22 15.88 22.68
Pr 4. 42 2.12 2.12 4. 09
Nd 18.96 7.76 8.46 20. 81
Sm 4. 97 2. 36 2. 34 6.59
Eu 1.92 0. 97 0. 67 1.83
Gd 5.55 2.51 2. 39 7.90
Tb 1 .08 0.56 0.40 1.36
Dy 7. 41 3.61 4.26 10.68
Ho 1.72 0. 90 0.63 2. 00
Er 5.22 2. 74 1.97 5. 38
Yb 5.11 2. 84 2.04 4. 80
Lu 0. 89 0. 53 0.44 0. 85
Ht 5. 45 4.02 3.20 4.25
Ta 0. 64 0. 65 0.79 0.46
Pb 2.19 10. 31 1.22 0.78
Th 2. 04 2. 24 2. 23 1.04
U 1.37 2. 01 0. 96 0.66

Table 2. continued

 

Sample 031024-4h 031024-4l P03-21-0202 PD1-21-0202
= = # #
Latltude (N) 1 1 .04 1 1 .04
Longitude (W) 85.67 85.67
Location name Playa Playa Playa Playa
Soley Soley Rajada Rajada
Sample type ignimbrite ignimbrite lava lava
SID, 76.56 80.16 84.28 79.99
TlO, 0.40 0.29 0.22 0.58
Aleg 1 1.84 10.43 9.20 1 1 .51
Fezoa 1.56 0.54 0.51 0.62
MnO 0.03 0.01 0.01 0.05
M90 0.11 0.01 0.01 0.00
CaO 1.22 1.39 0.91 1.48
Nazo 4.42 3.56 2.90 4.86
K20 2.06 1.77 2.16 0.73
P205 0.06 0.06 0.04 0.17
Totals 98.26 98.22 100.24 99.99
Cu (XRF) 0.00 7.00
Zn (XRF) 15.00 33.00
Rb (XRF) 36.30 19.20 ‘ 36.00 10.00
Sr (XRF) 147.10 127.50 84.00 154. 00
Zr (XRF) 182.00 129.40 134.00 119. 00
V 27.27 18.90 29.20 31.99
Cr 8. 98 19.53 7.54 9.06
Y 55.73 25.64 19.05 64.23
Nb 2. 96 2. 68 2.31 1 2.9
Ba 554. 45 600. 55 562.68 370. 01
La 15.18 6. 71 7.22 9. 32
Ce 26.93 14.22 15.88 22.68
Pr 4. 42 2.12 2.12 4. 09
Nd 18.96 7.76 8.46 20. 81
Sm 4.97 2.36 2.34 6.59
Eu 1.92 0.97 0.67 1.83
Gd 5.55 2.51 2.39 7.90
Tb 1.08 0.56 0.40 1.36
Dy 7. 41 3.61 4.26 10. 68
Ho 1 .72 0.90 0.63 2.00
Er 5. 22 2.74 1.97 5.38
Yb 5.11 2.84 2.04 4.80
Lu 0.89 0.53 0.44 0.85
Hf 5.45 4.02 3.20 4.25
Ta (1. 64 0. 65 0.79 0.46
Pb 2.19 10. 31 1.22 0.78
Th 2. 04 2. 24 2.23 1.04
U 1 .37 2.01 0.96 0.66

45

Table 2. continued

 

Sample PA5 PA6 PA8 PA9
+ + + -l-

Latltude (N)

Longitude (W)

Locatlon name Punta Punta Punta Punta

Samara Samara Samara Samara

Sample type lava lava lava lava
SIC, 62.19 54.16 48.39 50.24
TIC, 0.69 0.89 1 .03 0.83
N203 17.90 18.53 17.44 18.88
F920, 4.91 7.99 9.00 7.80
MnC 0.05 0.10 0.24 0.14
MgC 0.70 1.86 3.03 1 .73
Geo 7.19 9.32 1 1.83 1 1 .61
Map 3.64 3.68 3.45 3.20
K20 0.90 1.13 0.84 1.54
P205 0.23 0.18 0.25 0.24
Totals 98.40 97.84 95.50 96.21
Cu (XRF) 95.00 42.80 78.50 64.80
Zn (XRF) 49.10 77.00 89.20 82.70
Rb (XRF) 12.80 22.90. 19.30 31.00
Sr (XRF) 554.60 535.10 472.00 550.10
Zr (XRF) 92.00 60.70 79.40 64.10
V

Cr -0.50 256.90 13.50 13.20
Y 14.37 20.15 23.41 19.74
Nb 2.92 1 .67 2.42 2.03
Ba 676.04 420.63 383.26 501 .55
La 7.29 7.39 6.74 8.14
Ce 18.39 15.58 16.11 17.25
Pr 2.50 2.37 2.61 2.62
Nd 10.34 10.69 12.26 11.71
Sm 2.58 2.88 3.23 2.97
Eu 1.00 1.01 1.19 1.10
Gd 2.69 3.22 3.60 3.12
Tb 0.46 0.55 0.62 0.54
Dy 2.12 3.02 3.60 3.10
Ho 0.45 0.62 0.74 0.61
Er 1 .45 2.09 2.37 2.04
Yb 1 .40 2.16 2.24 1 .92
Lu 0.22 0.34 0.33 0.31
Hf 1.96 1.61 2.08 1.65
Ta 0.16 0.13 0.14 0.12
Pb 3.62 2.07 1.88 2.12
Th 0.74 0.52 0.52 0.82
U 0.37 0.18 0.21 0.26

46

Table 2. continued

 

Sample PA10 PA11 PA12 PA13
+ + + +
Latltude (N)
Longitude (W)
Locatlon name Punta Punta Punta Punta
Samara Samara Samara Samara
Sample type lava lava lava lava
SIC, 61 .64 56.46 59.09 53.78
TIC, 0.47 0. 83 0.86 0.73
Al,C, 16.28 19.28 19.12 18.10
Fe,03 6.57 5. 21 6.20 6.49
MnC 0.12 0.10 0.07 0.23
MgC 1 .39 0.91 0.97 1 .20
Geo 6.86 9.45 8.16 1 1 .60
Na,C 3.33 3.91 3.78 3.67
K,C 0.91 1 .20 1.15 1 .00
P,Cs 0.16 0. 25 0.18 0.33
Totals 97.73 97. 60 99.58 97.13
Cu (XRF) 42.30 75.30 99.40 83.80
Zn (XRF) 67.30 86.10 87.50 73.10
Rb (XRF) 13.60 23.20 19.80 18.00
Sr (XRF) 503.20 511.90 503.60 591.30
Zr (XRF) 66.50 94.10 94.00 87.30
V
Cr -0.10 4.40 10.40 2.80
Y 12.97 23.46 22.65 22.63
Nb 1.72 2.72 2.78 2.80
Ba 594. 68 566.04 535.88 613.50
La 5. 85 8. 54 7.15 10.62
Ce 13.27 20. 41 17.88 21.49
Pr 1 .99 3.13 2. 93 3.06
Nd 8.79 13.83 13.78 12.79
Sm 2. 22 3.55 3. 73 3.08
Eu 0. 86 1.29 1.26 1.13
Gd 2. 30 3.73 3. 80 3.24
Tb 0.42 0.63 0.67 0.57
Dy 1 .91 3.45 3.71 3.05
Ho 0.37 0.68 0.73 0.62
Er 1.15 2.38 2. 36 2.05
Yb 1.27 2.20 2. 20 2.06
Lu 0. 21 0. 33 0. 33 0.32
Hf 1.62 2. 24 2.52 2.14
Ta 0.09 0.16 0.16 0.16
Pb 2. 95 3.31 2. 53 2. 54
Th 0. 59 0.71 0. 76 0. 92
U 0.18 0.41 0. 36 0. 31

47

Table 2. continued

 

Sample PA14 PA15 PA18 PA19
+ + + +
Latltude (N)
Longitude (W)
Locatlon name Punta Punta Punta Punta
Samara Samara Samara Samara
Sample type lava lava lava lava
SIC, 51 .78 54.99 54.47 27.95
TIC, 0.73 0.71 0.87 0.39
AI,C, 18.17 19.21 18.82 10.35
Fe,C, 6.29 8.58 6.89 3.91
MnC 0.27 0.17 0.10 0.28
MgC 1.63 2.87 2.01 1.30
CaC 12.71 9.36 8.58 37.81
Na,C 3.40 2.68 4.15 1 .98
K,C 0.83 0.53 1.34 0.48
P,Cs 0.22 0.14 0.26 0.10
Totals 96.03 99.24 97.49 84.55
Cu (XRF) 69.70 169.10 125.00 49.30
Zn (XRF) 70.60 86.10 79.00 41 .30
Rb (XRF) 13.20 10.00. 24.20 7.50
Sr (XRF) 488.80 425.70 757.20 522.60
Zr (XRF) 73.70 43.70 95.80 34.40
V
Cr 5.20 9.00 19.00 21.70
Y 20.1 1 16.84 14.23 5.52
Nb 2.17 1.06 2.98 1.01
Ba 472.90 434.25 600.55 297.80
La 7.78 4.24 8.03 3.00
Ce 17.35 9.72 17.58 6.58
Pr 2.67 1.59 2.61 0.92
Nd 11.44 7.61 11.13 3.71
Sm 2.94 2.20 2.73 0.96
Eu 1 .12 0.89 1 .23 0.41
Gd 3.13 2.48 2.77 1.27
Tb 0.53 0.49 0.48 0.22
Dy 3.02 2.51 2.30 0.72
Ho 0.60 0.53 0.45 0.13
Er 1.83 1.65 1.45 0.38
Yb 1 .91 1 .76 1 .43 0.76
Lu 0.28 0.29 0.22 0.10
Ht 1.82 1.13 2.32 0.43
Ta 0.13 0.06 0.18 0.08
Pb 2.32 1.51 3.57 1.81
Th 0.78 0.44 1.38 0.18
U 0.36 0.17 0.47 0.86

48

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50

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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primitive arc lava samples.

51

characteristic of subduction zone related magmas (Figure 14). The rare earth
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elements (HREE).

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54

DISCUSSION

The major element variations (Figure 12) for these samples cannot be
evaluated for fractional crystallization trends because these samples are
volcaniclasts in a sedimentary deposit and the origin and relation of the lava
clasts is unknown. However several inferences about the subduction parameters
can still be made. Useful ratios and their ranges for each sample location are

given in Table 3.

Mantle Source

Trace element data can be used to determine mantle source (Woodhead
et al., 1998; Abratis and Worner, 2000). Dewatering of the subducting slab does
not seem to affect the Zr/Nb ratio and it has been used to discriminate between
mantle sources in arc related magmas (Woodhead et al., 1998; Abratis and
Worner, 2000). Woodhead et al. (1998) proposed that the mantle source in the
Marianas influenced the Zr/Nb ratio. In southern Costa Rica, Abratis and Worner
(2000) determined that the volcanics originated from a more enriched mantle
source based on the Zr/Nb ratios. Zirconium is less incompatible than Nb in
mantle minerals (Sun and McDonaugh, 1989), and in a partial melt of the mantle,
niobium will strongly partition to the melt. This leaves the mantle enriched in
zirconium relative to niobium. Therefore, a high Zr/Nb ratio is indicative of a
depleted mantle source, while a low Zr/Nb ratio variation signifies an enriched
mantle source. Thus, the Zr/Nb ratio of the volcanic products also can give an

indication as to how heterogeneous the mantle source is.

55

Table 3. Useful ranges of trace element ratios.

 

Punta Quebrada
Ratio Samara Isla Paloma Buenaventura Playa Soley
Zr/Nb 31.2 - 41.2 14.7 - 55.6 26.9 - 61.9 31.9 - 49.5
BalLa 56.9 - 102.4 62.7 - 111.7 62.5 - 118.5 54.3 - 137.8
Lale 2.4 - 5.6 2.3 - 6.5 2.5 - 7.3 2.9 - 7.3
Ba/Th 435.2 - 986.9 387.8 - 1530.8 5.8 - 15.8 314.4 - 2907.2
Ufl' h 0.32 - 0.58 0.25 - 0.53 0.31 - 0.59 0.51 - 1.05
Ce/Pb 4.9 - 8.6 6.6 - 39.2 308.0 - 1403.0 7.3 - 5.3

56

The primitive arc volcaniclasts have Zr/Nb ratios ranging from 15 to 62
(Figure 16). Patino et al. (2004) presented that the Zr/Nb ratios for lava clasts
from Punta Samara have a close range of values, and conclude that these
volcanic products originate from a homogenous mantle source. The
homogeneity in mantle source for Punta Samara is a contrast to the other sample
locations from the primitive arc included in this study. The Zr/Nb ratios are more
variable within each sample location, and indicate a more heterogeneous mantle
source than that found for volcaniclasts from Punta Samara (Figure 16). The
lavas from the primitive arc have a predominately depleted mantle source
influence. However, two lava samples from Isla Paloma, 040709-18 and 040709-
24, have the lowest Zr/Nb ratio, indicating that these two samples have a more
enriched mantle component contributing to the source of some of the primitive
arc volcanics.

In the modern arc, the volcanics from central and northern Costa Rica
have Zr/Nb ratios that range between approximately 10 to 19 and 5 to 70,
respectively (Figure 16)(Carr et al., 2003). The volcanics from central Costa Rica
have a restricted range in Zr/Nb ratios, indicating a more homogenous mantle
source than the volcanics from modern northern Costa Rica. The central Costa
Rica volcanics have been interpreted to be derived from a more enriched mantle
source than those in northern Costa Rica due to the subduction of the Cocos
Ridge in the proximity (Abratis and Worner, 2000; Herrstrom et al., 1995;

Feigenson et al., 1996, 2004). While the Zr/Nb ratios of the modern arc

57

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volcanics from northern Costa Rica indicate that these originated from a
heterogenous mantle source composed of both an enriched (lower Zr/Nb) and a
depleted (higher Zr/Nb) source (Abratis and Worner, 2000; Herrstrom et al.,
1995; Feigenson et al., 1996, 2004). In northern Costa Rica, the volcanics are
dominated by a depleted mantle source (Carr et al., 1990).

The primitive arc lava samples have a larger range of bulk rock
compositions than the samples included in the above mentioned studies.
However, the Zr/Nb ratio for the primitive arc lava samples does not correlate
with Si02 (Figure 17). Therefore, fractional crystallization does not have an effect
on the Zr/Nb ratio and can be used as a comparison for mantle source. The
Zr/Nb ratio for the primitive arc volcaniclasts (15 to 62) overlaps the range of
Zr/Nb ratios from the modern arc in northern Costa Rica (Figure 16). The two
lavas from Isla Paloma, with the lowest Zr/Nb ratios from the primitive arc sample
set, are similar to those from central Costa Rica. The samples from the primitive
arc most likely originate from contributions from both a depleted and enriched

mantle source, with the depleted source dominating.

Slab Signal

6 The slab signal in arc related lavas can be evaluated using trace element
ratios as long as the ratio in the lavas is significantly different relative to that in
mantle. Barium is highly mobile in hydrous fluids and La is considered immobile
in these fluids (Pearce and Peate, 1995). The BalLa ratio will be higher in arc

volcanics due to fluids generated from the sediments and altered oceanic crust

59

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Figure 17. The Zr/Nb (A) and Ba/La ratios (B) of the lava samples are not
correlated with Si02 (wt%).

60

 

from the subducting slab. For the modern Central American arc, Ba/La is
considered an excellent geochemical tracer of slab signal from the Cocos Plate
(Carr et al., 1990). The primitive arc lava samples have a larger range of bulk
rock compositions than the samples included in the other studies mentioned.
However, the Ba/La ratio for the primitive arc lava samples does not correlate
with Si02 (Figure 17). Therefore, fractional crystallization does not have an effect
on the Ba/La ratio and can be used as a comparison for slab signal.

The volcaniclasts from the primitive arc have Ba/La ratios higher than
MORB or OIB (<15) (Figure 18). The lavas from Playa Soley have the widest
range of Ba/La ratios for the primitive arc. The volcaniclasts from Quebrada
Buenaventura separate into two groups based on the Ba/La ratios (greater than
100 or less than 80), while at Isla Paloma and Punta Samara, the lava samples
have a continuous range of Ba/La ratios. For the modern arc in central Costa
Rica, the Ba/La ratios (17 - 35) are lower than those from the primitive arc
(Figure 18B). The primitive arc Ba/La ratios are mostly higher than those from
the modern arc in northern Costa Rica (33-60). This indicates a generally higher
slab contribution in the primitive arc than what is present in the modern arc in
Costa Rica.

Carr et al. (1990) infer the degree of melting along the modern Central
American volcanic arc using the variation of the slope of the REES through La/Yb
ratios. High La/Yb ratios indicate a lower degree of melting, and low La/Yb ratios
indicate a higher degree of melting. The Lale values vary along the modern arc

and the degree of melting correlates with the slab signal (Carr et al., 1990). The

61

160

120

BalLa

40

160

120

BalLaBO

40

 

 

 

 

 

Prlmltlve arc
4} Playa Soley
{-Quebrada Buenaventura
xlsla Paloma
A Punta Samara
Modern arc
i'L'SCentral Costa Rica (con)
-Northem Costa Rica (NCR)
-Westem Nicaragua (W Nic)
-Miocene arc Nicaragua (MNic)

 

 

 

 

 

 

l I I I l I I I r I I l
"' : Degreoofmaltlng "‘
- 4L 0 -i
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_ A“; m A .
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i-
.5
.o -
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Figure 18. The Ba/La ratios from the volcanics from the primitive arc (A) show a
correlation with the La/Yb ratio, similar to the modern arc volcanics (B).

62

highest slab signal and highest degree of mantle melting, based on Ba/La and
La/Yb ratios, occurs in western Nicaragua, and decreases along the volcanic
front away from this point. In the modern arc, in northern Costa Rica, the La/Yb
values range from 6 to 12, and in central Costa Rica, the La/Yb values range
from 7 to 38. The lower values for the slab tracers in the modern arc in Costa
Rica, combined with higher La/Yb values indicates less slab input into the
magmas, and a lower degree of mantle melting (Carr et al., 1990).

The correlation between degree of melting and slab signal is also present
in the primitive arc, the higher Ba/La ratios generally correlate with lower La/Yb
ratios (Figure 18A). The La/Yb ratios for the volcaniclasts from the primitive arc
are lower than those from the modern arc in central Costa Rica. The primitive arc
lavas have La/Yb ratios that are lower than, and overlap the lower range in La/Yb
values for the modern arc in northern Costa Rica (Figure 183). The higher
Ba/La values and lower La/Yb values for the primitive arc can be interpreted to
indicate that the primitive arc had a higher slab input and higher degree of mantle
melting than the modern arc in Costa Rica. The lavas from the primitive arc
display similar Ba/La and La/Yb ratios as what has been recorded from Miocene
Nicaraguan and modern western Nicaraguan volcanics (Figure 18B).

For the modern Central American volcanic arc, Carr et al. (1990) proposed a
model that links the regional variations in slab signal and degree of melting to the
changes in the subduction angle of the Cocos Plate. In Nicaragua, the
subduction angle is steeper and the angle shallows in Costa Rica. The steeper

angle will produce metasomatism over a smaller volume of mantle resulting in a

63

greater degree of melting. The shallow angle will melt a greater volume of
mantle, and it will melt to a lesser degree. Following this model, Patino et al.
(2004) concluded the primitive arc in Costa Rica is presumed to originate from a
steeper subduction angle than what is currently producing the modern arc in
Costa Rica. The Farallon Plate must have been older, colder, and denser than
the Cocos Plate in the region. Since the volcanics from the primitive arc have
similar Ba/La and La/Yb ratios to those found in western Nicaragua, this study

supports these conclusions.

Speculations on subducted sediment

Patino et al. (2000) characterized the geochemical changes in DSDP Site
495, a section on the Cocos Plate. The sediment package from the Cocos Plate,
for the modern arc, consists of an underlying Middle to Lower Miocence
carbonate sequence, approximately 200 meters thick, and an Upper Miocence to
Quaternary hemipelagic sequence, 177 meters thick. These sediments are from
the middle to lower Miocene, and upper Miocene-recent respectively. The
change in sediment type is dated near 10 Ma, and represents the “carbonate
crash” (Lyle et al., 1995). This occurred when the uplift of the Isthmus of
Panama changed the eastern Pacific Ocean circulation pattern and elevated the
carbonate compensation depth (Lyle et al., 1995). Each sediment type has a
geochemical signature that is thus reflected in the slab signal.

Patino et al. (2000) found that the carbonate sediment section on the

Cocos Plate has higher Ba/T h ratios than the hemipelagic section, and this ratio

64

is used as a tracer of the carbonate section in the modern Central American arc.
Due to a greater abundance of organic matter in the hemipelagic sequence, the
uranium concentrations are higher in the more recent sediments. Uranium is
enriched over other elements and U/La and WT h ratios are higher in the
hemipelagic sediment section than the carbonate sediment section from the
Cocos Plate (Patino et al., 2000).

The volcaniclasts from the primitive arc have Ba/T h ratios approximately
between 308 and 1,530, except sample 040710-19 from Playa Soley that has a
Ba/T h ratio of 2,907 (Figure 19A). The lavas from Punta Samara have the
smallest range of Ba/T h ratios. The primitive arc lavas have higher Ba/T h ratios
than those from central Costa Rica (~100 _- 262), and have higher ratios or
overlap the majority of the volcanics from northern Costa Rica (~296 - 808)
(Figure 16). The volcanics from the modern arc in western Nicaragua have
Ba/T h ratios (346 - 1,965) that are more similar to those from the primitive arc in
Costa Rica (Figure 19).

The U/T h ratios for the primitive arc sample locations (Figure 20) are
similar to the U/T h ratios present in the modern arc in central and northern Costa
Rica (0.32 to 0.56), excluding the high U/T h ratios for the Playa Soley sample
040710-19 (Figure 20).

The modern arc volcanics from Costa Rica have lower U/T h ratios than
those from the western Nicaraguan modern arc (0.67 - 0.97) (Carr et al., 2003).
In modern Costa Rica, the lower geochemical sediment signatures have been

attributed to underplating. This is a process where sediment is scraped off from

65

2800
2400
2000
1 600

Ba/Th
1200

2800

2400

1
Ba/Thsoo

1200

 

 

 

I I I

 

 

Prlmltlve are
‘3’ Playa Soley
Il-Quebrada Buenaventura
XIsla Paloma
A Punta Samara

Modern arc

l-_-aCentral Costa Rica (con)
-Northem Costa Rica (NCR)
-Westem Nicaragua (WNic)
-Miocene arc Nicaragua (MNic)

 

 

 

 

 

 
 

 

 

 

 

- <> 1
, Degree of meltlng
—- S —
a.
r- 5 ‘
.n
5
-.,, 33 .
_ -I- A 1
A +“
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r A + " 3 i
' 1 l 1 A 1
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La/Yb
w I I I
- ”T 1.117533" V Degree of melting ‘
__ 1' ‘f —i
_ Z: a
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- it? 8' ‘
1* 3
L- IIS“ 2 "
tn
:- I" '.| "
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._,‘ ' --------_-_-~ ’I '—-1
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0 10 30 40

Lale

Figure 19. The Ba/l' h variation for the primitive arc also decreases with an
increase in La/Yb ratio (A). The primitive arc lava samples have Ba/T h ratios
most similar to Miocene and modern western Nicaraguan volcanics (B).

66

 

.85. 980 589:
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67

the downgoing slab, resulting in a lower sediment input into the arc lavas
(Valentine et al., 1997). Although the primitive and modern arcs in Costa Rica
have similar U/T h values, the processes that lead to these values are different.
Patino et al. (2004) mention that in the primitive arc, the low U/T h ratios are most
likely due to the absence of hemipelagic sediments in the sediment package. If
underplating were the cause of the low U/T h ratios in the primitive arc lavas, the
Ba/La ratios would also be lower, as is the case in the modern arc volcanics in
Costa Rica. Given the high BalLa ratios for primitive arc volcaniclasts,
underplating is not the cause of the low U/T h ratios. The low U/T h ratios in the
additional volcanics from the primitive arc, included in this study, support the
conclusions of Patino et al. (2004). As Patino et al. (2004) also mentioned, a
more likely explanation for the U/T h values in the primitive arc is that the
hemipelagic sediments were absent from the subducting sediment package
because they are older than the “carbonate crash”.

Plank et al. (2002) found that older lavas from Nicaragua (Early- Middle
Miocene), also have lower U/T h values (0.38 - 0.60) than what is present in the
more recent western Nicaraguan volcanics. The difference between U/T h values
in volcanics from the Miocene and modern arcs in Nicaragua has been attributed
to the absence of hemipelagic sediments contributing to the Miocence lavas
because this occurred before the “carbonate crash”. While in the modern arc in
western Nicaragua, which occurs after the “carbonate crash”, subducted
hemipelagic sediments contribute to the magmas (Plank et al. 2002). The lavas

from the primitive arc in Costa Rica have similar U/T h ratios as presented by

68

Plank et al. (2002) for Miocene Nicaraguan lavas (Figure 20). In addition, the
Ba/T h ratios in the primitive arc correlate better with La/Yb ratios than the Ba/La
ratios. If higher Ba/T h ratios and lower U is a signature of the carbonate section
in the modern arc, then during the Paleogene carbonate sediments most likely

dominated the sediment being subducted.

Conclusions

The primitive arc volcaniclasts from the four locations in Costa Rica (Punta
Samara, Isla Paloma, Quebrada Buenaventura, and Playa Soley) range from
basaltic to dacitic in composition. The primitive arc lavas included in the
geochemical portion of the study are the result of subduction related magmatism,
when the Farallon Plate subducted underneath the Caribbean Plate (Late
Cretaceous to Eocene). Primitive arc magmas appear to be derived from a
mixture of a depleted and enriched mantle, dominated by the depleted mantle
component, similar to the modern arc in northern Costa Rica. However, two
lavas from the primitive arc originate primarily from a more enriched mantle
source, similar to what is observed in modern central Costa Rica. The slab input
and degree of melting is higher for the primitive arc than the modern arc in Costa
Rica, and more similar to what is present in the modern western Nicaraguan
volcanics. This may be reflection an increased angle of subduction because the
Farrallon Plate was older colder and denser than the Cocos Plate (Figure 21).
The sediment subducted may have been similar to the carbonate sediment that

is subducted in the modern arc. The primitive arc volcaniclasts lack the signature

69

of the hemipelagic sediment component present in the modern arc, comparable

to the Miocence Nicaraguan volcanics.

7O

 

carbonate

 

 

AOC

  

 

    
  

Crust
~~~~~ Mantle

c 1
1 _ %

1111 111111111111 08
1 1111111111111i p, ”f ,(DM+ EM)‘\ j
11 111111111111: 111111
. . 1111111111111' ,1 ‘ ‘ ‘ \\
hem'pelag'c '1‘1'1‘1‘1111‘11‘11‘11111111 _ ..
111111111111
11111111111 _
carbonate 111111 B. Modern Costa Rlca

 

 

1 11111111111I
. _ 11111111111111111111'
hem 1 pelag 1c 1111 11111111111111
1111111111111“
11'
11

 

 

carbonate C. Modern Nicaragua

   

Figure 21. Comparison of the primitive arc in Costa Rica (A) to the modern arc in
Costa Rica (B) and western Nicaragua (C). Diagram after Carr et al., 1990.

71

APPENDICES

72

APPENDIX A

Petrographic descriptions of altered samples.

73

Sample
PA8

PA1 0

PA1 9

Description
Sample PA8 is a lava sample from Punta Samara. The
phenocrysts comprise 60% of the sample. Subhedral
plagioclase phenocrysts comprise 40% of the sample and are
up to 2 mm in size. Plagioclase phenocrysts exhibit sericite.
Plagioclase phenocrysts are partially altered to chlorite. Ten
percent of the sample is comprised subhedral pyroxene grains
are up to 3.5 mm in size and most are broken. Anhedral
opaques comprise 5% of the sample and are most likely
secondary and are up to 0.75 mm in size. Approximately 5% of
the sample is comprised of phenocrysts replaced by calcite,
chlorite, opaques and other alteration products. The
groundmass comprises 55% of the sample. The groundmass
consists of the phenocryst assemblage and previously
mentioned alteration products. Calcite is also present in 10% of
the groundmass.

Sample PA10 is a lava sample from Punta Samara.
Phenocrysts comprise 45% of the section Subhedral plagioclase
phenocrysts are present in 35% of the sample, and are up to
mm in size. Plagioclase phenocrysts display sericite, alteration
to chlorite. The pyroxene phenocrysts comprise less than 1% of
the sample. The majority of pyroxene phenocrysts display
reaction rims of opaques. Approximately 10% of the
phenocrysts are completely altered to chlorite and possibly
amphibole, and have reaction rims of opaques. The
groundmass comprises 55% of the sample, and consists of 45%
cryptocrystalline material, which appears to be the phenocryst
assemblage in approximately the same proportions. Chlorite is
present in approximately 10% of the groundmass. Fissures of
calcite run across less than 1% of the section.

Sample PA19 is a sedimentary rock.

74

Sample
040709-19

040709-26

0407094

Description
Sample 040709— 19 is a lava sample from lsla Paloma. A large
portion of this thin section is missing. The remaining portion of
the section consists of 65% phenocrysts. Thirty-five percent of
the phenocrysts are subhedral plagioclase that are up to 1.3 mm
in length. Fifteen percent of the phenocrysts are euhedral
pyroxene that are up to 0.6 mm in size. Approximately 15% of
the phenocrysts have been completely altered to chlorite and
calcite. Partial alteration to chlorite and calcite is also present
on the phenocrysts. The groundmass comprises 35% of the
section that is present and consists of smaller versions of the
phenocrysts assemblage and alteration products and includes
approximately 5% opaques.

Is a lava sample from lsla Paloma. Phenocrysts compose 55%
of the section. Approximately 35% of the phenocrysts are
subhedral plagioclase that is up to 2.625 mm in length.
Plagioclase phenocrysts display sericite and alteration to
chlorite. Approximately 10% of the grains appear to be
amphibole. Some of this amphibole may be secondary and
display reaction rims. Less than 1% of the section is euhedral
pyroxene phenocrysts up to 0.75 mm. Approximately 1% of the
grains have altered to calcite. Transmineral fissures cross the
section in a few areas. The groundmass is 45% of the sample.
The groundmass is comprised of the 40% of the groundmass
consists of the phenocryst assemblage and alteration products
in approximately the same proportions and 5% is secondary
opaques.

This is a lava sample from Quebrada Buenaventura that was
eliminated from the geochemical portion of the study due to a
low total. The phenocrysts comprise 40% of the sample.
Subhedral plagioclase phenocrysts comprise 25% of the sample
and are up to 2.3 mm in length, The plagioclase phenocrysts
display sericite and contain intramineral fissures. Fifteen
percent of the phenocrysts are euhedral pyroxene that are up to
1.8 mm in length. Secondary opaques form reaction rims along
pyroxene grains. The groundmass includes 55% of the sample.
The groundmass is comprised of 20% calcite, 15% reddish
brown material, 10% opaques, and 10% of the phenocryst
assemblage.

75

Sample
040709-1 2

1 7-5

04071 0-1 2

04071 0-1 3

Description
Sample 040709-12 is a basaltic trachyandesite from Quebrada
Buenaventura. The sample is comprised of 55% phenocrysts.
Thirty percent of the phenocrysts are subhedral plagioclase that
display sericite, and are up to 2.5 mm in size. Calcite and
chlorite are partially and completely replacing plagioclase
grains. Ten percent of the phenocrysts are completely replaced
by calcite. Ten percent of the phenocrysts are subhedral
pyroxene that are up to 1.5 mm in size. Several of the grains
are broken and secondary opaques have formed. Anhedral
opaques are up to 0.5 mm in size and comprise 1% of the
phenocrysts. The groundmass composes 50% of the sample
and includes plagioclase, pyroxene, calcite, and opaques

Sample 17-5 is from Quebrada Buenaventura. The phenocrysts
comprise 60% of the sample. Thirty-five percent are subhedral
plagioclase phenocrysts that are up to 3 mm in length.
Plagioclases exhibit sericite. Subhedral pyroxene phenocrysts
comprise 15% of the section, are up to 1.3 mm. Secondary
opaques have formed near pyroxene grains and are up to 0.5
mm. Approximately 10% of the phenocrysts have been
completely replaced by calcite. Some phenocrysts have
partially altered to chlorite. The groundmass is 40% of the
section and contains cryptocrystalline material, opaques, calcite,
and chlorite.

Sample 040710-12 is a lava sample from Playa Soley. The

phenocrysts comprise 5% of the section. Anhedral feldspar
phenocrysts are up to 1.25 mm and comprise 5% of the section.
The feldspar appear to have reaction rims along some grain
boundaries. Less than 1% are anhedral quartz phenocrysts that
are up to 0.8 mm. Less than 1% of the phenocrysts are anhedral
opaques up to 0.5 mm. The groundmass is 95% of the sample
and is composed of cryptocrystalline material.

Sample 040710-13 is a fiamme sample from Playa Soley. The
phenocrysts comprise 15% of the section. Anhedral feldspar
phenocrysts are up to 2.25 mm and comprise 10% of the
phenocrysts. The feldspar appear to have reaction rims along
some grain boundaries. The remainder of the phenocrysts are
opaques or grains that have been altered with a reddish brown
material. The groundmass is 85% of the sample and is
composed of cryptocrystalline material including reddish brown
alteration products and opaques.

76

Sample
04071 0-14

04071 0-1 5

04071047

Description
Sample 040710-14 is a lava sample from Playa Soley. The
phenocrysts comprise 5% of the section. Anhedral feldspar
phenocrysts are up to 1.3 mm and comprise 5% of the section.
The feldspar appear to have reaction rims along some grain
boundaries. Less than 1% are anhedral quartz phenocrysts that
are up to 0.75 mm. Less than 1% of the phenocrysts are
anhedral opaques up to 0.5 mm. The groundmass is 95% of the
sample and is composed of cryptocrystalline material

Sample 040710-15 is from Playa Soley. The phenocrysts
comprise 15% of the section. Five percent are anhedral quartz
phenocrysts that are up to 1.0 mm. Anhedral feldspar

' phenocrysts are up to 2.0 mm and comprise 5% of the section.

Feldspar phenocrysts appear to have reaction rims along
boundaries. Approximately 5% consists of opaques and grains
that appear to have been replaced by opaques and alteration
materials. The groundmass is 85% of the sample and is
composed of cryptocrystalline material, opaques, and reddish
brown alteration products.

Sample 040710-17 is a lava sample from Playa Soley. The
phenocrysts comprise 15% of the section. Five percent are
anhedral quartz phenocrysts that are up to 1.5 mm. Anhedral
feldspar phenocrysts are up to 2.5 mm and comprise 5% of the
section. Approximately 5% consists of opaques and grains that
appear to have been replaced by opaques and alteration
materials. The groundmass is 85% of the sample and is
composed of cryptocrystalline material.

77

Sample
040710-18

04071 0-20

04071 0-23

Descrlptlon
Sample 040710-18 is from lsla Paloma. This sample is
composed of approximately 45% phenocrysts. Forty percent of
the phenocrysts are subhedral plagioclase an up to 2.8 mm in
length. The center of several plagioclase grains exhibit sericite
and alteration to chlorite. Secondary opaques have also formed
on some grains. Less than 1% of the phenocrysts have been
completely replaced by calcite and chlorite. Five percent of the
phenocrysts are replaced by alteration products and may have
been pyroxene, and are up to 1.7 mm in size. Less than 1% of
the sample contains quartz phenocrysts that are up to 0.8 mm in
size. Resorption/reaction rims are present along most grain
boundaries. The groundmass comprises 55% of the sample,
and consists of 35% cryptocrystalline material, some of which
may be the remnants of altered phenocrysts. Fifteen percent of
the groundmass is opaques, along with 5% chlorite and reddish
brown alteration.

Sample 040710-20 is a fiamme sample from Playa Soley. The
phenocrysts comprise 10% of the sample. Approximately 10%
are subhedral feldspar phenocrysts that are up to 2.3 mm in
size. Some feldspar phenocrysts contain fissures and display
sericite. Less than 1% of the phenocrysts are anhedral quartz
up to 1 mm in size. The groundmass comprises 90% of the
sample. The groundmass is composed of cryptocrystalline
material. In areas this material forms radial patterns. The
groundmass has traces of a reddish brown material and
opaques.

Sample 040710-23 is a fiamme sample from Playa Soley. The
phenocrysts comprise 5% of the sample. Approximately 5% are
subhedral feldspar phenocrysts that are up to 1.7 mm in size.
Some feldspar phenocrysts contain fissures, display sericite,
and appear to be partially resorbed. Less than 1% of the
phenocrysts appear to be replaced grains with opaques and
reddish brown alteration, possibly oxyhornblende. The
groundmass comprises 95% of the sample. The groundmass is
composed of cryptocrystalline material. The groundmass has
traces of a reddish brown material and opaques.

78

Sample
04071 0-24

031 024-4a

031 024-4b

031 0244c

031 024-4d

Descmatlon
Sample 040710-24 is a fiamme sample from Playa Soley. The
phenocrysts comprise 5% of the sample.’ Approximately 5% are
subhedral feldspar phenocrysts that are up to 1.8 mm in size.
Some feldspar phenocrysts contain fissures, display sericite,
and appear to be partially resorbed. Reaction rims appear
around phenocrysts. The groundmass comprises 95% of the
sample. The groundmass is composed of cryptocrystalline
material. The groundmass has traces of a reddish brown
material and opaques. In areas this appears in a radial pattern.

Sample 031024-4a is a fiamme sample from Playa Soley. The
phenocrysts comprise 5% of the sample. Approximately 5% are
subhedral feldspar phenocrysts that are up to 2.3 mm in size.
Some feldspar phenocrysts contain fissures, display sericite,
and appear to be partially resorbed. Approximately 2% of the
phenocrysts have altered to chlorite. Less than 1% of the
phenocrysts appear to be replaced grains with opaques. The
groundmass comprises 95% of the sample. The groundmass is
composed of cryptocrystalline material. The groundmass has
traces of a chlorite, reddish brown material, and opaques.

Sample 031024-4b is a fiamme sample from Playa Soley. The
sample contains approximately 5% phenocrysts of anhedral
feldspar that are up to 1.3 mm in size. The groundmass is
comprised of cryptocrystalline material that may be altered glass
and opaques.

Samp19031024-4c is a fiamme sample from Playa Soley. The
sample contains approximately 5% phenocrysts of anhedral
feldspar that are up to 1.5 mm in size. Approximately 5% of the
phenocrysts are grains that have been replaced by opaques and
reddish brown alteration products. Ninety percent of the section
is groundmass. The groundmass is comprised of
cryptocrystalline material that may be altered glass and opaques

Sample 031224-4d is a fiamme sample from Playa Soley. The
phenocrysts comprise 20% of the sample. Fifteen percent of the
phenocrysts are anhedral feldspar grains that are up to 2.25 mm
in length. Five percent of the phenocrysts are anhedral quartz
that are up to 1.5 mm. The groundmass comprises 75% of the
section. It contains cryptocrystalline material and opaques.

79

Sample
031024-4e

031 024-4f

031 024-49

031 024-4h

031 024-41

Descrlptlon
Sample 031024-4e is a fiamme sample from Playa Soley. The
phenocrysts comprise 15% of the section. Approximately 10%
of sample is anhedral feldspar up to 1.3 mm. The remaining 5%
of the phenocrysts consists of anhedral quartz phenocrysts that
are up to 1mm in size, and anhedral opaques that are up to 0.5
mm in size. The groundmass comprises 85% of the sample, and
consists of 80% cryptocrystalline material, and 5% of the
remnants of altered phenocrysts that have been disaggregated
and secondary opaques.

Sample 031024-4f is a fiamme sample from Playa Soley. The
phenocrysts comprise 15% of the sample. Approximately 10%
of sample is anhedral feldspar up to 1.5 mm. The remaining
phenocrysts appear to be phenocrysts that have been replaced
by reddish brown alteration and opaques. The groundmass is
85% of the section and consists of cryptocrystalline material that
may be altered glass.

Sample 031024-4g is a fiamme sample from Playa Soley. Ten
percent of the sample is comprised of phenocrysts. Anhedral
feldspar phenocrysts are up to 1.3 mm in size. Less than 1% of
the phenocrysts are anhedral quartz that are up to 1 mm in size.
The groundmass comprises 90% of the sample, and consists of
cryptocrystalline material and opaques.

Sample 031024-4h is a fiamme sample from Playa Soley.
Phenocrysts are present in 15% of the sample. Anhedral
feldspar comprises 10% of the phenocrysts, and are up to 1.5
mm in size. Anhedral quarts phenocrysts comprise 5% of the
sample and are up to 1.8 mm in size. Grains that have been
replaced by opaques and reddish brown alteration comprise less
than 1% of the section. The groundmass consists of
cryptocrystalline material and opaques.

Sample 031024-4i is a fiamme sample from Playa Soley. The *
phenocrysts comprise 10% of the sample, and are anhedral
feldspar up to 1.3 mm in size. The groundmass is 90% of the
section and consists of cryptocrystalline material that may be
altered glass.

80

Sample
PD1-21-0202

PD3-21 -0202

Description
Sample PD1-21-0202 is a lava sample from Playa Rajada, and
consists of 5% phenocrysts that are primarily anhedral feldspar
that are cracked and have reaction rims. The feldspar are up to
1.5 mm in size. The sample contains less than 1% of grains that
have been replaced by opaques and reddish brown material.
These replaced grains are up to 0.3 mm in size. The
groundmass comprises 95% of the section and is
cryptocrystalline material.

Sample PDa-21-0202 is a lava sample from Playa Rajada, and
consists of 100% groundmass. The groundmass is composed of
reddish brown material and opaques. There almost appears to
be a pattern similar to flow banding of opaques in areas. Less
than 1% of the sample appears to be grains that have been
replaced or altered with opaque and reddish brown material.

81

APPENDIX B

Minerals present in thin sections

82

Table 4. Minerals present in thin section.

Phen % is percentage of phenocrysts present in the section. Plag is plagioclase, Pyx is Pyroxene,
Opq is opaques, Qtz is quartz, Amph is amphibole, Cal is calcite, and Chi is chlorite. * is feldspar
in the sample that is most likely plagioclase.

 

Sample Phen % Plag Pyx Opq Qtz Amph Cal Chl

PA 040709-1 40 X X X X X

PA 040709-2 65 X X X X X

PA 040709-3 65 X X X X X

PA 040709-4 40 X X X X

PA 040709-5 40 X X X X

PA 040709-6 35 X X X X X

PA 040709-9 40 X X X X X

PA 040709—1 0 65 X X X X X

PA 040709-12 55 X X X X X

PA 040709-13 55 X X X X

PA 040709-14 15 X X X

17-5-1201 60 X X X X X

PA 040709-15 55 X X X

PA 040709-16 55 X X X

PA 040709-17 65 X X X X

PA 040709-18 65 X ‘ X X X

PA 040709-19 65 X X X X

PA 040709-20 40 X X X X

PA 040709-21 60 X X X X X

PA 040709-22 55 X X X X X

PA 040709-24 40 X X X X X

PA 040709-25 50 X X X X
1 PA 040709-26 35 X X X X X

14-11-1201 40 X X X X X

13-11-1201 35 X X X X X

PA 040710-12" 5 X X X

PA 040710-14“ 5 X X X

PA 040710-15' 15 X X X

PA 040710-16 40 X X X X

PA 040710-17‘ 15 X X X

PA 040710-18 45 X X X X X

PA 040710-19 50 X X X X

PA 040710-20‘ 10 X X

PA 04071043" 5 X X X

031024-4a“ 5 X X X

031024-4b* 5 X X

031024-4c“ 5 X X

031024~4d* 20 X X X

031024-4e' 15 X X X

03102441" 15 X X

03102449" 10 X X X

031024-4h" 15 X X X

031024-4l' 10 X

PD3-21-0202 0 X

83

Table 4 (continued)

 

Sample Phen % PM Pyx Opq Qtz Amph Cal Chl
PD1-21-0202* 5 X X

PAS 55 X X X

PA6 45 X X X X
PA8 60 X X X X X
PA9 55 X X X

PA10 45 X X X X X X
PA11 50 X X X X
PA12 45 X X X X
PA13 65 X X X X
PA14 45 X X X X
PA15 35 X X X X
PA18 35 X X X X

84

APPENDIX C

Anomalous Slab Signal (Ce/Pb)

85

Anomalous Slab Signal

The samples from the primitive arc in Costa Rica have Ce/Pb ratios that
range from 5 to 39 (Figure 22). According to Noll et al. (1986) arc related
magmas have Ce/Pb ratios between 1 and 10. In MORB and OIB, Ce/Pb ratios
are greater than 20, according to Hoffman et al. (1988), and near 25 according to
Sun and McDonaugh (1989). Both Pb and Ce have similar incompatibility in
mantle minerals, therefore, during melting these two elements should not be
fractionated. The characteristic Ce/Pb range of arc lavas has been explained by
the preferential extraction of Pb over Ce by fluids from the subducting oceanic
crust, resulting in a lower Ce/Pb ratio in arc magmas than what is present in
MORB or OIB (Miller et al., 1994; Brenan et al., 1995). Researchers have also
suggested that Ba and Pb behave similarly in subduction zone related
magmatism (e.g., Shibata and Nakamura, 1997). Lead has also been reported
to be more mobile than other LILE in fluids that are produced by the dehydration
of amphibolite (Kogiso et al., 1997). Therefore, if a high slab contribution is
present and BalLa ratios are high, Ce/Pb ratios would be expected to be low.

Five of the primitive arc lavas have Ce/Pb ratios greater than 14 (Figure 23)
due to lower Pb concentrations in the samples. However, a high slab
contribution is still evident in these lavas (Ba/La > 50) (Figure 22). The volcanics
from the modern arc in Central America have Ce/Pb ratios that range from <1 to
31 (Figure 22 and 23). However, the majority of the modern arc volcanics have
Ce/Pb ratios that are 14 or less. The higher range of Ce/Pb ratios (>14) from the

modern arc occur in central Costa Rican volcanics. The volcanics from central

86

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Costa Rica with high Ce/Pb differ from those from primitive arc in that modern
lavas do not have as high of a slab contribution (Ba/La < 50) (Figure 22).

One explanation for the low-Pb samples from the primitive arc is analytical
error. Lead is highly volatile and it can be lost during fusion of the samples/and
or during ablation. In addition, different sample matrices couple differently with
the laser during ablation. However, these samples were all fused for the same
amount of time as other samples with high Pb concentrations; therefore, any Pb
loss should be uniform. Also, the low-Pb lavas have similar major element
concentrations to the rest of the samples, with the exception of K20, which is low
among these samples. However, there are other lavas that have higher Pb
values and still have similar K20 contents. All of the low-Pb samples have Si02
< 70 wt% and were ablated as long as other samples with similar Si02 content.
There appears to be no correlation between the low-Pb samples and ablation
time. In addition, these samples were all analyzed in the LA lCP-MS on the
same day using the same regression lines to calculate the results.

Another explanation for the low Pb samples from the primitive arc is that
these volcanics are not arc related. Hoffman (1988) compiled an average for the
Pb concentrations in N-MORB (0.489 ppm) and Continental crust (8.0 ppm). In
addition, Sun and McDonaugh (1989) compiled the Pb concentrations in N-
MORB (0.30 ppm), E-MORB (0.60 ppm), and OIB (3.20 ppm). The primitive arc
Pb concentrations range from 0.27 to 5.44 ppm, and in the low-Pb samples the
concentration ranges from 0.27 to 0.80 ppm. The Pb concentrations from the

modern arc range from approximately 1.8 to 19 ppm, with the majority between 2

89

and 11 ppm. Other than the low Pb concentrations, the low-Pb samples from the
primitive arc have the chemical characteristics of subduction zone related lavas,
displaying enrichment of LILE over HFSE (Figure 24).

A third explanation for the low-Pb from the primitive arc samples is that the
slab contribution varied in these lavas. Patino et al. (2000) proposed a flux
model for the modern arc in Central America. The influx portion of the model
includes the hemipelagic sediment, carbonate sediment, and altered oceanic
crust. As previously discussed, it is unlikely that a hemipelagic sediment
component contributes to the primitive arc lavas. However, it is possible that a
carbonate sediment component and altered oceanic crust contribute to the
generation of the primitive arc volcaniclasts. To estimate the contribution from
the subducting oceanic crust in the modern arc, Patino et al. (2000) used DSDP
Site 504B. Site 5043 is dated at 7 Ma, and is 2 km in depth (Alt et al., 1996).
This site consists of an upper volcanic section that has been oxidized and
enriched in alkalis from the interaction with sea water. It is important to note that
the upper section of oceanic crust is enriched in Rb, K, and U (Alt and other,
1996; Staudigel et al., 1995). In the middle is a transition zone that has been
hydrothermally altered, and is enriched in chalcophile elements, including Pb,
which have been leached from the lower hydrothermally altered dikes (Alt et al.,
1996). Chauvel et al. (1995) and Alt et al. (1996) suggest that hydrothermal
alteration in the oceanic crust promotes the deposition of sulfide minerals in the
transition zone, thus adjusting the Pb content of the oceanic crust. In the modern

arc, Patino et al. (2000) found that B, Rb, Cs, and K are contributed from the

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upper segment of the AOC, while the majority of the Pb is contributed from the
hydrothermally altered portion of the AOC. Thorium and U are also contributed
from the AOC. The data set from the primitive arc does not include B and Cs,
however, in the low-Pb samples from the primitive arc Rb, K, U, and Th are near
the lower range of values observed (Figure 25). It is possible that the low-Pb
samples had a lower contribution from the AOC than the other primitive arc
samples.

There are of course several uncertainties with this third hypothesis. Although
the Rb, K, U, and Th concentrations for the low-Pb samples are amongst the
lowest from the primitive arc, there are other lavas from the primitive arc that
have similar values and do not have low-Pb concentrations. In addition, the Pb
concentrations presented by Alt et al. (1996) are not well constrained due to
contamination. Although close in geographic proximity to Central America, Site
504 is also younger than the primitive arc. Kelley et al. (2003) determined the
chemistry of oceanic crust from ODP site 801, which is dated near 179 Ma, and
is located in the western Pacific. The authors found that the oceanic crust at this
site was enriched in U, K, Rb, along with Li and Cs, but did not find high Pb
concentrations. Although the oceanic crust from ODP Site 801 is closer in age to
the primitive arc, it is geographically farther away than the locality of the primitive
arc, and is shallower (total of 470 m) than Site 504.

Due to the location and age of the samples, there is also the possibility that
secondary processes may have removed elements from the lavas, resulting in

the low-Pb primitive arc lavas. Noll et al. ( 1995) proposed that a reducing, acidic

92

hydrothermal fluid may preferentially mobilize chalcophile elements such as Pb
over LILE, due to the presence of sulfides and other complexes. A sulfide rich
hydrothermal fluid could have removed the Pb from the low-Pb lavas, and not

mobilized as much Ba, not altering the Ba/La ratio in these samples.

93

50

RD

20’

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20

 

 

 

 

 

 

 

 

 

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1|: Quebrada Buenaventura
x lsla Paloma
A Punta Samara

 

 

 

3.2

‘ 2.8

2.4
2.0
1.6 K90

Figure 25. Selected elements versus Pb. Notice that the low-Pb

samples (open symbols) also have low K20 (wt%), Rb. Th, and

U (ppm) compared to other primitive arc lava samples. However,
the low-Pb samples do not have the lowest concentrations of Ce

and Ba (ppm).

94

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