THROUGH THE MAGMA MUSH : TRACING MAGMA EVOLUTION FROM THE MANTLE, THROUGH THE CRUST, AND TO THE SURFACE USING THE GEOCHEMISTRY OF ERUPTED MELTS AND MINERALS

Summary

Studies synthesizing field work, numerical simulations, petrology, geochemistry, and geophysical observations indicate that the compositional diversity of lavas results from evolution of mantle-derived basaltic magmas by mixing, assimilation, and fractional crystallization. These studies indicate this evolution occurs within dispersed complexes called transcrustal magmatic systems, rather than tank-like magma chambers. The processes within these magmatic systems have implications for... Show moreStudies synthesizing field work, numerical simulations, petrology, geochemistry, and geophysical observations indicate that the compositional diversity of lavas results from evolution of mantle-derived basaltic magmas by mixing, assimilation, and fractional crystallization. These studies indicate this evolution occurs within dispersed complexes called transcrustal magmatic systems, rather than tank-like magma chambers. The processes within these magmatic systems have implications for understanding the evolution of continental crust, the breakup of continental landmasses, and the hazards associated with volcanism. We present three studies in various tectonic and magmatic settings, using the geochemistry of whole rocks and minerals to relate magmatic processes within transcrustal magma systems and their inputs from the mantle to large-scale plate tectonic and geodynamic questions. In a study of magnesium-rich andesites from the Taupo Volcanic Zone, I link primitive mineral compositions in Mg-depleted melts to the growth of magma accommodation zones in a rifting segment of arc crust. Mineral constraints on temperature and pressure indicate that the plumbing system first formed at mid- to lower-crustal pressures (3.5-7.0±2.8 kbar). I interpret the mafic mineralogy and presence of disequilibrium features as evidence that these andesites and their crystal cargo represent the products of a developing magmatic system in the middle to lower crust. This study addresses the question of how magmatic systems initially form and evolve. I examine lavas from the back arc of Patagonian Argentina, where volcanism is displaced from the magmatic arc due to subduction of oceanic spreading centers. I demonstrate using thermodynamic models that the source regions for melts contain volumes of pyroxenite (3-11%), and were generated at high pressures (2.6-2.7 GPa). These melting conditions have been consistent since the Eocene. I interpret these results as evidence of detachment of pyroxene-rich lithosphere that was created by magma-lithosphere interaction during the Mesozoic breakup of Gondwana. These results show a link between prior magmatic events the role of pyroxene-rich mantle lithologies in subsequent mantle melting episodes. Finally, I present a study that probes the evolution of late-stage magmas in the failed 1.1-billion-year-old Mid-Continent Rift that are analogous to packages of lava and sediment that are buried during the final stages of continental breakup. I present evidence showing magma mixing between primitive and evolved residual magma controls the magma composition in these lavas. The highly negative εHf and εNd isotopic characteristics of these magmas, supported by modeling outcomes, suggest extensive assimilation (15%) of continental crust accompanying fractional crystallization of olivine, plagioclase feldspar, and spinel at 4 kbar pressure and 1060° C. I interpret this as a renewed pulse of magma that has exploited a preexisting transcrustal magmatic system. I posit that this system had its magma supply exhausted at the end of the main stage of volcanism but remained partially molten due to residual heat from the main stage. The results of this study have implications for the persistence of these magmatic systems through time. Each individual study, while from disparate time periods and tectonic settings, demonstrates that the transcrustal magmatic system (and the transcrustal system’s link to the mantle) provides a suitable conceptual framework for modeling and describing the evolution of magmas. These systems are not static but rather change with time to respond to geodynamic forces and rates of magma delivery. Show less