Composition, Structure, and Formation of the Lower Crust in Continental and Oceanic Arc Settings: Insights from the Xenolith Record

Citation

Sosa, Emma Sofia (2024) Composition, Structure, and Formation of the Lower Crust in Continental and Oceanic Arc Settings: Insights from the Xenolith Record. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2arj-v665. https://resolver.caltech.edu/CaltechTHESIS:05242024-181307541

Abstract

The compositional variability of lavas erupted in subduction zone settings results from a multitude of deep crustal processes acting in concert, reflecting variations among arcs in source rock composition, water content, oxidation state (fO2), temperature, pressure, and crystallization sequence across arcs. Lower to mid-crustal xenoliths—deeply sourced rock fragments entrained and brought to the surface by ascending melts—provide a robust record of the chemistry and structure of their inaccessible source regions. This thesis combines a variety of laboratory techniques and modeling approaches to explore the origins of two xenolith suites from vastly different arc settings: the oceanic Aleutian Arc off the coast of Alaska and the continental Andean Arc in Colombia. While all samples are fully characterized in terms of their petrography and the major and trace element chemistry (of both minerals and whole-rock), special attention is given to stable Fe isotope ratios, which are particularly sensitive to the fractionation of important Fe-bearing minerals like olivine, magnetite, amphibole, and garnet. In the first chapter, we document the major and trace element compositions of 39 previously undescribed xenoliths from the Mt. Moffett and Mt. Adagdak volcanic centers on Adak Island, Central Aleutians. This data is then used to evaluate the P-T-fO2-H2O conditions under which the cumulates formed and interrogate the nature of their parental melts. The second chapter builds upon this first study, presenting Fe isotope data of whole-rock powders and mineral separates (spinel, clinopyroxene, olivine, amphibole, and magnetite) from the Adagdak xenoliths. Our data show that the Adak crust is stratified in terms of Fe isotopes, with an isotopically light lower-crust and an isotopically heavy middle to upper-crust. The implications of this compositional structure and its relation to the evolution of Adagdak magmas is then explored through mass-balance fractional crystallization modeling. In the final chapter, we apply the same methods used in the first two chapters to characterize a suite of lower to mid-crustal xenoliths from the Mercaderes region of Colombia in the Central Andean Cordillera. In contrast to Adagdak, the Mercaderes samples show a nearly constant whole-rock Fe isotope composition throughout our ~50 km crustal section. Through thermodynamic modeling, we show that the most likely explanation for this data is that the Mercaderes suite represents a prograde metamorphic sequence

Thesis Files