EVALUATING THE ROLE OF MASH PROCESSES AND THE GROWTH OF CONTINENTAL CRUST

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One of the unique features of Earth is that it contains large land masses, or continents, consisting of continental crust. Continental crust is important to society because it is enriched in minerals and rocks that are used in construction, manufacturing, agriculture and energy. Thus, understanding how continents form is vital to the success of our world economy. Continental crust is generally thought to form in subduction zones where two tectonic plates converge, and one plate is underthrusted beneath another. In these settings, melting of the mantle (the layer below the crust) is thought to result in the generation of continental crust. One of the major unresolved problems with this model is that direct melting of the mantle produces melts that are geochemically distinct relative to estimates for bulk continental crust. This conundrum leads to the central question of this proposal: what processes in the deepest parts of the crust lead to the transformation/enrichment of melts derived from the underlying mantle? This project addresses this problem by examining one of the few places on Earth where it is possible to directly observe the deep roots of continental crust in an ancient subduction zone. We focus on rocks exposed in Fiordland National Park located in the southwestern part of the South Island, New Zealand, where the deep crust of an ancient subduction zone is exposed to depths of 40 to 65 km paleodepth. Our group, in collaboration with New Zealand researchers, will use geochemical and isotopic data together with numerical modeling to determine the processes responsible for the geochemical evolution of these melts. The results of our project will provide new insights into the magmatic processes operating in the deep crust of modern subduction zones, which are inaccessible to geologists. The project will also advance societal outcomes through an innovative outreach program that provides mentorship through research to underrepresented students at California State University Northridge, a Hispanic Serving and an Asian American Native American-Pacific Islander Serving Institution in Los Angeles County. This project will also provide graduate and undergraduate students with international field experiences and exposure to cutting-edge analytical research facilities that will aid in the development of a diverse, globally competitive STEM workforce. Since first proposed by Hildreth and Moorbath in 1988, the "MASH" concept has had a profound impact on volcanological, petrological and tectonic thinking regarding the evolution of continental-arc systems. MASH zones as originally conceived were defined as "Melting, Assimilation, Storage and Homogenization" regions located at the crust-mantle boundary where arc magmas acquired their ?crustal? signature through intracrustal remelting and scavenging of existing crust. Although MASH zones are commonly invoked in continental-arc models, there have been few studies that have directly examined magmatic processes in the deep roots of continental arcs where MASH zones are located. The overarching goal of this project is to understand the origin of melts that enter deep-crustal "MASH" zones and the processes that lead to geochemical diversification of these melts. Our project involves targeted field mapping and sampling, whole-rock and mineral geochemistry, and high-precision U-Pb zircon dating of the Malaspina Pluton (Fiordland, New Zealand), which was emplaced at ~40-50 km paleodepth. We use new and existing samples to address three key questions specific to the lower crust of the Median Batholith:1) How and where do arc magmas acquire their "continental" crustal signature? 2) How efficient are MASH zones in homogenizing lower-crustal melts? and, 3) How long are melts stored and at what rate do they differentiate in lower-crustal MASH zones? The answers to these questions will provide insights into the processes that lead to the growth of continental crust in convergent margin settings. This project also provides continued support for the "ROCs" outreach program (Research Opportunities for CSUN students). Goals of this program are to 1) encourage aspiring CSUN geology undergraduate students, especially minorities and females, to pursue research projects with faculty mentors, 2) broaden students? understanding of career and graduate school opportunities in the geosciences, and 3) create a sense of community with the goal of increasing retention in geoscience. Phase 2 of the ROCs program will create a network of geoscience mentors from former ROCs participants who will return to CSUN to share their experiences in graduate school and industry. This effort will build ties between current underrepresented CSUN students and recent graduates in the Los Angeles area and beyond.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.