Collaborative Research: Is it MASH or MUSH? Testing Conflicting Models for Magma Diversification and Mobilization in Lower-Arc Crust

The Earth contains an outer layer of continental crust which is important to society because it is a major repository of rocks and minerals that are natural resources used in construction, manufacturing, agriculture and energy. Thus, understanding how continental crust forms is vital to the success of our national economy. Geoscientists believe that continental crust is generated in magmatic arcs above active subduction zones, where the convergence of tectonic plates results in melting of the mantle (the layer below the crust) and/or the lowermost part of the crust.

A key problem in understanding continental crust creation is that these processes occur at great depths within the Earth at 30-50 kilometers beneath the surface. Consequently, geoscientists have little direct information about the processes that occur in the roots of arcs except in rare locations where faulting has brought deep portions of the Earth to the surface.

This conundrum leads to the central question of this project, what processes occur in the deepest parts of the crust that lead to the creation of new continental crust? 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 above an ancient subduction zone, the Fiordland National Park located in the southwestern part of the South Island, New Zealand.

These investigators, in collaboration with New Zealand researchers, will link structural, geochemical and isotopic data together to determine the processes and timescales involved in the creation of continental crust. The project will advance societal outcomes through an innovative post-M.S. internship that provides mentorship and training to a recently graduate M.S. student in scanning electron microscopy 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.  

The lower crust of arcs is commonly considered to be the engine room for continental crust creation; however, there is strong disagreement about how magmas diversify and how they are extracted from the lower crust to form middle- and upper-crustal batholiths. This project investigates whether magma diversification in the lower crust mainly occurs in deep-crustal, crystal-rich mush zones, where magma mixing, hybridization, fractional crystallization and extraction of trapped melts from crystal-melt reservoirs are the dominant processes that lead to geochemical diversification of magmas.

The project involves linking field mapping and sampling, microstructural analysis using EBSD, bulk-rock and mineral geochemistry, and high-precision CA-ID-TIMS U-Pb zircon dating. This project focuses on testing three key questions specific to the Misty pluton, a large, superbly exposed lower-crustal intrusion that records igneous process from its base at ~40 km depth to its roof at ~25 km depth in Fiordland, New Zealand: 1) How do magmas diversify in the lower crust of arcs: are ‘MASH’ or ‘mush’ processes dominant in driving magmas to silicic compositions? 2) What mechanisms control the mobilization, segregation and extraction of melts in the lower crust?

And 3) Over what timescales do magma diversification processes occur in the lower crust? The answers to these questions will provide insights into magma diversification, segregation and continental crust construction in the roots of a continental arc. This project involves a researchers and students from under-represented groups in the geosciences, and students at California State University Northridge (a Hispanic Serving Institution) and the University of Vermont (an EPSCOR institution).

The project promotes international collaborations with New Zealand researchers and public engagement through a series of ‘Fiord’ talks aimed at the general, non-science community in New Zealand to explore the geology and unique environment of Fiordland National Park (an UNESCO World Heritage Site). Talks will be recorded and uploaded to YouTube for broad dissemination to the public.

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.