CAREER: Redefining the high field strength element systematics of subduction systems using non-traditional stable isotopes

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Reconstructing the nature of the geologic processes responsible for the evolution of our planet, including when and how continents formed and plate-tectonic cycles were established, has been a long-standing goal of Earth Scientists. Clues for answering these questions are ‘coded’ into the geochemistry of the ancient rock and mineral record, but interpreting these geochemical signatures is not always straightforward. Over billion-year timescales, the rocks that constitute Earth’s continents have been deformed and overprinted by younger geologic processes, significantly obscuring their original structural and chemical characteristics. In order to ‘see through’ this complex evolution, geoscientists use chemical signatures retained by elements that are not easily altered and/or remobilized, and thus that can provide clues into primary rock-forming processes despite subsequent overprinting. A group of elements particularly suited for these investigations are the so-called high-field strength elements (HFSE), a group of transition metals with unique geochemical characteristics which make them not only important to understand crust formation but also very resilient to alteration. In particular, the isotopic compositions of these elements, which can now be measured with great accuracy and precision, can provide unique glimpses into the geochemical processes taking place in convergent tectonic margins and during formation of continental crust. Nevertheless, making geologically meaningful interpretations from these data requires a robust understanding of the processes that control the observed isotopic signatures, and developing such framework is the central research goal this project will undertake.This CAREER project will leverage recent analytical developments in the field of non-traditional stable isotopes to i) conduct a detailed study of the mass-dependent isotope fractionations that characterize the HFSE titanium, zirconium, and hafnium in subduction environments; and ii) utilize these isotopic variations for better understanding the processes leading to their fractionation and fluxes during formation of continental crust. To achieve this, the research team will generate a series of combined Ti-Zr-Hf isotopic datasets from key geologic components and petrologic processes characterizing the ‘subduction factory’ and that are known to influence the chemistry of arc-related magmatic systems. Isotopic fractionations at the bulk-rock and mineral scales will be determined in various global localities of orogenic peridotites, mid ocean ridge basalts, high-pressure/low-temperature subduction complexes, lower-crustal arc cumulates, and arc-related basalts and differentiated volcanic rocks, to understand HFSE mass-transfer and isotopic fractionations across the entire subduction cycle. Samples will be measured using high-accuracy methods involving calibrated double-spikes and MC-ICP-MS measurements. In addition to the broader scientific impacts that will result from this research, the PI will develop educational activities that foster greater equity, diversity, and inclusivity within the Geosciences starting from a pre-college stage. These include the development of an afterschool bilingual program for Hispanic/Latinx high-school students in collaboration with the Tucson Unified School District (TUSD), active recruitment of URM undergraduates into meaningful research experiences, and training of diverse graduate students and postdocs. As a result of this project, the PI will also develop an accessible on-line resource for researchers and students interested in learning more about non-traditional stable isotopes, constructed with active student involvement.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.