CSEDI Collaborative Research: C-O-H Volatile Metasomatism in the Cratonic Mantle - Implications for Mid-Lithospheric Discontinuities

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Earth is the only planet in the Solar System whose surface is divided into oceans and continents. Hence understanding the origin of continents and how portions of them remain stable for billions of years is central to understanding the evolution of the Earth. The old, stable cores of continents (cratons) are underlain by thick layers of mantle (the silicate layer of the Earth that lies below the crust), and these mantle roots are thought to contribute to continental stability. Yet, it remains debated how the continental mantle formed and evolved. In particular, the roles of magma or fluid interaction with the mantle, either during or after the continent formation, are unclear, although widespread layering observed in the cratonic mantle by seismic imaging may have been created by magma and fluid related processes. This research will constrain the formation and evolution of cratons by bracketing scenarios of melt/fluid and mantle reactions that are consistent with geophysical observations at mid-lithospheric depths. The research findings - disseminated through peer-reviewed publications, conference presentations and colloquia, and class room lectures and activities - will impact a wide range of Earth science sub-disciplines including petrology-geochemistry, mineral physics, geodynamics, and seismology. The proposal will support female PhD students in high P-T experimental petrology, mineral physics, and seismology, and support the research activities of several undergraduate students, including students from underrepresented groups. In addition to their own research, all the students will participate in discussions and write research articles together with all 3 PIs, learning how to combine concepts and methods from three different yet related sub-disciplines of solid Earth science. The proposal will thus directly further the educational experience and professional development of several students at various stages of their careers. The proposed research will also support an early career PI in the field of mineral physics and will aid the investigators in engaging K-12 students in science.The stability of the continental interiors for 2-3 billion years is a key feature of the Earth's dynamical and thermal evolution. Understanding the geophysical structure, petrology, and material properties of cratonic mantles and the tectonic history that produced them is therefore one of the grand challenges of Earth science. A curious feature of subcontinental mantles is the presence of distinct reductions in seismic shear velocity at depths of 60-150 km, often called mid-lithospheric discontinuities (MLDs). Given the widespread occurrence of MLDs within cratons, their origins may be linked to craton formation; however, the causal link between MLD and craton formation is yet to be established, and MLD origins remain highly debated. One hypothesis is that MLDs represent zones enriched in CO2-H2O volatiles, which led to stability of hydrous and/or carbonate minerals or partial melts. This project will test this hypothesis by integrating results from experimental petrology, mineral physics, and seismology. Through laboratory experiments, this project will constrain the phase equilibria of cratonic mantle peridotites fluxed by mixed CO2-H2O volatiles at MLD depths, taking into consideration how the agent of volatile introduction may vary with craton formation models. The goal will be to determine the stability of hydrous and carbonate mineral phases over partial melts and obtain the compositions and proportions of mineral phases as a function of depth, temperature, and melt/fluid:peridotite ratio for each of the cases. Guided by the phase equilibria experiments and compiled xenolith data, thermoelastic properties of the appropriate end members and intermediate solid solutions of amphibole and mica will be determined using Resonant Ultrasound Spectroscopy (RUS) and first principles simulation. Using the mineral assemblages from experiments and updated elasticity data from new mineral physics, mantle velocity models will be predicted. These will be compared to mantle models for southern African craton (to be obtained by joint inversion of surface wave, Sp and Ps data) and North America, and used to calculate synthetic Sp and Ps receiver function stacks that will be compared to MLD observations globally. The proposed research will test whether one or several scenarios of fluid/melt infiltration into cratons, if any, satisfy the geophysical and petrologic characters of continental mantles at mid-lithospheric depths. Our research will also lead to specific products such as solidus parameterizations of depleted mantle+CO2+H2O; metasomatized mantle mineral assemblages as a function of P-T, infiltrated fluid/melt composition and fluid/melt:rock ratio; codes for computing seismic velocities for MLD-related assemblages with updated thermoelastic data; and observed mantle discontinuity parameters and mantle velocity models.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.