Renewal: Chemistry of the Earth's Deep Interior

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Intellectual Merit. Recent discoveries of the Cmcm post-perovskite (ppv) phase of (Mg,Fe)SiO3 and the high-spin-low-spin (HS-LS) transition of iron in lower-mantle pressure-temperature conditions have set the stage for a new paradigm in deep Earth research. Previously supported research results reveal that (Mg0.6Fe0.4)SiO3 crystallizes in another ppv structure (Pmma) and its iron is in a mixed spin state and ordered in two distinct crystallographic sites. Thus mantle mineralogy is more complex than previously thought, and several questions must be addressed: What are the crystal structures of the major mineral phases within the lower mantle? What are the effects of pressure-induced electronic and magnetic transitions on the crystal chemistry of lower mantle phases? What are the pressure-temperature-composition phase relations? How do major (Fe, Mg, Si) and minor (Ca, Al) components partition among different solid and melt phases and between different crystallographic sites? What are the geochemical and geodynamic consequences of this new paradigm of mineral physics? Recent progress in high-pressure technology now makes it possible to tackle these challenging questions. The development of X-DAC and portable laser-heating system provide the platform for versatile and robust high pressure-temperature environments. The sub-?Êm x-ray probe allows in situ single-crystal xray diffraction studies with nominally polycrystal samples. A battery of newly developed synchrotron xray spectroscopic probes, including NRXS, XES, XANES, XMCD, and XRS allows in-depth study of the oxidation states, spin states, and site occupancy of Fe, as well as chemical bonding of light elements (O, Si, Mg, Al). This accomplishment-based project will address these and related questions that are fundamental for understanding large-scale processes and phenomena including the nature of geochemical reservoirs, the fate of deeply subducted slabs, the origin of plumes, the nature of the core-mantle boundary, the differentiation of the early Earth, and the continuing dynamic geochemical cycles today. Broader Impacts. The research outlined in this proposal will impact a broad range of disciplines within the Earth sciences, including seismology, geodynamics, geodesy and tectonics, as well as petrology and geochemistry. This work will also improve understanding of material properties as a whole under extreme conditions, and as a consequence the research will illuminate areas beyond the geosciences: in physics, chemistry, materials science, planetary science, and biology. The research will enhance capabilities at synchrotron radiation and other major national facilities with the development of new experimental techniques for the broader community and training of new scientists at these facilities. A wide cross?]section of participants will benefit from the work, from high school and undergraduate students in internship programs, to graduate students (predoctoral fellows), postdoctoral associates and visiting investigators. As such, the project leverages existing infrastructure to prepare young scientists for careers in academia, national laboratories, and industry. The results and implications of the proposed work will be featured in popular articles and lectures, as with previous awards. By advancing the scientific frontiers of geoscience with newly developed techniques, the potential for materials applications, the training of new scientists, and educating the public, the proposed work will broadly impact both science and society.