Collaborative project: CSEDI- Understanding Si and Fe differentiation in Earth's mantle and core through experimental and theoretical research in geochemistry and mineral physics

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To first order, the Earth is divided into three concentric shells of different nature: the metallic core, the rocky mantle, and the fluid atmosphere/hydrosphere. While samples are available from the mantle and atmosphere/hydrosphere, the nature and composition of the core remain poorly understood. In particular, the core is known to be less dense than pure iron-nickel alloy, indicating that another light element is present in the core, possibly oxygen, silicium, or sulfur. The conditions (pressure and temperature) under which Earth's core formed and the nature of the light element in Earth's core are two major unresolved questions in planetary sciences. Because no core samples are directly available for study, scientists rely on remote seismic observations or other indirect methods to address those questions. In the proposed work, the approaches of geochemistry (the chemistry of the Earth), mineral physics (solid state physics applied to natural materials), and computational techniques will be combined to set limits on the temperature condition during core formation and the nature of the light element in Earth's core. This will be achieved by examining the extent to which different isotopic flavors of silicon and iron were partitioned between metal and silicate when the core formed. The work will involve synthesizing minerals in the laboratory and compressing them to pressure conditions relevant to the deep Earth by confining the samples between two diamonds, measuring the strength of the iron bonds in those minerals at a synchrotron source that produces very energetic X-rays, and examining, through computer calculations, the behavior of matter under high pressure and temperature. This work can impact many fields of science, ranging from the origin of Earth's dynamo to characterization of extrasolar planets through measurement of their mass. All PIs will actively engage in training and educating graduate students, undergraduate students, and postdocs in the proposed research projects. The PIs will continue developing SciPhon, a user-friendly, free software for NRIXS data reduction. This program will be made available to various communities studying different aspects of NRIXS, including geochemistry, mineral physics, material sciences, condensed matter physics, and biochemistry. All PIs will be actively involved in outreach programs including the UTeach Outreach Program that conducts academic summer camps for underrepresented K-12 kids from the Austin and southwest Texas area. The mass of the Earth and its accretion history are such that core-mantle differentiation was probably unavoidable but considerable uncertainties remain as to how and when this took place. Our limited understanding of this major event arises from our lack of sampling of Earth's deep interior. Scientists have devised indirect approaches to address this shortcoming by relying on (1) mineral physics experiments to reproduce the high pressure-temperature conditions prevailing in Earth's interior, (2) theoretical calculations to mimic those same conditions, and (3) geochemical measurements of the composition of mantle rocks to search for telltale signatures of core formation. These strongly interweaved approaches have led to significant progress but first-order unanswered questions remain, such as under what pressure-temperature conditions did the core form, what is the nature of the light element in the core, and did core formation fractionate Si and Fe isotopes. Terrestrial basalts have non-chondritic Si and Fe isotopic compositions, which could reflect partitioning of these elements into the core, although other interpretations exist. The investigators propose to establish Si and Fe isotope fractionation factors using high-pressure nuclear resonant inelastic X-ray scattering (NRIXS) and theoretical calculations at deep mantle conditions via collaborative approaches in geochemistry (Dauphas), theoretical ab initio calculations (Wentzcovitch), and experimental mineral physics (Lin). The derived force constants of Si and Fe bonds in basaltic glasses, lower-mantle minerals (bridgmanite and ferropericlase), and Fe alloys will allow us to build a deep-Earth geochemical model to evaluate if the specific Si and Fe isotopic compositions of the silicate Earth reflect core partitioning, and if they do, put constraints on important aspects of core formation such as temperature or the presence of Si as a light element in the core. The experimental results will serve as a benchmark for ab initio calculations of Si and Fe isotopic fractionation between relevant metal and silicate phases at high pressure and temperature. The theoretical work will in turn guide and refine the experimental and geochemical modelling efforts, focusing in particular on nuclear resonant measurements, force constant derivations, anharmonic and spin crossover effects. The exchanges and feedbacks between geochemists and experimental and theoretical physicists involved in this project will provide a holistic view of Si and Fe isotopic fractionation during core formation.