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Abstract

The production of metallic Fe via the disproportionation of Fe2+ to Fe0 and Fe3+ in the deep Earth has been a long debated topic with important implications for the geochemistry of the lower mantle. The presence of disproportionated metallic Fe can affect the siderophile element geochemistry of the lower mantle, notably through its impact on isotopic tracers such as Os, and on platinum group element distributions. Metallic iron could also serve as a likely host for volatile elements in the lower mantle, such as C, S, and H, impacting the mantle’s carbon and hydrogen budgets. It is understood that bridgmanite is the dominant phase in the lower mantle, and it has been shown that the presence of Al promotes the partitioning of Fe3+ into the perovskite structure as an FeAlO3 component, charge balanced by metallic iron. Frost et al. (2004) proposed that this disproportionation process occurs in the lower mantle, where the formation of aluminous perovskite implies the precipitation of approximately 1 wt% metallic Fe-rich alloy. However, literature data conflict on the pressure, temperature, and composition space in which this reaction occurs, and there has been little subsequent study to confirm this process at deeper lower mantle conditions.This thesis focuses on the experimental detection and theoretical prediction of the iron disproportionation reaction across a range of lower mantle conditions. First, I describe modifications I made to the thermodynamic database of Stixrude & Lithgow-Bertelloni (2022), which contains thermodynamic parameters for the calculation of various lower mantle phase equations of state. I use the modified database to model high pressure and temperature phase equilibria with the PerpleX Gibbs energy minimization software (Connolly, 2009). I find that the disproportionation reaction can be successfully modeled with the updated database, and I identify several results from the literature where the disproportionation reaction was incorrectly overlooked. Next, I explore the occurrence of the iron disproportionation reaction from 25 to 65 GPa in a natural almandine-pyrope-grossular garnet with in-situ X-ray diffraction in the laser- heated diamond anvil cell and with ex-situ scanning electron microscopy techniques. Examination of the samples recovered between 39-64 GPa by scanning electron microscopy analysis reveals the presence of nm-scale disproportionated iron metal grains as an additional product of this reaction that was not detectable in the X-ray diffraction patterns. I use volume compression data of the synthesized bridgmanite to estimate the FeAlO3 content of the bridgmanite in this composition, which I confirm with PerpleX thermodynamic modeling. Finally, I investigate the iron disproportionation reaction in a lower mantle pyrolite composition from 27 to 132 GPa along the geotherm with in-situ X-ray diffraction and ex-situ scanning electron and transmission electron microscopy techniques. I demonstrate that disproportionated metallic Fe can be detected in all assemblages recovered across the range of lower mantle conditions. Through TEM image analysis and PerpleX thermodynamic modeling, I determine that the amount of disproportionated metallic Fe decreases by a factor of ~5 from the top of the lower mantle to its base, dropping from ~0.6 vol% to 0.1 vol%.

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