Ferric Iron Evolution During Crystallization of the Earth and Mars

Abstract Magma ocean crystallization models that track f O 2 evolution can reproduce the D/H ratios of both the Earth and Mars without the need for exogenous processes. Fractional crystallization leads to compositional evolution of the bulk oxide components. Recent work suggests that metal‐saturated magma oceans may contain near‐present‐day Fe 3+ concentrations. We model the fractional crystallization of Earth and Mars, including Fe 2+ and Fe 3+ as separate components. We calculate Fe 3+ partition coefficients for lower mantle minerals and compare the results of fractional crystallization for both Earth and Mars. We calculate oxygen fugacity ( f O 2 ) at the surface as the systems evolve and compare them to constraints on the f O 2 of the last magma ocean atmosphere from D/H ratios, both with and without metal saturation. For Earth, we find that Fe 3+ likely behaves incompatibly in the lower mantle in order to match the D/H constraint for whole mantle models, but shallow magma ocean models also provide reasonable matches. Disproportionation in whole mantle magma oceans likely overpredicts the amount of Fe 3+ and metal that form or require subsequent reduction to return to present‐day values. For Mars, we cannot match the D/H constraints on last f O 2 unless the magma ocean begins with <50% of the predicted Fe 3+ , but better match the present day mantle redox. We show that Fe 3+ partitioning has a measurable effect on magma ocean redox, and that it evolves throughout the magma ocean's lifetime. We highlight the need for additional experimental constraints on ferric iron mineral/melt partitioning and more thermodynamic data for the Fe‐disproportionation reaction.