Ferric Iron Evolution During Crystallization of the Earth and Mars

Magma ocean crystallization models that track fO₂ 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 Fe³⁺ concentrations. We model the fractional crystallization of Earth and Mars, including Fe²⁺ and Fe³⁺ as separate components. We calculate Fe³⁺ partition coefficients for lower mantle minerals and compare the results of fractional crystallization for both Earth and Mars. We calculate oxygen fugacity (fO₂) at the surface as the systems evolve and compare them to constraints on the fO₂ of the last magma ocean atmosphere from D/H ratios, both with and without metal saturation. For Earth, we find that Fe³⁺ 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 Fe³⁺ 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 fO₂ unless the magma ocean begins with <50% of the predicted Fe³⁺, but better match the present day mantle redox. We show that Fe³⁺ 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.