Ferric iron evolution during crystallization of the Earth and Mars

Abstract

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. Metal-saturated magma oceans have long been thought to contain negligible ferric iron oxide (Fe3+O1.5), but recent work suggests they may contain near-present-day Fe3+ concentrations. We model the fractional crystallization of Earth and Mars, including Fe2+ and Fe3+ as separate components. We use two independent equations of state (Deng, Armstrong EOS) to calculate Fe3+ partition coefficients for lower mantle minerals and compare results of fractional crystallization for different magma ocean configurations for both Earth and Mars. We calculate the 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. For Earth, we find that Fe3+ must behave incompatibly in the lower mantle to match the D/H constraint for whole mantle models, but shallow magma ocean models also provide reasonable matches to the constraints. For Mars, both EOSs produce near identical results but cannot match the D/H constraints on last fO2 unless the magma ocean begins with less than 50% of the predicted Fe3+. This model shows that Fe3+ partitioning has a measurable effect on magma ocean atmosphere redox state, which is not a static quantity but evolves throughout the magma ocean’s lifetime. We highlight the need for additional experimental constraints on ferric iron partitioning.