Ferric iron stabilization at deep magma ocean conditions

Fe2O3 produced in a deep magma ocean in equilibrium with core-destined alloy sets the early redox budget and atmospheric composition of terrestrial planets. Previous experiments (≤28 GPa) and first-principles calculations indicate that a deep terrestrial magma ocean produces appreciable Fe3+, but predict Fe3+/ΣFe that conflict by an order of magnitude. We present Fe3+/ΣFe of glasses quenched from melts equilibrated with Fe-alloy at 38-71 GPa, 3600-4400 K, analyzed by synchrotron Mössbauer spectroscopy. These indicate Fe3+/ΣFe of 0.056-0.112 in a terrestrial magma ocean with mean alloy-silicate equilibration pressures of 28-53 GPa, producing sufficient Fe2O3 to account for the modern bulk silicate Earth redox budget and surficial conditions near or more oxidizing than the iron-wüstite buffer, which would stabilize a primitive CO- and H2O- rich atmosphere.