Evolution of the Steam Atmosphere and Early Oxidation of the Silicate Earth

The largest differentiation event on Earth was the separation of the metallic core and gaseous atmosphere from the silicate mantle. Mantle abundances of siderophile elements constrain the chemical conditions (e.g., the fO2) of metal-silicate equilibration during early differentiation events. The oxygen fugacity inferred on this basis is a few (~2-3) log units below the iron-wüstite buffer [e.g., 1]. At such low fO2, the atmosphere blanketing the magma ocean would be dominated by H2 [2]. By contrast, the composition of gases emanating from modern volcanoes are much more oxidizing (logfO2≃IW+4,QFM) and dominated by H2O [3]. Such an oxidized state for Earth's mantle was apparently established early, by 3.9-4.4 Ga [4,5], motivating the steam atmosphere concept [6,7]. Some process apparently oxidized the silicate Earth above the redox state at which core formation occurred. This still unknown process was a defining event for Earth history and climate.

There are, at present, two ideas for how this oxidation took place. Both involve magma oceans: self-oxidation via Fe disproportionation during core formation, currently subject to experimental study [8], and oxidation via selective escape of atmospheric H over O, which has been proposed [9] but not subject to dedicated study. Here, we describe the first models of the magma ocean-primordial atmosphere that encompass the range from reducing (H2-rich) to oxidizing (H2O-rich) atmospheres. The models capture hitherto neglected effects including thermal blanketing via H2 collision-induced absorption, redox-dependent H volatility in the magma ocean, and H/O fractionation due to condensation clouds. These effects determine the lifetime of the magma ocean and the efficacy of oxidation via hydrogen escape. Using the results, we subject the hydrogen loss hypothesis for the oxidation of silicate Earth to quantitative tests.

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