[P-PS07] Planetary Sciences

[PPS07-P07] Oxidation of Earth’s proto-atmosphere during the great mantle oxidation event induced by disproportionation of ferrous iron in a deep magma ocean

*Tatsuya Yoshida¹, Naoki Terada¹, Kiyoshi Kuramoto² (1.Tohoku University, 2.Hokkaido University)

Keywords:proto-atmosphere, magma ocean, Earth, mantle redox state

Earth is expected to have acquired a reduced proto-atmosphere enriched in H2 and CH4 through impact degassing of volatiles and simultaneous accretion of reduced materials enriched in metallic iron during the formation period (e.g., Kuramoto and Matsui, 1996; Shaefer and Fegley, 2010; Zahnle et al., 2020). If a deep magma ocean was formed by a giant impact during accretion, the magma ocean is expected to have been oxidized through the disproportionation of ferrous iron (Fe2+) to metallic iron and ferric iron (Fe3+) which is a powerful oxidant under the lower mantle pressure, followed by differentiation of metallic iron and homogenization of ferric iron by vigorous convection in the whole magma ocean. Assuming that the magma ocean and the proto-atmosphere were in a chemical equilibrium state, it has been suggested that the reduced proto-atmosphere formed during the accretion rapidly evolved into an oxidized composition enriched in H2O and CO2 through the great mantle oxidation event (e.g., Hirschmann, 2012; Pahlevan et al., 2019). However, the oxidation of the proto-atmosphere can be limited by the diffusion of atmospheric components in the boundary layer between the atmosphere and the magma ocean since their diffusion rates in silicate melt are very small (Zhang et al., 2010).

In this study, we developed a gas-rock reaction-diffusion model in silicate to estimate the oxidation rate of the proto-atmosphere associated with mantle oxidation. The model incorporates H2 as a representative example of reduced atmospheric components and considers the diffusion of H2 and Fe3+ in silicate as well as their redox reaction. According to the calculation results, the H2 oxidation is limited by its diffusion flux in the boundary layer. Particularly when a primitive crust is formed due to a decrease in internal heat flow, the oxidation of the proto-atmosphere hardly proceeds. As a result, the proto-atmosphere may have maintained a reduced state of chemically non-equilibrated with the upper mantle depending on the initial volatile mass and cooling time of the magma ocean.