Core formation and the oxidation state of the Earth

Oxidation during accretion is an important component of heterogeneous accretion models, but the mechanisms proposed are generally untestable. Here we propose that an increase in oxidation state is a natural consequence of the size of the earth. Magnesium silicate perovskite is the principal phase in the earth's lower mantle. This phase forces the disproportionation of ferrous iron into ferric iron plus metal. When perovskite started to grow at the base of the magma ocean, dissolution and reprecipitation acted as an 'oxygen pump' injecting oxidized ferric iron into the upper mantle. As the earth continued to grow, infalling metal was oxidized by the ferric iron into ferrous iron, raising the iron content of the magma ocean and the oxygen fugacity of metal-silicate equilibration. This process did not occur to any great extent on Mars because of the limited stability of perovskite in the smaller planet