Ferric Iron Content of Majorite Coexisting With Reducing Melt at 18 GPa: Implications for the Mantle Oxygen Fugacity of Mars and Earth

Knowledge of the Fe 3+ /ΣFe ratio of mantle minerals crystallized from a magma ocean under high pressures is important to constrain the initial distributions of Fe 3+ /ΣFe ratio and oxygen fugacity () of the mantle of terrestrial planets, such as Mars and Earth. Here we experimentally investigated the Fe 3+ /ΣFe ratio of majorite coexisting with reducing melts at 18 GPa, corresponding to the middle part of the Earth's mantle transition zone and the base of the Martian mantle. The results show that majorite coexisting with melts has Fe 3+ /ΣFe ratios ranging from 0.11 to 0.21, with typical analytical uncertainties of ±0.01–0.02, under metal‐ and diamond‐saturated conditions. These values are higher than those of the upper mantle minerals and silicate melts, but lower than in bridgmanite under identical conditions. These results suggest the formation of a vertically heterogeneous distribution of Fe 3+ /ΣFe ratio in the mantle during magma ocean crystallization. For Earth, after the magma ocean crystallization, the phase transition of Fe 3+ ‐rich bridgmanite to majorite in an upwelling mantle flow could release an excess amount of Fe 3+ and oxidize the mantle transition zone, generating redox heterogeneity in this region. For Mars, the Fe 3+ /ΣFe ratio of majorite coexisting with melt under metal‐saturated conditions may explain the apparent discrepancy in between the molten silicate layer above the metallic core and the oxidized surface magmas recorded in Martian meteorites.