Oxidation state of the lower mantle: In situ observations of the iron electronic configuration in bridgmanite at extreme conditions

• We collect Mössbauer spectra of bridgmanite in situ at lower mantle pressure and temperatures.
• We establish iron valence and spin states in bridgmanite at P, T conditions of the lower mantle.
• A large proportion of iron is in the ferric state at lower mantle conditions.
• Fe3+ spin crossover plays a negligible role in determining mantle properties.

We have investigated the electronic configuration of iron in Fe-, Al-containing magnesium silicate perovskite, i.e., bridgmanite, the main component of the lower mantle, at conditions of the deep Earth's interior using the energy domain Synchrotron Mössbauer Source technique. We show that the high ferric iron content observed previously in quenched samples is preserved at high temperatures and high pressures. Our data are consistent with high-spin to intermediate-spin (HS-IS) crossover in Fe2+ at high pressures and ambient temperature. We see no evidence of spin crossover in Fe3+ occupying the A-position of bridgmanite. On laser heating at pressures above ∼40 GPa we observe a new doublet with relative area below 5% which is assigned to Fe3+ in the octahedral (B-site) position in bridgmanite. We conclude that at lower mantle conditions Fe3+ remains predominantly in the HS state, while Fe2+ occurs solely in the IS state.