Perovskite as a possible sink for ferric iron in the lower mantle

The lower mantle constitutes more than half the Earth's interior by volume, and is believed to consist predominantly of (Mg,Fe)SiO3 perovskite with up to approximately 20% (Mg,Fe)O. In the system FeO-MgO-SiO2, iron partitions preferentially into (Mg,Fe)O relative to the perovskite phase and has been believed to be nearly all in the form of Fe2+ on the basis of experiments in the MgO-FeO-SiO2 system(1-4). Here, however, we present a Mossbauer study of (Mg,Fe)SiO3 perovskite containing 3.3 mol% Al2O3, which shows that approximately 50% of the iron is Fe3+. These results, combined with evidence from other experiments, suggest that the proportion of iron present as Fe3+ in the lower-mantle perovskite phase is probably much higher than is currently believed. Because the oxidation state of iron in the perovskite phase affects the electrostatic charge balance and equilibrium defect concentration, the presence of Fe3+ is Likely to significantly affect physical and chemical properties of the lower mantle such as sub- and super-solidus phase relations, transport properties, mechanical behaviour, trace-element partitioning and the concentration of species such as hydroxide ions.