Ferrous iron partitioning between magnesium silicate perovskite and ferropericlase and the composition of perovskite in the Earth's lower mantle

Abstract

We have investigated the exchange of Fe and Mg between magnesium silicate perovskite (Mg‐Pv) and ferropericlase (Fp) at 25 GPa and 2400 to 2600 K using a Kawai‐type multianvil apparatus. Each experiment was performed with coexisting metallic Fe, which buffered the oxygen fugacity at the lowest possible level. As the system was Al‐free the presence of metallic Fe ensures low ferric iron (Fe3+) contents in all phases. The results are used to extract thermodynamic data to describe Fe2+‐Mg partitioning. A thermodynamic assessment and modeling of the available high‐pressure partitioning data indicates that the influence of a Fe‐spin transition in Fp on Fe‐Mg partitioning may be more subtle than previously proposed. Furthermore, we demonstrate that a comparison between perovskite Fe2+ contents predicted by the thermodynamic model and previously reported perovskite analyses can be used to estimate Mg‐Pv Fe3+ concentrations of both Al‐bearing and Al‐free phases in the previous studies. These estimates show that the Fe3+ content of Al‐free Mg‐Pv depends strongly on oxygen fugacity, and varies accordingly with the capsule materials used in experiments. The relationship between Fe3+ and Al concentrations in Al‐bearing Mg‐Pv indicates that the substitution mechanism of Fe3+ and Al changes with Al content. Chemical heterogeneities in the lower mantle will result in the formation of Mg‐Pv with quite different Al and bulk Fe concentrations, which will cause important differences in Fe3+ and oxygen vacancy concentrations in Mg‐Pv.