A decrease in the Fe3+/∑Fe ratio of bridgmanite with temperature at the top of the lower mantle

Abstract

Bridgmanite, the most abundant mineral in the lower mantle, readily incorporates Fe3+ even under very reducing conditions where it is in equilibrium with metallic iron. This has led to the proposal that charge disproportionation of Fe2+ to produce Fe3+ and metallic iron may occur in the lower mantle. The amount of Fe3+ in bridgmanite is important for understanding the oxidation state of the mantle, as charge disproportionation of Fe2+ to form Fe3+ and metallic iron, followed by loss of metallic iron to the core, could have raised the Fe3+ content of the mantle as a whole after core formation. Here, the Fe oxidation state in bridgmanite was studied as a function of temperature at 27 GPa and an oxygen fugacity of ∼2.0 log units above the iron-wüstite oxygen buffer (ΔIW+2) using multi-anvil techniques. Bridgmanite Fe3+/∑Fe ratios were determined by Mössbauer spectroscopy and decrease from ∼80% at 1800 K to 50% at 2600 K. A decreasing Fe3+/∑Fe ratio with increasing temperature indicates that the level of charge disproportionation would have been lower under the higher temperature conditions of the early Earth. However, if the redox state of the upper lower mantle is buffered by coexisting iron and ferropericlase, then the relative oxygen fugacity will have changed very little during secular cooling.