Pressure stabilizes ferrous iron in bridgmanite under hydrous deep lower mantle conditions

Abstract

Earth’s lower mantle is a potential water reservoir. The physical and chemical properties of the region are in part controlled by the Fe ³⁺/ΣFe ratio and total iron content in bridgmanite. However, the water effect on the chemistry of bridgmanite remains unclear. We carry out laser-heated diamond anvil cell experiments under hydrous conditions and observe dominant Fe ²⁺ in bridgmanite (Mg, Fe)SiO ₃ above 105 GPa under the normal geotherm conditions corresponding to depth > 2300 km, whereas Fe ³⁺-rich bridgmanite is obtained at lower pressures. We further observe FeO in coexistence with hydrous NiAs-type SiO ₂ under similar conditions, indicating that the stability of ferrous iron is a combined result of H ₂O effect and high pressure. The stability of ferrous iron in bridgmanite under hydrous conditions would provide an explanation for the nature of the low-shear-velocity anomalies in the deep lower mantle. In addition, entrainment from a hydrous dense layer may influence mantle plume dynamics and contribute to variations in the redox conditions of the mantle.

Abstract

The large low-shear-velocity anomalies in the deep lower mantle below 2300 km depth may relate to H ₂O-induced ferrous iron stability in bridgmanite, according to laser-heated diamond anvil cell experiments.