Mineralogy of the deep lower mantle in the presence of H ₂O

Abstract

Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800–2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H ₂O to form a previously unknown (Mg, Fe)O ₂H _x ( x ≤ 1) phase. The (Mg, Fe)O ₂H _x has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg _0.6Fe _0.4)O or beyond 2300 km for (Mg _0.7Fe _0.3)O. The (Mg, Fe)O ₂H _x is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg ²⁺, Fe ³⁺ and H ⁺). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.