Interaction between water and ferropericlase in Earth’s lower mantle

Abstract:

The water budget in Earth’s lower mantle is still under investigation. However, even minor presence of water will induce significant effects on the structural and physicochemical properties of minerals. Here, we conduct high pressure-temperature experiments to mimick the conditions between ~1400-2890 km depth of the Earth along relatively cold geotherm. We observe ferropericlase, the second most abundant mineral in the lower mantle, reacts with H₂O to form new hexagonal and pyrite-type phases. The hexagonl phase has a composition of (Mg,Fe)₄O_7±0.33 and it becomes the stable phase approximately from 1400 km depth. The hydrogen bearing pyrite-type (Mg,Fe)O₂Hx has the same structure as the pyrite-FeO₂Hx and is preferrably stable at higher pressures (e.g. > 80 GPa). Both phases show excessive amount of oxygen and we found they coexist with the dominant silicate phases like bridgmanite and post-perovskite. Those exotic reactions between water and ferropericlase suggest the redox state in the lower mantle can be locally heterogenuous. Products from water chemistry can potentially formulate deep-oxygen reservoirs, cause seismic anomalies and modulate water budget in the lower mantle.