Chemical reaction between ferropericlase (Mg,Fe)O and water under high pressure-temperature conditions of the deep lower mantle

The presence of water may contribute to compositional heterogeneities observed in the deep lower mantle. Mg-rich ferropericlase (Fp) (Mg,Fe)O in the rock-salt structure is the second most abundant phase in a pyrolitic lower mantle model. To constrain water storage in the deep lower mantle, experiments on the chemical reaction between (Mg,Fe)O and H2O were performed in a laser-heated diamond-anvil cell at 95-121 GPa and 2000-2250 K, and the run products were characterized combining in situ synchrotron X-ray diffraction measurements with ex-situ chemical analysis on the recovered samples. The pyrite-structured phase FeO2Hx (x <= 1, Py-phase) containing a negligible amount of Mg (< 1 at%) was formed at the expense of iron content in the Fp-phase through the reaction between (Mg,Fe)O and H2O, thus serving as water storage in the deepest lower mantle. The formation and segregation of nearly Mg-free Py-phase to the base of the lower mantle might provide a new insight into the deep oxygen and hydrogen cycles.

Automated summary

The presence of water may cause compositional heterogeneities in the deep lower mantle. Experiments on the chemical reaction between (Mg,Fe)O and H2O confirmed the formation of FeO2Hx (x ≤ 1, Py-phase) in the deepest lower mantle, serving as a water storage.