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.

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