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As the reaction product of subducted water and the iron core, FeO 2  with more oxygen than hematite (Fe 2 O 3 ) has been recently recognized as an important component in the D” layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe) 2 O 3+   δ   (0 <  δ  < 1, denoted as ‘OE-phase’). It forms at pressures greater than 40 gigapascal when (Mg, Fe)-bearing hydrous materials are heated over 1500 kelvin. The OE-phase is fully recoverable to ambient conditions for  ex situ  investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe 3+ ) as in Fe 2 O 3  but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H 2 O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO 2 H x  which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1000 km in the Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial or subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.

Evidence for oxygenation of Fe-Mg oxides at mid-mantle conditions and the rise of deep oxygen