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

Jin Liu; Chenxu Wang; Chaojia Lv; Xiaowan Su; Yijin Liu; Ruilian Tang; Jiuhua Chen; Qingyang Hu; Ho-Kwang Mao; Wendy L Mao

doi:10.1093/nsr/nwaa096

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

Natl Sci Rev. 2020 May 9;8(4):nwaa096. doi: 10.1093/nsr/nwaa096. eCollection 2021 Apr.

Authors

Jin Liu¹, Chenxu Wang², Chaojia Lv¹, Xiaowan Su³, Yijin Liu⁴, Ruilian Tang¹, Jiuhua Chen⁵, Qingyang Hu¹, Ho-Kwang Mao¹, Wendy L Mao²

Affiliations

¹ Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094, China.
² Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA.
³ School of Earth and Space Sciences, Peking University, Beijing 100871, China.
⁴ SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA.
⁵ Center for Study of Matter at Extreme Conditions, Department of Mechanical and Materials Engineering, Florida International University, Miami, FL 33199, USA.

Abstract

As the reaction product of subducted water and the iron core, FeO₂ with more oxygen than hematite (Fe₂O₃) 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)₂O₃₊ _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 (Fe³⁺) as in Fe₂O₃ but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H₂O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO₂Hx 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.

Keywords: deep oxygen; hydrous phase; lower mantle; oxygen-excess oxides; subducting slab.