The distribution, accumulation and circulation of oxygen and hydrogen in Earth’s interior dictate the geochemical evolution of the hydrosphere, atmosphere and biosphere1. The oxygen-rich atmosphere and iron-rich core represent two end-members of the oxygen–iron (O–Fe) system, overlapping with the entire pressure–temperature–composition range of the planet. The extreme pressure and temperature conditions of the deep interior alter the oxidation states1, spin states2 and phase stabilities3,4 of iron oxides, creating new stoichiometries, such as Fe4O5 (ref. 5) and Fe5O6 (ref. 6). Such interactions between O and Fe dictate Earth’s formation, the separation of the core and mantle, and the evolution of the atmosphere. Iron, in its multiple oxidation states, controls the oxygen fugacity and oxygen budget, with hydrogen having a key role in the reaction of Fe and O (causing iron to rust in humid air). Here we use first-principles calculations and experiments to identify a highly stable, pyrite-structured iron oxide (FeO2) at 76 gigapascals and 1,800 kelvin that holds an excessive amount of oxygen. We show that the mineral goethite, FeOOH, which exists ubiquitously as ‘rust’ and is concentrated in bog iron ore, decomposes under the deep lower-mantle conditions to form FeO2 and release H2. The reaction could cause accumulation of the heavy FeO2-bearing patches in the deep lower mantle, upward migration of hydrogen, and separation of the oxygen and hydrogen cycles. This process provides an alternative interpretation for the origin of seismic and geochemical anomalies in the deep lower mantle, as well as a sporadic O2 source for the Great Oxidation Event over two billion years ago that created the present oxygen-rich atmosphere.
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We thank Y. Fei for providing haematite powder samples; A. Goncharov for conducting the laser-heating treatment; and D. Z. Zhang, J.-F. Shu, J. Smith, Y. Kono, K. Yang, S. Yan, Z. H. Yu, Y. Yuan, M.Q. Hou and L. Xu for beamline technical support. XRD measurements were performed at the High Pressure Collaborative Access Team (HPCAT 16-IDB and 16-BMD) Advanced Photon Source (APS), Argonne National Laboratory, and the BL15U1 beamline, Shanghai Synchrotron Radiation Facility in China. Part of the experiments was performed at the 13BM-C experimental station of the GeoSoilEnviroCARS facility at the APS. HPCAT operations are supported by the DOE-NNSA under award number DE-NA0001974 and by the DOE-BES under award number DE-FG02-99ER45775, with partial instrumentation funding by the NSF. 13BM-C operation is supported by COMPRES through the Partnership for Extreme Crystallography (PX2) project, under NSF Cooperative Agreement EAR 11-57758. APS is supported by the DOE-BES, under contract number DE-AC02-06CH11357. Q.H. and H.-K.M. were supported by NSF grants EAR-1345112 and EAR-1447438. L.Z. was supported by the Foundation of President of China Academy of Engineering Physics (grant no. 201402032) and the National Natural Science Foundation of China (grant no. 41574080). This work was also supported in part by the National Natural Science Foundation of China (grant number U1530402).
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Journal of Earth Science (2018)