FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles

Abstract

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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Figure 1: The FeO2 phase.
Figure 2: Integrated XRD pattern and Rietveld refinement of the pyrite-type FeO2 phase.
Figure 3: Raman peak of the hydrogen Q1 vibron in dense neon.
Figure 4: Crystal structure search results.

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Acknowledgements

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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Authors

Contributions

W.Y., L.Y. and Q.H. carried out the experiment. Y.M. conducted the laser heating for the first synthesis of FeO2. W.Y., L.Z., Q.H. and H.-K.M. performed the experimental data analysis. D.Y.K. predicted the FeO2 structure and performed the computer simulation. H.-K.M. conceived and designed the project and directed the calculations and experiments. Q.H. and H.-K.M. wrote the manuscript. All authors contributed to the discussion of the results and revision of the manuscript.

Corresponding author

Correspondence to Ho-Kwang Mao.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Bonding lengths and angles in pyrite-type FeO2 at 76 GPa.

The structure is viewed along the x axis of the experimental FeO2 unit cell.

Extended Data Figure 2 XRD pattern series by decompressing the P-phase.

The P-phase becomes weak in intensity at 41 GPa and totally disappears at 31 GPa and below. The decompressed sample eventually recovered to the α-Fe2O3 phase at low pressure. P indicates the P-phase FeO2; O indicates solid O2; H indicates α-Fe2O3 (haematite). Fe2O3 contains post-perovskite type and Aba2-structured high-pressure phases.

Extended Data Figure 3 Synthesis pressuretemperature conditions for FeO2.

a, Open circles indicate the coexistence of Fe2O3 and O2. Solid squares indicate the appearance of P-phase FeO2. FeO2 was synthesized between 72 GPa and 75 GPa. b, Open circles indicate FeOOH. Decomposition pressure was constrained between 78 GPa and 87 GPa. Sample temperature was measured using the spectroradiometric method and errors are estimated from the goodness of fit to the spectroradiometric profile.

Extended Data Figure 4 XRD patterns of FeOOH through the experimental pressuretemperature path.

a, Goethite sample (G) in neon and Re gasket at 0.9 GPa. b, As in a, compressed to 35 GPa. The XRD peaks include goethite, solidified Ne, and the Re gasket. c, As in a, compressed to 92 GPa. The sample peaks remain and shift to higher Q. d, After laser-heating and quenching to ambient temperature, the pressure dropped to 87 GPa, and the goethite peaks disappeared. The new pattern consists of peaks of the P-phase, Ne and minor amounts of ε-FeOOH (red stars).

Extended Data Figure 5 Phonon dispersion relations of FeO2 P-phase.

a, At 0 GPa; b, At 300 GPa. The P-phase is mechanically stable at 0 GPa and 300 GPa.

Extended Data Table 1 Additional multigrain XRD data was obtained at 13BM-C
Extended Data Table 2 The lattice parameters and atomic positions of pyrite-type FeO2 at 76 GPa
Extended Data Table 3 Bonding lengths and angles in pyrite-type FeO2 at 76 GPa

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Hu, Q., Kim, D., Yang, W. et al. FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles. Nature 534, 241–244 (2016). https://doi.org/10.1038/nature18018

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