Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones

Abstract

Ultralow-velocity zones (ULVZs) at Earth’s core–mantle boundary region have important implications for the chemical composition and thermal structure of our planet, but their origin has long been debated1,2,3. Hydrogen-bearing iron peroxide (FeO2Hx) in the pyrite-type crystal structure was recently found to be stable under the conditions of the lowermost mantle4,5,6. Using high-pressure experiments and theoretical calculations, we find that iron peroxide with a varying amount of hydrogen has a high density and high Poisson ratio as well as extremely low sound velocities consistent with ULVZs. Here we also report a reaction between iron and water at 86 gigapascals and 2,200 kelvin that produces FeO2Hx. This would provide a mechanism for generating the observed volume occupied by ULVZs through the reaction of about one-tenth the mass of Earth’s ocean water in subducted hydrous minerals with the effectively unlimited reservoir of iron in Earth’s core. Unlike other candidates for the composition of ULVZs7,8,9,10,11,12, FeO2Hx synthesized from the superoxidation of iron by water would not require an extra transportation mechanism to migrate to the core–mantle boundary. These dense FeO2Hx-rich domains would be expected to form directly in the core–mantle boundary region and their properties would provide an explanation for the many enigmatic seismic features that are observed in ULVZs1,13,14.

Figure 1: XRD pattern of reaction products of iron and water.
Figure 2: Comparison of the pressure–volume relationships for FeO2Hx at 300 K.
Figure 3: NRIXS data for FeO2Hx and determination of VD at 133 GPa.
Figure 4: Schematic diagram of the formation of FeO2Hx-bearing domains at the CMB.

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Acknowledgements

We thank A. Shahar for providing the 57Fe-enriched hematite (Fe2O3 with 57Fe of >96.5%) powder samples. We acknowledge C. Kenney-Benson, L. X. Yang, T. T. Gu, B. Li, W. G. Yang, Y. Wu, B. Chen, E. Ohtani, X. Y. Tong and M. M. Li for experimental assistance and discussion, and E. Greenberg for beamline technical support. NRIXS and XRD measurements were performed at the High Pressure Collaborative Access Team (HPCAT 16-IDB and 16-IDD), Advanced Photon Source, Argonne National Laboratory. Some of the XRD experiments were performed at GeoSoilEnviroCARS (Sector 13ID-D) at the Advanced Photon Source. HPCAT operations are supported by the Department of Energy (DOE)-NNSA under award DE-NA0001974, with partial instrumentation funding by NSF. Y.X., P.C. and Y.M. acknowledge the support of the DOE-BES/DMSE under award DE-FG02-99ER45775. GeoSoilEnviroCARS is supported by the National Science Foundation (NSF)—Earth Sciences (EAR-1128799) and the DOE Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the US DOE, Office of Science, Office of Basic Energy Sciences, under contract number DE-AC02-06CH11357. W.L.M., Q.H. and J.L. acknowledge support from the Geophysics Program by the NSF (EAR 1446969) and the Deep Carbon Observatory. H.-K.M. and Q.H. were supported by NSF grants EAR-1345112 and EAR-1447438. This work was also partially supported by the National Natural Science Foundation of China (grant number U1530402). Some of the computations were conducted at the Supercomputing Center of the University of Science and Technology of China. Z.W. and W.W. acknowledge the support of the Natural Science Foundation of China (41590621) and State Key Development Program of Basic Research of China (2014CB845905). We thank M. Walter for comments and suggestions.

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J.L., Q.H., Y.X., Y.M., V.B.P. and P.C. carried out the experiment. J.L., W.L.M. and H.-K.M. performed the experimental data analysis. D.Y.K., Q.H., Z.W. and W.W. performed the theoretical simulation. H.-K.M. and W.L.M. conceived and designed the project and directed the calculations and experiments. J.L., W.L.M. and H.-K.M. wrote the manuscript. All authors contributed to the discussion of the results and revision of the manuscript.

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Correspondence to Ho-Kwang Mao or Wendy L. Mao.

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Extended data figures and tables

Extended Data Figure 1 XRD pattern of FeO2Hx at 133 GPa and 2,600 K.

The sharp diffraction lines in the integrated pattern (right panel) are from the synthesized FeO2Hx and H2O (ice X). FeO2Hx was first synthesized from Fe2O3 and H2O at 90–100 GPa and 2,000–2,200 K. XRD patterns were collected with increasing pressure up to 133 GPa. At each increasing pressure step, the sample was laser annealed to 2,000 K to release any possible deviatoric strain that may have developed. Miller indices (hkl) for FeO2Hx in the pyrite structure are labelled above corresponding peaks. Green arrows indicate locations of peaks for H2O in the ice X structure. The inset shows a photomicrograph of the sample chamber at 133 GPa after laser heating at 2,600 K. The sample was surrounded by a cubic boron nitride (cBN) gasket insert. A two-dimensional XRD image (left panel) of FeO2Hx was collected at 133 GPa and 300 K after laser heating, which corresponds to the integrated XRD pattern. Source Data for Extended Data Figs 1, 2, 3, 4 accompany the online version of the paper. Source data

Extended Data Figure 2 Sound velocities of FeO2Hx at high pressure and 300 K.

Filled symbols show the VP and VS of FeO2 at 81 GPa (black) and FeO2Hx at 133 GPa (blue) from NRIXS experiments; solid and dashed curves show FeO2 (black) and FeO2H (red) as a function of pressure from theoretical calculations. All error bars are derived from standard error propagation and represent ±1σ. Error bars smaller than the symbols are not shown for clarity. Source data

Extended Data Figure 3 Temperature effects on VP and VS of FeO2 and FeO2H.

Sound velocities of FeO2 (a, b) and FeO2H (c, d) at high pressure and high temperature obtained by theoretical calculations. Source data

Extended Data Figure 4 Density of FeO2Hx as a function of temperature at CMB pressures.

Diamonds show FeO2Hx at 133 GPa from XRD experiments; black and red curves show FeO2 and FeO2H at 130 GPa, respectively, from theoretical calculations. All error bars are derived from standard error propagation and represent ±1σ. Error bars of the density derived from XRD experiments are smaller than the symbol. Source data

Extended Data Table 1 Sound velocities of FeO2 and FeO2Hx at high pressure

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Liu, J., Hu, Q., Young Kim, D. et al. Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones. Nature 551, 494–497 (2017). https://doi.org/10.1038/nature24461

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