Letter

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

Received:
Accepted:
Published online:

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.

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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.

Author information

Author notes

    • Jin Liu
    •  & Qingyang Hu

    These authors contributed equally to this work.

Affiliations

  1. Department of Geological Sciences, Stanford University, Stanford, California 94305, USA

    • Jin Liu
    • , Qingyang Hu
    •  & Wendy L. Mao
  2. Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China

    • Qingyang Hu
    • , Duck Young Kim
    •  & Ho-Kwang Mao
  3. Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China

    • Zhongqing Wu
    •  & Wenzhong Wang
  4. High Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, Illinois 60439, USA

    • Yuming Xiao
    • , Paul Chow
    •  & Yue Meng
  5. Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60437, USA

    • Vitali B. Prakapenka
  6. Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA

    • Ho-Kwang Mao
  7. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Wendy L. Mao

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Contributions

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.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

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

Reviewer Information Nature thanks M. Walter and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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