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Evidence for an oxygen-depleted liquid outer core of the Earth

Nature volume 479, pages 513–516 (2011)Cite this article

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Abstract

On the basis of geophysical observations, cosmochemical constraints, and high-pressure experimental data, the Earth’s liquid outer core consists of mainly liquid iron alloyed with about ten per cent (by weight) of light elements1,2. Although the concentrations of the light elements are small, they nevertheless affect the Earth’s core: its rate of cooling, the growth of the inner core, the dynamics of core convection, and the evolution of the geodynamo3,4. Several light elements—including sulphur, oxygen, silicon, carbon and hydrogen—have been suggested2, but the precise identity of the light elements in the Earth’s core is still unclear. Oxygen has been proposed as a major light element in the core on the basis of cosmochemical arguments and chemical reactions during accretion5,6. Its presence in the core has direct implications for Earth accretion conditions of oxidation state, pressure and temperature. Here we report new shockwave data in the Fe–S–O system that are directly applicable to the outer core. The data include both density and sound velocity measurements, which we compare with the observed density and velocity profiles of the liquid outer core. The results show that we can rule out oxygen as a major light element in the liquid outer core because adding oxygen into liquid iron would not reproduce simultaneously the observed density and sound velocity profiles of the outer core. An oxygen-depleted core would imply a more reduced environment during early Earth accretion.

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Figure 1: The density–pressure relationships for Fe92.5O2.2S5.3 and Fe90O8S2.
Figure 2: The bulk sound velocity as a function of density for Fe92.5O2.2S5.3 and Fe90O8S2.
Figure 3: Density versus pressure and bulk sound velocity for Fe92.5O2.2S5.3, Fe90O8S2, Fe90O0.5S9.5 and pure iron along the adiabatic geotherm, compared with the PREM model24.

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Acknowledgements

We thank C. D. Dai, J. D. Weng and Q. S. Wang from the Institute of Fluid Physics for help with the experiments, and Q. Shen from the Sate Key Lab of Advanced Technology for Materials Synthesis and Processing and C. H. Shen from the Materials Research and Testing Center for the preparation of the samples. We also thank C. Seagle, L. Zhang and V. Hillgren for comments. This work was supported by the National Natural Science Foundation of China (grant numbers 41074056 and 40604007), the Fundamental Research Funds for the Central Universities, and the National Basic Research of China (grant number 2010CB731600). Support by the National Science Foundation (grant number EAR-0809539 to Y.F.) and the Carnegie Institution of Washington is also acknowledged.

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Authors and Affiliations

  1. School of Sciences, Wuhan University of Technology, Wuhan, 430070, Hubei, China

    Haijun Huang, Fuqian Jing & Xiaojun Hu

  2. Geophysical Laboratory, Carnegie Institution of Washington, Washington, 20015, District of Columbia, USA

    Yingwei Fei

  3. School of Earth and Space Sciences, Peking University, Beijing, 100871, China

    Yingwei Fei

  4. National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China ,

    Lingcang Cai & Fuqian Jing

  5. Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550002, China ,

    Hongsen Xie

  6. State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China ,

    Lianmeng Zhang

  7. National Key Laboratory of Science and Technology on Reliability and Environment Engineering, Beijing Institute of Spacecraft Environment Engineering, Beijing, 100094, China

    Zizheng Gong

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Contributions

H.H. and Y.F. designed the study, analysed the data and wrote the manuscript. H.H. and X.H. conducted the experiments. L.C. made arrangements and preparations for the experiments. F.J., H.X., L.Z. and Z.G. contributed to discussions and made suggestions for the experiments.

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Correspondence to Yingwei Fei.

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The authors declare no competing financial interests.

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Huang, H., Fei, Y., Cai, L. et al. Evidence for an oxygen-depleted liquid outer core of the Earth. Nature 479, 513–516 (2011). https://doi.org/10.1038/nature10621

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