Letter | Published:

Crystallization of silicon dioxide and compositional evolution of the Earth’s core

Nature volume 543, pages 99102 (02 March 2017) | Download Citation

Abstract

The Earth’s core is about ten per cent less dense than pure iron (Fe), suggesting that it contains light elements as well as iron. Modelling of core formation at high pressure (around 40–60 gigapascals) and high temperature (about 3,500 kelvin) in a deep magma ocean1,2,3,4,5 predicts that both silicon (Si) and oxygen (O) are among the impurities in the liquid outer core6,7,8,9. However, only the binary systems Fe–Si and Fe–O have been studied in detail at high pressures, and little is known about the compositional evolution of the Fe–Si–O ternary alloy under core conditions. Here we performed melting experiments on liquid Fe–Si–O alloy at core pressures in a laser-heated diamond-anvil cell. Our results demonstrate that the liquidus field of silicon dioxide (SiO2) is unexpectedly wide at the iron-rich portion of the Fe–Si–O ternary, such that an initial Fe–Si–O core crystallizes SiO2 as it cools. If crystallization proceeds on top of the core, the buoyancy released should have been more than sufficient to power core convection and a dynamo, in spite of high thermal conductivity10,11, from as early on as the Hadean eon12. SiO2 saturation also sets limits on silicon and oxygen concentrations in the present-day outer core.

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Acknowledgements

We thank H. Ozawa, Y. Kidokoro and K. Yonemitsu for their assistance in high high-pressure, high-temperature experiments. Discussions with J. Badro and R. Deguen helped develop the model for core energetics.

Author information

Affiliations

  1. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8550, Japan

    • Kei Hirose
    • , Ryosuke Sinmyo
    • , Koichio Umemoto
    • , John Hernlund
    •  & George Helffrich
  2. Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR CNRS 7590, Sorbonne Universités—Université Pierre et Marie Curie, CNRS, Muséum National d’Histoire Naturelle, IRD, 4 Place Jussieu, 75005 Paris, France

    • Guillaume Morard
  3. Université de Lyon, École normale supérieure de Lyon, Université Lyon-1, CNRS, UMR 5276 LGL-TPE, F-69364 Lyon, France

    • Stéphane Labrosse

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Contributions

K.H. and G.M. designed the project. K.H., G.M. and R.S. performed experiments, K.U. carried out ab initio calculations, G.H. developed the thermodynamic model, and J.H. and S.L. did the dynamical modelling. The manuscript was written by all authors.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Kei Hirose.

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

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https://doi.org/10.1038/nature21367

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