Letter | Published:

Spin crossover and iron-rich silicate melt in the Earth’s deep mantle

Nature volume 473, pages 199202 (12 May 2011) | Download Citation

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Abstract

A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation1,2. The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core–mantle boundary region3. Recent theoretical calculations4 combined with estimates of iron partitioning between (Mg,Fe)SiO3 perovskite and melt at shallower mantle conditions5,6,7 suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments8. Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than 76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg0.95Fe0.05)SiO3 glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO3 liquid becomes more dense than coexisting solid at 1,800 km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to 1,000 km in thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO3 perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core–mantle boundary region.

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Acknowledgements

We thank R. Sinmyo for support with TEM analyses and K. Shimizu for preparing the glass sample. Discussion with R. Caracas and comments from D. Frost were helpful. C.-C. Chen is acknowledged for XES measurements at BL12XU Taiwan Beamline, SPring-8. Some of the melting experiments were conducted at BL10XU (SPring-8 proposal no. 2009B0087). J.H. was supported by the National Science Foundation (NSFEAR0855737).

Author information

Affiliations

  1. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

    • Ryuichi Nomura
    • , Haruka Ozawa
    • , Shigehiko Tateno
    •  & Kei Hirose
  2. Department of Earth and Planetary Sciences, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan

    • Ryuichi Nomura
  3. Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa 237-0061, Japan

    • Haruka Ozawa
    •  & Kei Hirose
  4. Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA

    • John Hernlund
  5. Department of Materials, Physics and Energy Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan

    • Shunsuke Muto
  6. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

    • Hirofumi Ishii
    •  & Nozomu Hiraoka

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Contributions

R.N., H.O., S.T. and K.H. performed high-pressure experiments and chemical analyses. S.M. carried out the ELNES measurements. H.I. and N.H. supported the XES study at SPring-8. R.N., K.H. and J.H. wrote the paper. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Kei Hirose.

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https://doi.org/10.1038/nature09940

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