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Deep penetration of molten iron into the mantle caused by a morphological instability

Nature volume 492, pages 243–246 (2012)Cite this article

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Abstract

The core–mantle boundary of Earth is a region where iron-rich liquids interact with oxides and silicates in the mantle1. Iron enrichment may occur at the bottom of the mantle, leading to low seismic-wave velocities and high electrical conductivity2,3,4,5, but plausible physical processes of iron enrichment have not been suggested. Diffusion-controlled iron enrichment is inefficient because it is too slow6, although the diffusion can be fast enough along grain boundaries for some elements7. More fundamentally, experimental studies and geophysical observations show that the core is under-saturated with oxygen, implying that the mantle next to the core should be depleted in FeO. Here we show that (Mg,Fe)O in contact with iron-rich liquids leads to a morphological instability, causing blobs of iron-rich liquid to penetrate the oxide. This morphological instability is generated by the chemical potential gradient between two materials when they are not in bulk chemical equilibrium, and should be a common process in Earth’s interior. Iron-rich melt could be transported 50 to 100 kilometres away from the core–mantle boundary by this mechanism, providing an explanation for the iron-rich regions in the mantle.

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Figure 1: Distribution of Mg and Fe in the annealed couples of molten Fe and solid (Mg,Fe)O.
Figure 2: Instability wavelength λ plotted against thickness of chemical boundary layer L.
Figure 3: Effective diffusivity D MRL corresponding to the migration of iron-rich liquid blobs compared with the Fe–Mg interdiffusion coefficient in (Mg,Fe)O.
Figure 4: Influence of liquid iron blobs on geophysically observable properties.

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Acknowledgements

We are grateful to Z. Du, T. Hiraga, T. Kawazoe, K. Tsuno and T. Yoshino for discussions and J. Eckert, G. Amulele, Z. Du, D. Wang and R. Farla for technical support. We thank M. Zuber, J. Schubert and S. Peale for a discussion on Mercury. B. Buffett provided a useful comment on the magnetic coupling between the core and the mantle. B. Watson provided constructive criticism. This research was financially supported by the National Science Foundation under grant number EAR-0809330.

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  1. Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA,

    Kazuhiko Otsuka & Shun-ichiro Karato

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  1. Kazuhiko Otsuka
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  2. Shun-ichiro Karato
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Contributions

Experimental studies were conducted by K.O. Theoretical interpretation and geophysical applications were done by both K.O. and S.K. Both authors wrote the paper.

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Correspondence to Shun-ichiro Karato.

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The authors declare no competing financial interests.

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Otsuka, K., Karato, Si. Deep penetration of molten iron into the mantle caused by a morphological instability. Nature 492, 243–246 (2012). https://doi.org/10.1038/nature11663

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