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The plastic deformation of iron at pressures of the Earth's inner core

Nature volume 405, pages 1044–1047 (2000)Cite this article

Abstract

Soon after the discovery of seismic anisotropy in the Earth's inner core1, it was suggested that crystal alignment attained during deformation might be responsible2. Since then, several other mechanisms have been proposed to account for the observed anisotropy3,4, but the lack of deformation experiments performed at the extreme pressure conditions corresponding to the solid inner core has limited our ability to determine which deformation mechanism applies to this region of the Earth5. Here we determine directly the elastic and plastic deformation mechanism of iron at pressures of the Earth's core, from synchrotron X-ray diffraction measurements of iron, under imposed axial stress, in diamond-anvil cells. The ε-iron (hexagonally close packed) crystals display strong preferred orientation, with c-axes parallel to the axis of the diamond-anvil cell. Polycrystal plasticity theory predicts an alignment of c-axes parallel to the compression direction as a result of basal slip, if basal slip is either the primary or a secondary slip system. The experiments provide direct observations of deformation mechanisms that occur in the Earth's inner core, and introduce a method for investigating, within the laboratory, the rheology of materials at extreme pressures.

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Figure 1: Experimental set-up.
Figure 2: X-ray diffraction spectra of ε-iron recorded at 220 GPa and 300 K for different rotation angles, χ.
Figure 3: Inverse pole figures illustrating preferred orientation patterns of ε-iron deformed in axial compression.
Figure 4: Calculated activities of basal and prismatic slip in ε-iron as a function of compressive strain.

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Acknowledgements

We thank A. K. Singh, S. Merkel and J. Hu for discussions and assistance. The work was supported by IGPP-LANL and NSF.

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Authors and Affiliations

  1. Department of Geology and Geophysics University of California, Berkeley, 94720, California, USA

    H.-R. Wenk & S. Matthies

  2. Geophysical Laboratory and Center for High-Pressure Research, Carnegie Institution of Washington, Washington, 20015, DC, USA

    R. J. Hemley, H.-K. Mao & J. Shu

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Correspondence to H.-R. Wenk.

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Wenk, HR., Matthies, S., Hemley, R. et al. The plastic deformation of iron at pressures of the Earth's inner core . Nature 405, 1044–1047 (2000). https://doi.org/10.1038/35016558

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