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Strength of iron at core pressures and evidence for a weak Earth’s inner core

Nature Geoscience volume 6, pages 571–574 (2013)Cite this article

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Abstract

The strength of iron at extreme conditions is crucial information for interpreting geophysical observations of the Earth’s core and understanding how the solid inner core deforms1,2. However, the strength of iron, on which deformation depends, is challenging to measure and accurately predict at high pressure. Here we present shear strength measurements of iron up to pressures experienced in the Earth’s core. Hydrostatic X-ray spectroscopy and non-hydrostatic radial X-ray diffraction measurements of the deviatoric strain in hexagonally close-packed iron uniquely determine its shear strength to pressures above 200 GPa at room temperature. Applying numerical modelling of the rheologic behaviour of iron under pressure3, we extrapolate our experimental results to inner-core pressures and temperatures, and find that the bulk shear strength of hexagonally close-packed iron is only ∼ 1 GPa at the conditions of the Earth’s centre, 364 GPa and 5,500 K. This suggests that the inner core is rheologically weak, which supports dislocation creep as the dominant creep mechanism influencing deformation.

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Figure 1: Shear modulus for hcp iron derived from NRIXS phonon density of states data collected under hydrostatic conditions.
Figure 2: Caked rXRD pattern of hcp iron at 113 GPa (λ = 0.4246 Å) showing detector azimuth (°) versus 2θ (°).
Figure 3: Strength of hcp iron versus pressure and extrapolated to core temperatures.
Figure 4: Average dislocation velocity and strain rate as a function of applied stress τ.

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Acknowledgements

The authors are supported by the Geophysics Program at NSF (EAR0738873). Portions of this work were performed at Sectors 16-ID-D, 16-BM-D and 3-ID-B within XOR, Advanced Photon Source, ANL supported by the Office of Basic Energy Sciences of the US Department of Energy and by NSF Division of Materials Research under DE-AC02-06CH11357 and W-31-109-Eng-38. Beamline 12.2.2 of the Advanced Light Source, LBNL is supported by the Office of Basic Energy Sciences, US Department of Energy, under DE-AC02-05CH11231 and in part by COMPRES through NSF. The authors are grateful for discussion and assistance from H-K. Mao (Geophysical Laboratory), L. Gao (APS), J-F. Shu (Geophysical Laboratory) and L. Miyagi (University of Montana). Special thanks to B. Buffett (University of California, Berkeley) for invaluable revision insight.

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Authors and Affiliations

  1. Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA

    A. E. Gleason & W. L. Mao

  2. Photon Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    W. L. Mao

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  1. A. E. Gleason
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Contributions

A.E.G. and W.L.M. contributed equally to this work. Both authors conducted the experiments, discussed the results and implications and commented on the manuscript at all stages. A.E.G. analysed the data and wrote the paper.

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Correspondence to A. E. Gleason.

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Gleason, A., Mao, W. Strength of iron at core pressures and evidence for a weak Earth’s inner core. Nature Geosci 6, 571–574 (2013). https://doi.org/10.1038/ngeo1808

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