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

• Published in: Nature (London) Vol. 405; no. 6790; pp. 1044 - 1047
• Main Authors: Wenk, H.-R, Matthies, S, Hemley, R. J, Mao, H.-K, Shu, J
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 29-06-2000; Nature Publishing Group
• Subjects: Anisotropy; Crystals; Deformation; Earth; Earth core; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geology; Internal geophysics; Iron; Pressure; Rheology; Solid-earth geophysics, tectonophysics, gravimetry; X-ray diffraction; experimental studies; synchrotrons; rheology; anisotropy; X-ray diffraction; plasticity; preferred orientation; high pressure; plastic deformation; iron; diamond anvil cells; inner core; seismic waves
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/35016558

Soon after the discovery of seismic anisotropy in the Earth's inner core, it was suggested that crystal alignment attained during deformation
might be responsible. Since then, several other mechanisms
have been proposed to account for the observed anisotropy,
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 Earth. 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 -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.