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

Soon after the discovery of seismic anisotropy in the Earth's inner core(1) , it was suggested that crystal alignment attained during deformation might be responsible(2). Since then, several other mechanisms have been proposed to account for the observed anisotropy(3,4), but the lack of deformation e xperiments performed at the extreme pressure conditions corresponding to th e solid inner core has limited our ability to determine which deformation m echanism applies to this region of the Earth(5). 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, unde r imposed axial stress, in diamond-anvil cells. The epsilon-iron (hexagonal ly close packed) crystals display strong preferred orientation, with c-axes parallel to the axis of the diamond-anvil cell. Polycrystal plasticity the ory predicts an alignment of c-axes parallel to the compression direction a s a result of basal slip, if basal slip is either the primary or a secondar y slip system. The experiments provide direct observations of deformation m echanisms that occur in the Earth's inner core, and introduce a method for investigating, within the laboratory, the rheology of materials at extreme pressures.