Primary slip system of epsilon-iron and anisotropy of the Earth's inner core

Models which attribute the seismic anisotropy of the Earth's inner core to lattice preferred orientation due to plastic deformation, rely on the assum ption that the slip systems are the same for hexagonal epsilon-iron and for titanium, taken as an analogue because of a similar c/a ratio. However, me tals with similar c/a ratio may predominantly slip on different planes. In the absence of experimental deformation data on epsilon-iron, we have estim ated its primary slip system using a theoretical criterion which has been s hown to give correct results in all the cases where it has been tested. The criterion depends on the values of the stacking fault energies on basal an d prism planes, which we have calculated from first principles. We find it probable that epsilon-iron predominantly slips on the basal plane, rather t han on the prism planes as titanium. The consequences on the seismic anisot ropy of the preferred orientation caused by plastic deformation are analyze d. It is suggested that the observed seismic anisotropy is unlikely to be p roduced by low-order convection in the inner core. (C) 1999 Elsevier Scienc e B.V. All rights reserved.