Elasticity and rheology of iron above 220 GPa and the nature of the Earth's inner core

Abstract

Recent numerical-modelling and seismological results have raised new questions about the dynamics and magnetism of the Earth's core. Knowledge of the elasticity and texture of iron at core pressures is crucial for understanding the seismological observations, such as the low attenuation of seismic waves, the low shear-wave velocity and the anisotropy of compressional-wave velocity. The density and hulk modulus of hexagonal- close-packed iron have been previously measured to core pressures by static and dynamic methods. Here we study, using radial X-ray diffraction and ultrasonic techniques, the shear modulus, single-crystal elasticity tensor, aggregate compressional- and shear-wave velocities, and orientation dependence of these velocities in iron. The inner core shear-wave velocity is lower than the aggregate shear-wave velocity of iron, suggesting the presence of low-velocity components or anelastic effects in the core. Observation of a strong lattice strain anisotropy in iron samples indicates a large (~24%) compressional-wave anisotropy under the isostress assumption, and therefore a perfect alignment of crystals would not be needed to explain the seismic observations. Alternatively the strain anisotropy may indicate stress variation due to preferred slip systems.