Simon Hunt, Andrew Walker , Oliver Lord, Stephen Stackhouse, Lewis Schardong, Lora Armstrong, Andrew Parsons, Geoffrey Lloyd, Matthew Whitaker

The Earth’s inner core exhibits significant anisotropy in both seismic velocity and attenuation as well as hemispherical and depth variations. These observations point to an inner core that is both complex and dynamic. However, interpretation of these observations without knowledge of the attenuation processes active in the inner-core is difficult. To address this we have used zinc, as a low-pressure analogue of the hexagonal close pack (hcp) structured iron that forms the inner core, to provide first-order constraints on the anelasticity of hcp metals at seismic frequencies and high temperatures. A D-DIA apparatus was combined with X-radiography were utilised to measure the anelastic response of zinc up to a homologous temperature (T/Tm) of ≈ 0.8. To analyse the data we developed an improved image processing method that reduces systematic errors and gives up to 3 orders of magnitude improvement in strain measurement precision. Using this algorithm, in the frequency range 0.1 to 0.003Hz, significant dissipation and softening of zinc’s Young’s modulus is observed. The softening occurs in the absence of significant impurities or a fluid phase and appears to be caused by, or related to, recrystallisation of the samples in response to the stress. The recrystallisation results in a steady-state grain-size and low dislocation density. The predicted reduction in shear wave speed is 2-3 times greater than that of for compressional waves, which is consistent with anelasticity playing a significant role in the seismic velocity of the inner core. Therefore anelastic effects in hcp iron must be considered in the interpretation of inner-core.