Anelasticity of the HCP Metal Zinc: a Key to Understanding the Dynamics of Earth’s Core

Wednesday, 17 December 2014
Simon A Hunt, University College London, London, United Kingdom, Andrew Walker, University of Bristol, Bristol, United Kingdom, Oliver T Lord, University of Bristol, Bristol, BS8, United Kingdom, Stephen Stackhouse, University of Leeds, School of Earth and Environment, Leeds, United Kingdom, Lora S Armstrong, University of Minnesota Twin Cities, Minneapolis, MN, United States, Andrew J Parsons, University of Leeds, Leeds, LS2, United Kingdom, Geoffrey E Lloyd, University of Leeds, COMET, School of Earth and Environment, Leeds, United Kingdom and Matthew L Whitaker, Mineral Physics Institute, Stony Brook University, Stony Brook, NY, United States
The solid inner core is the most remote and inaccessible part of our planet but its structure and composition may provide a key record needed to reveal the timing and nature of the onset of Earth’s protective magnetic field generated and even of long-term changes in the way the mantle convects driving surface dynamics. Key to developing our understanding of the inner core is our ability to use seismic observations to constrain its structure on all scales. Seismic wave velocities are mostly sensitive to the atomic scale crystal structure, temperature and composition. On a larger scale the microstructure of the inner core, reflecting its deformation and crystallization history, can be probed by seismic studies of elastic anisotropy and anelasticity [Makinen & Deuss (2013) Geophys. J. Int. 194:401]. The inner core is at temperatures in excess of ~0.95Tm and interpretation of the properties and history of the inner core must therefore include careful understanding of the anelastic properties of HCP iron and its alloys.

The most recent study of the anelasticity of iron and iron alloys is now over a decade old [Jackson et al., (2000) J. Geophys. Res. 105:23605] and is limited to low pressure where iron adopts the body centered cubic (BCC) or face centered cubic (FCC) structure. It is now widely, although not universally, accepted that iron in the core adopts the hexagonally close packed (HCP) epsilon-iron structure stable above 10 GPa and there are currently no results that reveal the anelasticity of this core-forming phase.

We have used Zinc as a low pressure analogue for HCP-iron and measured its anelastic response as a function of frequency (periods 10-300s), temperature and pressure (P<7GPa). Our experiments use the D-DIA to apply a sinusoidally varying strain to the sample and a corundum elastic standard. We image changes in length of the sample and standard in response to the driving strain X-radiographically. The amplitude and phase of sample length change relative to that of the elastic standard gives us the effective Young’s modulus (amplitude) and internal friction (frequency dependent phase lag). Above ~0.7Tm, we observe significant reduction in the sample’s effective Young’s modulus and an increase in internal friction; both of these are frequency dependent.