High-P,T Elasticity of Hcp Iron: Reinvestigation of the Applicability of Hcp Iron to the Earth’s Inner Core

Abstract:

Earth’s inner core (329~364 GPa and 5000~6000 K) is thought to be composed of hexagonal closed pack (hcp) structured solid Fe-Ni alloy (e.g., Mao et al., 1998; Kuwayama et al., 2008; Sha & Cohen, 2010). Thermoelasticity of hcp (ε) iron is therefore a key to interpreting seismological information of the inner core: density, seismic wave velocities, and their anisotropy. However, several studies reported that hcp iron has a shear modulus distinctly larger than that of the inner core (e.g., Mao et al., 1998; Vocadlo et al., 2009). This large Poisson ratio of the inner core is one of the remaining inexplicable features of the deep Earth, and it suggests the presence of mechanisms to lower the S-wave velocity in the inner core, such as a low-velocity component (Prescher et al., 2015), pre-melting effect (Martorell et al., 2013), anelasticity, and so on.

In this study, we perform ab initio molecular dynamics simulations employing a supercell larger than in previous calculations (Vocadlo et al., 2009; Martorell et al., 2013). Also computations are conducted in a wide P,T range including, but not limited to, the inner core conditions to clarify the P,T effects on the elasticity of the hcp iron more comprehensively, and to provide an internally-consistent thermoelastic model. In addition to checking the validity of the Birch’s law, the obtained Poisson ratio and aggregate anisotropy, with and without the pre-melting effect, are compared against seismological constraints to reinvestigate the viability of hcp iron in the inner core.

Research supported by KAKENHI (JSPS) and the X-ray Free Electron Laser Priority Strategy Program (MEXT).