Strong Premelting Effect in the Elastic Properties of hcp-Fe Under Inner-Core Conditions

Abstract

The observed shear-wave velocity VS in Earth's core is much lower than expected from mineralogical models derived from both calculations and experiments. A number of explanations have been proposed, but none sufficiently explain the seismological observations. Using ab initio molecular dynamics simulations, we obtained the elastic properties of hexagonal close-packed iron (hcp-Fe) at 360 gigapascals up to its melting temperature Tm. We found that Fe shows a strong nonlinear shear weakening just before melting (when T/Tm > 0.96), with a corresponding reduction in VS. Because temperatures range from T/Tm = 1 at the inner-outer core boundary to T/Tm ≈ 0.99 at the center, this strong nonlinear effect on VS should occur in the inner core, providing a compelling explanation for the low VS observed.

Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models

“…In particular, our results argue for combined effects of high temperature and inner core impurities (such as nickel and light elements) that slightly reduce V P of pure hcp-Fe at constant density. This can be potentially satisfied by assuming that both alloying and high temperature only very moderately lower V P , which, however, is not supported by recent results (Badro et al, 2007;Antonangeli et al, 2010;Sha and Cohen 2010;Mao et al 2012;Martorell et al, 2013). Alternatively, and most likely, effects of impurities (combined effect of nickel and light elements) and high temperature counterbalance, the first increasing velocity at constant density (e.g.…”

“…This is because anharmonic and/or pre-melting effects at high temperature (e.g. Laio et al, 2000;Vočadlo et al, 2009;Sha and Cohen, 2010;Martorell et al, 2013), as well as the possible presence of fluid inclusion in the core (Vočadlo, 2007), or frequency-dependent viscoelastic relaxations (Jackson et al, 2000), are all expected to more strongly influence V S than V P .…”

Molybdenum sound velocity and shear modulus softening under shock compression

“…Shear-wave velocity in the Earth's inner core also decreases near the melting pressuretemperature. Martorell et al [36] attributed this decrease in elastic properties of hcp-Fe. For Mo in particular, our sound speed results are consistent with calculations by Cazorla et al [17] showing that the bcc phase is more stable than the fcc phase up to 3.5 Mbars.…”

Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe 7 C 3

“…Understanding the elastic anisotropy and rheology of hot and dense Fe 7 C 3 would allow further tests of our model. For an inner core consisting of Fe 7 C 3 , it is unnecessary to invoke partial melting (3) or the postulated large temperature effect (32,33) to explain the anomalous seismic properties. A fully frozen Fe 7 C 3 inner core would have unique properties and dynamics from one that contains partial melt.…”