Pre-Melting in Iron and Iron Alloys at Earth’s Core Conditions: Results from Ab Initio Molecular Dynamics Calculations

Abstract:

Seismically determined S-wave velocities in the Earth’s inner core are observed to be much lower (10-30%) than those generally inferred from mineral physics. This is a remarkably large discrepancy - mineralogical models for the mantle and the outer core match the observed velocities to around 1%. In no other large volume of the Earth does such a difference exist. There have been a number of arguments put forward over the years to account for the difference, but none have been universally accepted and our inability to explain the seismic velocities of the inner core remains an uncomfortable truth.

Here, we present results from ab initio molecular dynamics calculations performed at 360 GPa and core temperatures on hcp and fcc iron, and on fcc-Fe alloyed with nickel and hcp-Fe alloyed with silicon. The calculated shear modulus, and therefore seismic velocities, of pure hcp-Fe reduces dramatically just prior to melting, providing an elegant explanation for the observed velocities. Calculations on fcc-Fe show no such strong reduction in V_S, with a transformation to an hcp-type structure prior to melting; addition of 6.5 atm% and 13 atm% Ni to fcc-Fe raises the temperature of this transition.

When silicon is added to hcp-Fe, the pre-melting behaviour is found to be very similar to that of pure hcp-Fe with a strong nonlinear shear weakening just before melting and a corresponding reduction in V_S. Because temperatures range from T/Tm = 1 at the inner-outer core boundary to T/Tm ≈ 0.99 at the centre, this strong nonlinear effect on V_S should occur in the inner core, providing a compelling explanation for the low V_S observed.