Wide-ranged multiphase equation of state for iron and model variations addressing uncertainties in high-pressure melting

Abstract

We describe the construction of a wide-ranged multiphase equation of state for elemental iron, consisting of four solid phases and one fluid phase. The free-energy models for the phases are constrained by fitting to a broad swath of ambient, static, and dynamic high-pressure experimental data, including recent ramp-compression and shock--ramp recrystallization experiments. In order to better describe the conditions near Earth's inner-core boundary, ab initio electronic-structure calculations of various types have been carried out for the hcp and liquid phases and used as additional constraints. Given the variability of reported iron melting temperatures in the 1--3 Mbar range, we construct EOS model variations based on two separate ${T}_{\mathrm{melt}}(P)$ curves: lower-${T}_{\mathrm{melt}}$ and higher-${T}_{\mathrm{melt}}$. Our analysis shows that only the higher-${T}_{\mathrm{melt}}(P)$ data are consistent with reported shock-melting pressure measurements. Furthermore, our examination of the recrystallization data from a recent shock--ramp x-ray diffraction study motivates a family of EOS models that address experimental uncertainties and provide a range of possible ${T}_{\mathrm{melt}}(P)$ curves that have a somewhat lower melt temperature above 5 Mbar than the one reported in that study. This family of multiphase EOS models allow us to suggest new experiments to further reduce the uncertainties on the high-pressure melt curve, and they should also enable more accurate predictions of complex high-$P,T$ processes involving iron. We show their potential utility by applying these EOSs to the investigation of properties of interest to planetary science, such as the vaporization entropy, critical temperature and density, Earth inner-core boundary temperature plus inner- and outer-core density deficits.

Topics & Concepts