Shock temperatures and melting curve of an Fe–Ni–Cr alloy up to 304 GPa

Abstract

The melting temperatures of Fe–Ni alloys and their densities in the liquid state at relevant pressure–temperature (P–T) conditions present in the core are of great importance for understanding the composition and thermal structure of the Earth's core. We measured shock temperatures of the Fe–11Ni–18Cr (wt. %) alloy up to ∼304 GPa using a special target configuration, a quasi-spectral pyrometer, and velocimeter diagnostics in a two-stage light-gas gun. The present results show that Fe–11Ni–18Cr starts to melt at the pressure of 210 (8) GPa and 4700 (300) K and completes at the pressure of 280 (10) GPa and 5250 (350) K under shock loading, which is ∼1000 K lower than some previous shock temperature measurements. The melting temperatures of the Fe–11Ni–18Cr alloy are 4100 (250) K and 5500 (450) K at the pressures present at the core-mantle boundary (∼136 GPa) and inner-core boundary (∼330 GPa), respectively, which are slightly lower than that of pure iron. Combined with the previous results of the pressure–density measurements at Hugoniot states, our results indicate that the presence of limited amounts of Ni and Cr into Fe has a minor effect on its melting curve and density in the liquid state, suggesting that sufficient light elements are required in the outer core to satisfy both the core density deficit and the reduced melting temperature.