Shock Compression and Melting of an Fe‐Ni‐Si Alloy: Implications for the Temperature Profile of the Earth's Core and the Heat Flux Across the Core‐Mantle Boundary

Abstract

AbstractUnderstanding the melting behavior and the thermal equation of state of Fe‐Ni alloyed with candidate light elements at conditions of the Earth's core is critical for our knowledge of the region's thermal structure and chemical composition and the heat flow across the liquid outer core into the lowermost mantle. Here we studied the shock equation of state and melting curve of an Fe‐8 wt% Ni‐10 wt% Si alloy up to ~250 GPa by hypervelocity impacts with direct velocity and reliable temperature measurements. Our results show that the addition of 10 wt% Si to Fe‐8 wt% Ni alloy slightly depresses the melting temperature of iron by ~200–300 (±200) K at the core‐mantle boundary (~136 GPa) and by ~600–800 (±500) K at the inner core‐outer core boundary (~330 GPa), respectively. Our results indicate that Si has a relatively mild effect on the melting temperature of iron compared with S and O. Our thermodynamic modeling shows that Fe‐5 wt% Ni alloyed with 6 wt% Si and 2 wt% S (which has a density‐velocity profile that matches the outer core's seismic profile well) exhibits an adiabatic profile with temperatures of ~3900 K and ~5300 K at the top and bottom of the outer core, respectively. If Si is a major light element in the core, a geotherm modeled for the outer core indicates a thermal gradient of ~5.8–6.8 (±1.6) K/km in the D″ region and a high heat flow of ~13–19 TW across the core‐mantle boundary.