Compression experiments to 126 GPa and 2500 K and thermal equation of state of Fe3S: Implications for sulphur in the Earth's core

Abstract

Pressure-volume-temperature (P-V-T) experiments on tetragonal Fe3S were conducted to 126 GPa and 2500 K in laser-heated diamond anvil cells (DAC) with in-situ X-ray diffraction (XRD). Seventy nine high-T data as well as four 300-K data were collected, based on which new thermal equations of state (EoS) for Fe3S were established. The room-T data together with existing data were fitted to the third order Birch-Murnaghan EoS, which yielded, K0=126±2 GPa and K′=5.1±1 with V0 fixed at 377.0 Å3. A constant αKT term in the thermal pressure equation, Pth = αKT(T-300), fitted the high-T data well to the highest temperature, which implies that the contributions from the anharmonic and electronic terms should be minor in the thermal pressure term. The high-T data were also fitted to the Mie-Grüneisen-Debye model; γ0=1.01±0.03 with θ0 and q fixed at 417 K and 1 respectively. Calculations from the EoS show that crystalline Fe3S at 4000-5500 K is denser than the Earth's outer core and less dense than the inner core. Assuming a density reduction due to melting, liquid Fe3S would meet the outer core density profile, which however suggests that no less than 16 wt%S is needed to reconcile the observed outer core density deficit. The S-rich B2 phase, which was suggested to be a potential liquidus phase of an Fe3S-outer core above 250 GPa, namely the main constituent of its solid inner core, would likely be less dense than the Earth's inner core. As such, while the outer core density requires as much sulphur as 16 wt%, the resulting liquidus phase cannot meet the density of the inner core. Any sulphur-rich composition should therefore be rejected for the Earth's core.