THERMAL EQUATION OF STATE OF FE3P BY X-RAY DIFFRACTION: IMPLICATIONS FOR PHOSPHORUS IN THE PLANETARY CORES

As informed by numerous geophysical constraints, the Earth’s core is predominantly 80%-90% iron (Fe) with one or more light elements. Light elements such as silicon (Si), phosphorus (P), sulfur (S), oxygen (O), carbon (C), and hydrogen (H) have been proposed. Phosphorus is one of the candidates partly due to its high metal/silicate partitioning coefficients and its abundance of iron phosphide minerals, such as schreibersite (Fe, Ni)₃P, in iron nickel meteorite. In addition, the abundance of phosphorus in the Earth’s mantle is much lower than that estimated from the planetary volatility trend, indicating the incorporation of phosphorus in the core. The thermoelastic properties of Fe₃P, crucial for extrapolating the room-temperature mineral physics data to high temperature conditions, were rarely investigated. In this study, we performed synchrotron powder X-ray diffraction (XRD) measurements using externally-heated diamond anvil cells to determine the densities of Fe₃P up to 70 GPa and 700 K. We observed a magnetic transition accompanied by a volume collapse at about 18 GPa at 300 K or 16 GPa at 600K. The P-V-T data set for Fe₃P could be well represented by a high-temperature Birch-Murnaghan equation of state. The thermoelastic properties of Fe₃P will provide insights into the crystal chemistry of the binary Fe-P system and the ternary Fe-S-P system as Fe₃P can form solid solution with the isostructural Fe₃S. The determined thermoelastic property of Fe₃P will permit stringent test of a Fe-P core composition model and contribute to the mineral physics database for iron alloys.