New constraints on the abundances of phosphorus and sulfur in the lunar core: High-pressure and high-temperature experimental study of the Fe[sbnd]S[sbnd]P ternary system

Abstract

High-pressure and high-temperature experiments for the FeSP ternary system were performed at 3–5 GPa and 1173–1873 K. We systematically investigated the effect of pressure, temperature, and bulk composition on the phase relationships, on the core crystallization sequences, and on the presence of sulfur and phosphorous in the lunar core. Our experimental results indicate that while up to < 1 wt% phosphorus can be dissolved in solid iron in the FeSP ternary system at 3 and 5 GPa, S dissolution in solid iron is near negligible. On the iron rich (S + P < 10 wt%) side of the FeSP phase diagram completely miscible FeSP liquids were observed. Combined with previous experimental results, the relationship of the sulfur content in the liquid metal (XSliquid) and the partitioning coefficient of phosphorus (DP) between the solid and liquid metal follows an equation of lgDP=-1.8286-17.87×lg1-XSliquid. Tradeoff between the liquidus of the FeSP system and the (S + P) content of the lunar core well constrain the upper limit of the (S + P) content in the liquid lunar outer core to the concentrations between 8.7 and 13.1 wt%. Using the result of the phosphorus coefficient and our partitioning model, we further assessed the abundances of 6.08–7.15 wt% S, 0.54 ± 0.01 wt% P in the lunar liquid outer core, and 0.05 ± 0.01 wt% S, 0.07 ± 0.01 wt% P in the lunar solid inner core, respectively. Integrating the observed lunar core adiabat and the pressure dependence of the FeSP liquidus temperature, we propose that the solidification regime in the lunar core will switch from bottom-up to top-down once the abundance of (S + P) in the liquid outer core exceeds 3.5 wt% as the core evolves.