The phase diagram of the Fe-P binary system at 3 GPa and implications for phosphorus in the lunar core

Abstract Phosphorus is a potential candidate in the metallic core of the Moon. The phase diagram of the Fe-P binary system was investigated at the pressure of 3 GPa and temperatures of up to 1600 °C. Up to 3.0 wt% and 10.4 wt% phosphorus can dissolve in the solid iron and liquid Fe-P phases at 1100 °C and 3 GPa, respectively. The eutectic temperature on the iron-rich side was determined as 1085 °C at 3 GPa. The solubility of phosphorus in the iron decreases from ∼1.4 wt% at 1100 °C to ∼0.7 wt% at 1500 °C and 3 GPa. Structure of the solid iron in the quenched sample is the body-center cubic, corresponding to α-Fe phase. Extending the phosphorus solubility in the solid iron to the present lunar core conditions yields a maximum phosphorus concentration in a fully crystallized iron core of 0.85 ± 0.15 wt%. If there are Ni and C in the core, the value would be depressed to 0.4 ± 0.1 wt%. In addition, based on a simple siderophile mass balance between the bulk Moon (BM) and bulk silicate Moon (BSM) and a modeled phosphorus partition coefficient, D P-Moon core/mantle (40–200) for the lunar magma ocean, a bulk silicate Earth-like P content (82 ± 8 ppm) in the initial Moon yields a lunar core with D P SM/LM ) was found to be 0.10 ± 0.03 at 3 GPa. Taking the sulfur into account, the D P SM/LM increase to 0.18 ± 0.02 at 5 GPa in the S-rich liquid metal (∼8 wt%). In the case of a solid lunar inner core and S-bearing liquid outer core, their P contents were assessed to be less than 0.09 ± 0.01 wt% and 0.51 ± 0.01 wt%, respectively, when the lunar core’s storage of P is