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

摘要

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 degrees C. Up to 3.0 wt% and 10.4 wt% phosphorus can dissolve in the solid iron and liquid Fe-P phases at 1100 degrees C and 3 GPa, respectively. The eutectic temperature on the iron-rich side was determined as 1085 degrees C at 3 GPa. The solubility of phosphorus in the iron decreases from similar to 1.4 wt% at 1100 degrees C to similar to 0.7 wt% at 1500 degrees C and 3 GPa. Structure of the solid iron in the quenched sample is the body-center cubic, corresponding to alpha-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 <0.3 wt% P. Some other potential light elements such as S and C could reduce the P content in the lunar core. Furthermore, the partition coefficient of phosphorus in the iron and liquid melt (D-p(SM/)LM) was found to be 0.10 +/- 0.03 at 3 GPa. Taking the sulfur into account, the D s p m / Lm increase to 0.18 +/- 0.02 at 5 GPa in the S-rich liquid metal (similar to 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 <0.3 wt%. The moderate phosphorus solubility in the solid iron, combined with the assumption of abundant phosphorus in the bulk Moon, indicates tha