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

1,2Yuan Yin,1,2Zeming Li,1Shuangmeng Zhai
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.037]
1Key Laboratory of High-temperature and High-pressure Study of the Earth’s Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
2University of Chinese Academy of Sciences, Beijing 100049, China
Copyright Elsevier

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, Dcore/mantle P-Moon(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 (DSM/LM P) was found to be 0.10 ± 0.03 at 3 GPa. Taking the sulfur into account, the DSM/LM P 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 <0.3 wt%. The moderate phosphorus solubility in the solid iron, combined with the assumption of abundant phosphorus in the bulk Moon, indicates that the phosphorus concentration in the lunar core could higher than previously thought.

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