Experimental investigation of the partitioning of phosphorus between metal and silicate phases: implications for the Earth, Moon and Eucrite Parent Body

Abstract

The solid metal/silicate melt partition coefficient for P, D( P), has been determined experimentally at 1190°C and 1300°C. The dependence of the partition coefficient on oxygen fugacity has been investigated, and is consistent with a valence state of 5 for P in the silicate melt. The experimental partition coefficients are given by: log D(P) = −1.21 log ƒO 2 − 15.95 at 1190°C log D(P) = −1.53 log ƒO 2 − 17.73 at 1300°C The experimentally determined partition coefficients may be used to interpret the low P La ratios of the Earth, Moon and eucrites relative to C1 chondrites. The low P La ratios in the eucrites may be explained by partitioning of P into 5% to 25% of a sulfur-bearing metallic liquid assuming equilibration and separation of the liquid metal from the silicates at low degrees of partial melting of the silicates. The low W La ratios in the eucrites compared to C1 chondrites require the separation of an additional 2% to 10% solid metal. The lowering of both P La and W La ratios in the Moon may be explained by partitioning of P and W into metal during formation of a small core by separation of liquid metal from silicate at low degrees of partial melting of the silicates. The W La ratios in the Earth and Moon are virtually indistinguishable, while P La ratios differ by a factor of two. The concentrations of FeO also appear to be different. These observations are difficult to reconcile with the hypothesis of a terrestrial origin of the Moon following formation of the Earth's core, but are consistent with independent formation of the Earth and Moon.