The Fate of Sulfur and Chalcophile Elements During Crystallization of the Lunar Magma Ocean

To assess the viability of sulfide liquid saturation during crystallization of the lunar magma ocean (LMO), we present a new data set describing both the sulfur (S) concentration at sulfide liquid saturation (SCSS) and sulfide liquid-silicate melt partition coefficients of many trace elements for various differentiated lunar magmas at lunar-relevant conditions. Using these parameterizations, we model the SCSS and the distribution of the most chalcophile elements with progressive LMO crystallization in the absence and presence of sulfide liquids. Modeling results for different modes of LMO crystallization show that for proposed lunar mantle S abundances FeS sulfide liquid saturation is expected to occur between 96% and 98% of LMO crystallization. This is decreased to >91% for Fe-S liquids with 30% Ni or Cu. Saturation of S-poor sulfide liquids can occur at >75% of LMO crystallization. The timing of sulfide liquid saturation depends most strongly on the assumed S content of the lunar mantle following formation of the lunar core and on the sulfide liquid composition. Modeled abundances of chalcophile elements indicate that sulfide-liquid saturation during late-stage LMO crystallization would yield much lower abundances of Ni and Cu than observed in KREEP basalts and estimated for the urKREEP reservoir, as well as lower Ni/Co than observed in the latter. Sulfide liquids therefore did not affect moderately siderophile and chalcophile element fractionation within the LMO, supporting the hypothesis that the nonvolatile, siderophile element abundances of the lunar mantle reflect a phase of core formation and/or the addition of a meteoritic late veneer.