Estimating the lunar mantle water budget from phosphates: Complications associated with silicate-liquid-immiscibility

Fractional crystallization trends have been used in order to assess the conditions under which magmas are likely to undergo SLI. Predicted liquid lines of descent indicate that it is the late-stage residual liquids of lunar basalts with relatively low-Mg# (e.g., <∼50) that intersect the two-liquid field during crystallization, forming conjugate liquids that are chemically and mineralogically distinct. However, residual liquids for basaltic magmas with high-Mg# (e.g., >∼50) may not intersect the two-liquid field depending upon the fractionation trends in the late-stage mesostasis pockets. For samples that undergo SLI, the apatite/melt-partition coefficients required for back-calculating water abundances of the parental melts are compromised by the generation of two populations of apatites-merrillites from conjugate immiscible liquids. This process highlights an important complexity inherent to all water back-calculations that use apatite, as this requires an additional set of partition coefficients. We emphasize that the complex petrologic nature and common development of SLI of apatite-bearing, late-stage mesostasis pockets have not been considered in published apatite-volatile data. These factors, in additional to other considerations, illustrate why water back-calculations to model the primary melts from such data must be viewed with caution.