Partitioning in REE-saturating minerals: Theory, experiment, and modelling of whitlockite, apatite, and evolution of lunar residual magmas

We present compositions, including REEs determined by ion microprobe, of apatite and whitlockite in lunar rock assemblages rich in incompatible trace elements. Total concentrations of REE oxides in whitlockites range from 9-13 wt%, and those in apatites range from 0.15 to 1 wt%. Ratios of REE concentrations in whitlockite to those in coexisting apatite range from ~ 10 to 60. The distribution of Mg and Fe between apatite and whitlockite is correlated to that of coexisting mafic silicates: Magnesium is strongly preferred by whitlockite, and Fe is preferred by apatite. Incorporation of REEs in whitlockite is dominated by the coupled substitution of 2REE³⁺ in Ca(B) sites + vacancy in Ca(IIA) for 2Ca⁺² in Ca(B) sites and (Ca²⁺,Na⁺) in Ca(IIA). Other substitutions account for only a small portion of the REEs in whitlockite over the observed concentration range; thus, REE concentrations become partially saturated as the primary substitution approaches its stoichiometric limit of two REEs per fifty-six oxygens, leading to reduced whitlockite/melt distribution coefficients e.g., decreasing from twenty-five to ten for Nd. The REE concentrations of lunar residual melts are not depleted by whitlockite crystallization in assemblages consisting mainly of other minerals in typical proportions. Distribution coefficients for the REEs in lunar apatite appear to be low and variable e.g., ~0.2-0.8 for Nd. Variations in the modal ratio of whitlockite to apatite, specifically the abundance of whitlockite, lead to a range of REE concentrations in the phosphates and variations in the magnitude of REE concentration ratios between whitlockite and apatite. If apatite crystallizes before whitlockite or in the absence of whitlockite, as textures in several samples indicate, then apatite zoned in REEs and apatite crystals of different REE concentrations may occur in a given sample, provided there is some amount of fractional crystallization and apatite does not later equilibrate. This may occur because, in the absence of whitlockite in the crystallizing assemblage, the REEs are highly incompatible relative to the crystalline assemblage, so REE concentrations in lunar residual melts increase strongly during small increments of late-stage crystallization. Once whitlockite begins to crystallize, bulk distribution coefficients for the REEs, although still < 1, are only mildly incompatible, so the change in REE concentrations of residual melts with further crystallization is small, consistent with the lack of REE zoning in whitlockite. The REE concentrations in lunar whitlockites are modelled as resulting mainly from equilibrium crystallization of the assemblages in which they occur; metasomatism or other secondary metamorphic processes are not indicated. Local melt-pocket equilibrium at advanced stages of crystallization may lead to variable REE concentrations and variable whitlockite /apatite concentration ratios within the same sample. Parent melts with extremely high REE concentrations are not required in order to crystallize REE-rich lunar whitlockite if modal proportions of whitlockite are low.