Partitioning in REE-saturating minerals: Theory, experiment, and modellmg 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 whit&kite 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 2Ca2+ in Ca(B) sites and (Ca’+,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 whit&kite/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 whit&kite and apatite. If apatite crystallizes before whitlockite or in the absence of whit&kite, 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 whit&kite 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 whitlockitcs 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. INTRODUCTION AND PREVIOUS WORK THE PHOSPHATES APATITE and whitlockite ( merrillite ) , are common accessory minerals in many lunar rocks ( FRONOEL, 1975; PAPIKE et al., 199! ). Whitlockite has very high REE concentrations and accounts for much of the REE content of rocks in which it is found. Concentrations of REEs (lanthanides plus yttrium; e.g., HASKIN and GEHL, 1962) in whit&kite exceed those of coexisting apatite by one to two orders of magnitude. This observation, based mainly on highquality ion microprobe analyses (LINDSTROM et al., 1985, 1991; GWDRICH et al., 1985a,b), led to the suggestion that such apatite and whitlockite in lunar rocks might not be at equilibrium with each other and that some process such as metasomatism, metamorphic alteration, or sequential crystallization is required to explain the REE distributions. Early experimental studies of phosphateliquid partitioning involving apatite and whitlockite indicated that REEs might not favor whit&kite relative to apatite as strongly as the natural assemblages seem to indicate. For example, whitlockite/melt distribution coefficients for the LREEs of 10 (DICKINSON and HESS, 1983) do not seem high enough relative to typical apatite/melt D values in terrestrial systems. Experimentally determined whitlockite/apatite REE concentration ratios < 5, reported by MURRELL et al. ( 1984), are much lower than measured concentration ratios between the lunar phosphates. That the apparent distribution of REEs between apatite and whitlockite in lunar rocks differs from that indicated by early experimental work led to the speculation that one or the other phosphate mineral was produced by infiltration metasomatism or metamorphic alteration ( LINDSTROM et al., 1985; NEAL and TAYLOR, 199 1; SNYDER et al., 1992 ) . Apatite and whitlockite occur in vugs in Apollo 14 breccias, presumably deposited by hot vapors within basinformed ejecta deposits (MCKAY et al., 1972), so it is reasonable that these minerals might have formed by metasomatism in igneous rocks or that one replaced the other in thermally metamorphosed rocks.