PARTITIONING OF F, CL, AND H2O BETWEEN APATITE AND A SYNTHETIC BASALTIC LIQUID: IMPLICATIONS FOR QUANTIFYING WATER CONTENTS OF LUNAR MAGMAS FROM APATITE
The apatite-melt partition coefficients for H2O range from about 0.025-0.25, for F they range from about 0.15-40, and for Cl they range from 0.3-5. Consequently, mineral-melt D values are not appropriate to use for quantifying volatile abundances in magmatic liquids from apatite composition alone. This is to be expected because F, Cl, and OH are essential structural constituents in apatite. However, we observed that exchange coefficients are not as variable and change systematically as a function of temperature and melt composition. In fact, the exchange coefficients indicate that F is preferred in the apatite structure by a factor of 100 compared to OH and by a factor of 20 compared to Cl. Cl is preferred by a factor of 20 over OH. Based on these results, the estimates of water contents in lunar magmas, which primarily used and apatite-melt D value of 0.25, are appropriately conservative for estimating lower limits on magmatic water abundances in late-stage lunar liquids.