Apatites in lunar KREEP basalts: The missing link to understanding the H isotope systematics of the Moon

Recent re-analyses of lunar samples have undoubtedly measured indigenous water, challenging the paradigm of a dry Moon, and arguing that some portions of the lunar interior are as wet as some regions of the Earth's mantle and that water in both planetary bodies likely share a common origin. Mare basalts indirectly sample the lunar mantle and are affected by petrogenetic processes such as crystallization and degassing that can modify characteristics of indigenous water in primary mantle melts. Analyses of apatite in phosphorus-rich KREEP (K + REE [rare earth elements] + P) basalts may provide more reliable estimates for the water content of lunar magmas, as some apatites likely crystallized before substantial degassing occurred. In lunar KREEP basalt sample 15386, apatite H2O content and H isotopic composition suggest that degassing occurred during apatite crystallization, the lowest delta D value of 90 parts per thousand +/- 100 parts per thousand representing an upper limit for the isotopic composition of water in the parental magma. Interpretation of the data for KREEP basalt 15386 suggests that this basalt is characterized by relatively elevated H2O contents and CI chondrite-type delta D values, similar to those proposed for other mare basalts and pyroclastic glasses. On the other hand, most of the apatites in lunar KREEP basalt 72275 and lunar meteorite NWA 773 crystallized before degassing and H isotope fractionation, and their D/H ratios thus directly reflect those of their source regions. These apatites have an average delta D value of -130 parts per thousand +/- 50 parts per thousand, suggesting the presence of a water reservoir in the Moon characterized by moderate H2O contents and H isotopic composition similar to that of Earth's interior. These findings imply that significant amounts of water in the Moon were inherited from the proto-Earth, surviving the purported Moon-forming impact event.