ABSTRACT

While it is now recognized that the Moon has indigenous water and volatiles, their total abundances are unclear, with current literature estimates ranging from nearly absent to Earth-like levels. Similarly unconstrained is the source of the Moon’s water, which could be cometary, chondritic, or the primordial nebula. Here we measure H₂O and D/H in olivine-hosted melt inclusions in lunar mare basalts 12018, 12035, and 12040, part of the consanguineous suite of Apollo 12 olivine basalts that differ primarily because of cooling rate (Walker et al., 1976). We find that the water contents are higher in the more rapidly cooled 12018 (62–740 ppm H₂O) compared to the more slowly cooled basalts 12035 (28–156 ppm H₂O) and 12040 (27–90 ppm H₂O), suggesting that lunar basalts may have been dehydrating during slow cooling. D/H is similar in the olivine-hosted melt inclusions in all three samples, and indistinguishable from terrestrial water (δD = –183 ± 212‰ to +138 ± 61‰). When we compare the D/H of olivine-hosted melt inclusions to D/H of apatite in the same samples, the evolution of δD and water content can be better constrained. We propose that lunar magmas first exchange hydrogen with a low D/H reservoir during cooling, and then ultimately lose their water during extended subsolidus cooling. Due to high diffusion rates of hydrogen in olivine, it is likely that all basaltic olivine-hosted melt inclusions from the Moon exchanged hydrogen with a low D/H reservoir in near-surface magma chambers or lava flows. The most likely source of the low D/H reservoir on the Moon is the lunar regolith, which is known to have a significant solar wind hydrogen component.