Coordinated microanalysis of volatiles in apatite and silicate minerals in ancient lunar basalts

Despite being essentially water-free, nominally anhydrous minerals such as plagioclase and pyroxene represent the biggest reservoir of water in most lunar rocks due to their sheer abundance. Apatite, which incorporates F, Cl, and OH into its mineral structure as essential crystal components, on the other hand, is the only other volatile-bearing phase common in lunar samples. Here, we present the first coordinated study of volatiles (e.g., H ₂O, Cl, F, and S) in nominally anhydrous minerals combined with isotopic measurements in apatite from the ancient lunar basalt fragments from meteorite Miller Range (MIL) 13317. Apatite in MIL 13317 basalt contains ∼ 2000 ppm H ₂O and has an elevated δD values (+ 523–737 ‰), similar to Apollo mare basalts, but has high δ ³⁷Cl values (+ 29–36 ‰), similar to apatite found in several KREEP-rich samples. MIL 13317 is unique compared with other lunar basalts; it has both elevated δD and δ ³⁷Cl values currently only observed in highlands sample 79215 (a granulitic impactite). Based on measurements of H ₂O in nominally anhydrous minerals and in apatite, the source magma of MIL 13317 basalt is estimated to contain ∼ 130–330 ppm H ₂O. Assuming reasonable levels of partial melting of the lunar mantle and magmatic degassing during eruption of the basalt, the Moon contained at least one reservoir with < 100 ppm H ₂O, a δD value of < 0 ‰ similar to carbonaceous chondrites, and extensively fractionated Cl isotopes prior to 4.332 Gyr, the crystallization age of the MIL 13317 basalt.