The H2O concentration and H2O/Ce ratio in olivine-hosted melt inclusions are high in lunar pyroclastic sample 74220 (H2O up to 1410 ppmw; H2O/Ce up to 77) but lower (H2O 10 to 430 ppmw; H2O/Ce 0.3 to 9.4) in all other lunar samples studied before this work. The difference in H2O concentration and in H2O/Ce ratio is absent for other volatile elements (F, S, and Cl) in melt inclusions in 74220 and other lunar samples. Because H2O (or H) is a critical volatile component with significant ramifications on the origin and evolution of the Moon, it is important to understand what causes such a large gap in H2O/Ce ratio between 74220 and other lunar samples. Two explanations have been advanced. One is that volcanic product in sample 74220 has the highest cooling rate and thus best preserved H2O in melt inclusions compared to melt inclusions in other samples. The other explanation is that sample 74220 comes from a localized heterogeneity enriched in some volatiles. To distinguish these two possibilities, here we present new data from two rapidly cooled lunar samples with glassy melt inclusions: olivine-hosted melt inclusions (OHMIs) in 79135 regolith breccia (unknown cooling rate but with glassy MIs similar in texture with those in 74220), and pyroxene-hosted melt inclusions (PHMIs) in 15597 pigeonite basalts (known high cooling rate, second only to 74220 and 15421). In addition, we also investigated new OHMIs in sample 74220. If the gap is due to the difference in cooling rates, samples with cooling rates between those of 74220 and other studied lunar samples should have preserved intermediate H2O concentrations and H2O/Ce ratios. Our results show that melt inclusions in 79135 and 15597 contain high H2O concentrations (up to 969 ppmw in 79135 and up to 793 ppmw in 15597) and high H2O/Ce ratios (up to 21 in 79135 and up to 13 in 15997), bridging the big gap in H2O/Ce ratio among 74220 and other lunar samples. Combined with literature data, we confirm that H2O/Ce ratios of different lunar samples are positively correlated to the cooling rates and independent of the type of mare basalts. We hence reinforce the interpretation that the lunar sample with the highest cooling rate best represents pre-eruptive volatiles in lunar basalts due to the least degassing. Based on Ce concentration in the primitive lunar mantle, we estimate that H2O concentration in the primitive lunar mantle (meaning bulk silicate Moon) is 121 +/- 15 ppmw. Our new data also further constrain F/P, S/Dy and Cl/Ba ratios in lunar basalts and the lunar mantle. Estimated F, P, and S concentrations in the lunar primitive mantle are 4.4 +/- 1.1 ppmw, 22 +/- 8 ppmw, and 67+67 33 ppmw, respectively.

In the 50 years since the first lunar sample return, the investigation of H2O in the Moon has experienced several stages of developments and paradigms. In the early years since Apollo sample return, only bulk soil and bulk rock samples were analyzed for H2O as well as other volatiles. From 1970 to 2007, it was thought that the Moon is essentially devoid of innate H2O, containing probably less than 1 ppb. New technologies gradually enabled the measurements of H2O in lunar glass beads, soil glass, minerals such as apatite and anorthite, and olivine-hosted melt inclusions. The advancements in measurement techniques led to improved data and new insights. Starting from 2008, significant H2O in deep-sourced lunar rocks has been reported, resulting in a paradigm shift from a bone-dry Moon to a fairly wet Moon, although there is still debate about whether the bulk silicate Moon contains similar to 100 ppm of H2O (similar to that in the Earth's MORB mantle) or only a few ppm H2O. The advances on our knowledge of H2O in the Moon is accompanied by increased understanding of other volatiles in the Moon. Gradually, the degrees of depletion of various volatiles in the Moon relative to the Earth were inferred. Using assessed data from available lunar samples, mostly the melt inclusions, and also bulk rock analyses, it is found that the inferred degrees of depletion for volatile elements in the Moon relative to the Earth do not vary much and are independent of the condensation temperature. It is proposed that an early veneer delivered the volatiles to both the Earth and the Moon, but the Moon received proportionally less of the early veneer planetesimals. In addition to H2O in the interior of the Moon, significant surface H2O in the form of ice in lunar polar regions and structural OH in agglutinate glass in lunar regolith originating from solar wind implantation has also been gradually quantified.