WATER BUDGET IN THE MOON

Recent studies have shown that the moon contains much more hydrogen than previously thought (e.g., Saal et al. (2008) and Hauri et al. (2011) found lunar glass inclusions with H concentration up to 1410 ppm, by weight as H₂O; Hui et al. (2013) found 6 ppm H₂O in lunar anorthosites). In this study, we report new measurements of H and trace elements in olivine-hosted melt inclusions in lunar rocks.

We used Secondary Ion Mass Spectrometry to measure H (as OH), F, Cl, S and other trace elements in 42 melt inclusions from seven lunar samples: 10020,49, 12008,5, 12040,199, 15016,47, 15647,22, 74220,871, and 74235,22, and one glass bead from each of 15421,22 and 74220,871. Some of the melt inclusions examined in this study are naturally glassy (in 74220,871 and some inclusions in 10020,49), but most are crystalline. We rehomogenized 8 crystalline melt inclusions. H concentrations in melt inclusions from each sample is generally highly variable, and the variation does not always correlate with other elements, presumably due to loss of water from melt inclusions during magma evolution or during rehomogenization. Hence, our H concentrations should be considered as minimum.

The highest H content is 820 ppmw H₂O in 74220,871, consistent with Hauri et al. (2011). Melt inclusions in 10020,49 contain up to 340 ppmw H₂O. Melt inclusions in all the other samples contain lower H₂O concentrations (≤110 ppmw, often 30-20 ppmw). An orange glass bead in 74220,871 and a green glass bead in 15421,22 show weakly zoned profiles with H₂O concentrations in the range of 30-20 ppmw.

Our measurements of trace elements in inclusions and glass beads constrain diagnostic ratios such as H₂O/Ce. The H₂O/Ce ratio in fresh and undegassed terrestrial MORB’s is commonly 100-300, and considered typical of the depleted upper mantle. The measured H₂O/Ce ratios in the lunar melt inclusions are highly variable, but the maximum (130±10 in 74220,871) is within range of the MORB values. The Ce concentration in the lunar mantle has been estimated to be 2 times that of the terrestrial mantle (e.g., Anders 1977, Taylor 1982, McDonough and Sun 1995). If the highest H₂O/Ce ratio in lunar melt inclusions represents the true ratio in the lunar mantle, it is possible that H concentration in the lunar mantle is the same as or even higher than that in the terrestrial mantle.