The Chlorine Isotope Composition of the Moon and Implications for an Anhydrous Mantle

Over the Moon Based on recent analyses of lunar rocks, it has been argued that the lunar interior contained much more water than previously thought. Sharp et al. (p. 1050, published online 5 August) measured the chlorine isotope content of lunar samples returned by the Apollo missions and found that the spread in their chlorine isotope composition is 25-fold greater than for rocks and minerals that have been measured from Earth and meteorites. This result implies that the hydrogen content of the Moon (and therefore its water content) is much lower than suggested by recent studies. The range of chlorine isotope values of the Moon is distinct from those of Earth and meteorites, indicating that the Moon is dry. Arguably, the most striking geochemical distinction between Earth and the Moon has been the virtual lack of water (hydrogen) in the latter. This conclusion was recently challenged on the basis of geochemical data from lunar materials that suggest that the Moon’s water content might be far higher than previously believed. We measured the chlorine isotope composition of Apollo basalts and glasses and found that the range of isotopic values [from –1 to +24 per mil (‰) versus standard mean ocean chloride] is 25 times the range for Earth. The huge isotopic spread is explained by volatilization of metal halides during basalt eruption—a process that could only occur if the Moon had hydrogen concentrations lower than those of Earth by a factor of ~104 to 105, implying that the lunar interior is essentially anhydrous.

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