Alberto E. Saal

Sulfur isotope variations in mantle-derived lavas provide important constraints on the evolution of planetary bodies. Here, we report the first in situ measurements of sulfur isotope ratios dissolved in primitive volcanic glasses and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. The new data reveal large variations in ³⁴S/³²S ratios, which positively correlates with sulfur and titanium contents within and between the distinct compositional groups of volcanic glasses analyzed. Our results uncover several magmatic events that fractionated the primordial sulfur isotope composition of the Moon: the segregation of the lunar core and the crystallization of the lunar magma ocean, which led to the formation of the heterogeneous sources of the lunar magmatism, followed by magma degassing during generation, transport, and eruption of the lunar lavas. Whether the Earth’s and Moon’s interiors share a common ³⁴S/³²S ratio remains a matter of debate.

Common Water: The Moon has been traditionally considered bone-dry, but in recent years a number of studies have shown that during mantle melting, the lunar mantle had as much water as Earth's upper mantle. Saal et al. (p. 1317 , published online 9 May; see the cover) measured the isotopic composition of hydrogen dissolved in volcanic glass and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. Lunar magmatic water was indistinguishable from the bulk water in carbonaceous chondrites and similar to terrestrial water, implying a common origin for the water contained in the interiors of Earth and the Moon.

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO(2), H(2)O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.

U‐Th decay chains in young volcanic rocks have become a unique geochemical tool with the potential to constrain the physical processes involved in the generation and transport of magmas. Among the different isotopes of the decay series, the measured disequilibrium between ²²⁶Ra and ²³⁰Th isotopes in basaltic melts has been a crucial player in the interpretation of the U‐series data as a whole. However, the key assumption proposed in all previous models, that the origin of ²²⁶Ra‐²³⁰Th disequilibrium in oceanic basalts is only produced during mantle melting processes, has remained largely unchallenged. Here we present an alternative process for the origin of the ²²⁶Ra‐²³⁰Th disequilibrium in oceanic basalts. We argue that diffusive interaction of magmas with cumulates formed in the oceanic crust beneath mid ocean ridges or oceanic islands may be responsible for the observed ²²⁶Ra‐²³⁰Th disequilibrium in oceanic basalts. Hence the inferences drawn from (²²⁶Ra)/(²³⁰Th) activity ratios about mantle melting and melt transport processes beneath mid‐ocean ridges and ocean islands are at best equivocal.