Water in lunar anorthosites and evidence for a wet early Moon

The Moon was thought to be anhydrous since the Apollo era1, but this view has been challenged by detections of water on the lunar surface2–4 and in volcanic rocks5–9 and regolith10. Part of this water is thought to have been brought through solarwind implantation2–4,7,10 and meteorite impacts2,3,7,11, long after the primary lunar crust formed from the cooling magma ocean12,13. Here we show that this primary crust of the Moon contains significant amounts of water. We analysed plagioclase grains in lunar anorthosites thought to sample the primary crust, obtained in the Apollo missions, using Fourier-transform infrared spectroscopy, and detected approximately 6 ppm water. We also detected up to 2.7 ppm water in plagioclase grains in troctolites also from the lunar highland upper crust. From these measurements, we estimate that the initial water content of the lunar magma ocean was approximately 320 ppm; water accumulating in the final residuum of the lunar magma ocean could have reached 1.4 wt%, an amount sufficient to explain water contents measured in lunar volcanic rocks. The presence of water in the primary crust implies a more prolonged crystallization of the lunar magma ocean than a dry moon scenario and suggests that water may have played a key role in the genesis of lunar basalts. Dissolved water in silicates can alter their structure, and hence significantly change their physical and chemical properties14–17, which can further influence geologic processes. One of the most important conclusions resulting from theApollo and Lunamissions was that no water was detected in returned samples or at the surface of the Moon1. The Moon was thought to have lost its volatiles as it formed from ejecta of the impact of a Mars-size planetesimal with the proto Earth, the favouredMoon formation scenario18, and during degassing of an early planet-wide magma ocean12,13. This notion has been included in most geophysical and geochemical models of formation and evolution of the Moon12,18. The view of a dry lunar interior, however, has been challenged by recent discoveries of water in picritic glass beads5, apatites6–8 and olivine melt inclusions9, which were facilitated by the improvements of the analytical detection limit of hydrogen. Indigenous water is suggested to be heterogeneously distributed in the lunar interior and some parts of lunar mantle may contain as much water as Earth’s uppermantle5,9. Hydrogen isotopic compositions of apatites in mare basalts have been interpreted to indicate a hybrid source of the water, that is, a combination of lunar mantle, comets and solar-wind protons7. The chlorine isotope compositions in the lunar pyroclastic deposits, however, have been interpreted as suggesting an essentially anhydrous lunar interior19. It has been further suggested usingmagma ocean crystallizationmodelling that