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, 3, 4 and in volcanic rocks5, 6, 7, 8, 9 and regolith10. Part of this water is thought to have been brought through solar-wind implantation2, 3, 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.
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