Recent detection of indigenous hydrogen in a diversity of lunar materials, including volcanic glass (Saal et al., 2008), melt inclusions (Hauri et al., 2011), apatite (Boyce et al., 2010; McCubbin et al., 2010), and plagioclase (Hui et al., 2013) suggests water played a role in the chemical differentiation of the Moon. Water contents measured in plagioclase feldspar, a dominant mineral in the ancient crustal lunar highlands have been used to predict that 320 ppm water initially existed in the lunar magma ocean (Hui et al., 2013) whereas measurements in apatite, found as a minor mineral in lunar rocks, representing younger potassium-enriched melt predict a bulk Moon with <100 ppm water. Here we show that water in alkali feldspar, a common mineral in potassium-enriched rocks, can have ∼20 ppm water, which implies magmatic water contents of ∼1 wt. % in chemically evolved rhyolitic magmas. The source for these wet, potassium-rich magmas probably contained ∼1000 ppm H2O. Thus, lunar granites with ages from 4.3-3.9 Ga (Meyer et al., 1996) likely crystallised from relatively wet melts that degassed upon crystallisation. Geochemical surveys by the Lunar Prospector (Jolliff et al., 2011) and Diviner Lunar Radiometer Experiment (Glotch et al., 2010; Jolliff et al., 2011) indicating the global significance of evolved igneous rocks suggest that the formation of these granites removed water from some mantle source regions, helping to explain the existence of mare basalts with <10 ppm water, but must have left regions of the interior relatively wet as seen by the water content in volcanic glass and melt inclusions. Although these early-formed evolved melts were water-rich, their petrogenesis supports the conclusion that the Moon's mantle had <100 ppm water for most of its history
