Although ~10% of Earth’s water resides within continents, HO distribution throughout the continental lithosphere and partitioning of HO in nominally anhydrous minerals (NAMs) remain poorly constrained. Models of continent formation and destruction depend on HO content. We report HO contents in NAMs measured on petrographic thin sections by secondary ion mass spectrometry (SIMS) of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average HO contents of 560, 347, and 85 ppm, respectively; reconstructed bulk rock HO ranges from ~75 to ~600 ppm. Inter-mineral HO ratios overlap experimental mineral/melt D values, and are used to calculate HO of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% HO) melt at high (~1 GPa) pressures, similar to conditions in modern subduction zones, and potentially associated with widespread arc accretion that formed the core of North America in the Precambrian.