Water is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile-rich state and calc-alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution-in the deep crust-remains poorly constrained. Here, we report H 2 O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H 2 O contents ranging from 75-760, 233-410, and 42-139 ppm, respectively. Reconstructed bulk rock H 2 O contents range from~60 to~650 ppm. Intermineral H 2 O ratios overlap experimental mineral/melt D values and are used to calculate H 2 O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H 2 O) 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.

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