Because subducted serpentinites may release significant quantities of volatiles, high-pressure phase equilibria were computed for two end-member hydrothermally altered mantle harzburgite protoliths (ophicarbonates): calcite + antigorite + brucite and calcite + antigorite + talc. For both bulk compositions, most of the H2O released by metamorphic dehydration occurs at subarc depths; thus dehydration of serpentinites could be a major source for H2O in arc magmas. In contrast, for both model compositions a significant fraction of the original carbonate is retained to depths exceeding 200 km. Consequently, deep subduction of ophicarbonate rocks of the oceanic lithosphere and/or downward drag of mantle wedge ophicarbonates provide a mechanism for carbonating the mantle and thus a potentially significant CO2 source for deep mantle melts. The probable CO2 sources for arc magmas are metamorphic decarbonation of marine sediments and/or carbonated mafic volcanics in the subducted slab.