High water content of arc magmas recorded in cumulates from subduction zone lower crust

We report secondary ion mass spectrometry measurements of volatile (H2O, F, P, S, Cl) abundances in lower-crustal cumulate minerals from the Kohistan paleo-arc (NW Pakistan), and determine H2O abundances of melts from which the cumulates crystallized. Pyroxenes retained magmatic H2O abundances and record damp (less than 1% wt.% H2O) to hydrous (up to 10 wt.% H2O) primitive melts. Subsequent crystal fractionation led to formation of super-hydrous melts with approximately 12–20 wt.% H2O, predicted petrologically yet virtually absent from the melt inclusion record. Porphyry copper deposits are likely a natural eventuality of fluid exsolution from super-hydrous melts, corroborating a growing body of evidence.