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

Magmatic volatiles (for example, water) are abundant in arc melts and exert fundamental controls on magma evolution, eruption dynamics and the formation of economic ore deposits. To constrain the H 2 O content of arc magmas, most studies have relied on measuring extrusive products and mineral-hosted melt inclusions. However, these methods have inherent limitations that obfuscate the full range of H 2 O in arc magmas. Here, we report secondary-ion mass spectrometry measurements of volatile (H 2 O, F, P, S, Cl) abundances in lower-crustal cumulate minerals from the Kohistan palaeo-arc (northwestern Pakistan) and determine H 2 O abundances of melts from which the cumulates crystallized. Pyroxenes retained magmatic H 2 O abundances and record damp (less than 1 wt% H 2 O) to hydrous (up to 10 wt% H 2 O) primitive melts. Subsequent crystal fractionation led to formation of super-hydrous melts with approximately 12–20 wt% H 2 O, predicted petrologically yet virtually absent from the melt-inclusion record. Porphyry copper deposits are probably a natural eventuality of fluid exsolution from super-hydrous melts, corroborating a growing body of evidence.