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 H₂O 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 H₂O in arc magmas. Here, we report secondary-ion mass spectrometry measurements of volatile (H₂O, F, P, S, Cl) abundances in lower-crustal cumulate minerals from the Kohistan palaeo-arc (northwestern 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 probably a natural eventuality of fluid exsolution from super-hydrous melts, corroborating a growing body of evidence.