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

The water content of arc magmas in the lower crust can reach up to 20 wt% during crystallization, according to geochemical analyses of minerals from the Kohistan palaeo-arc, Pakistan, underscoring the role of water in porphyry deposits formation. 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.

Automated summary

According to geochemical analyses, the water content of arc magmas can reach up to 20 wt% in the lower crust, highlighting the importance of water in the formation of porphyry deposits. By measuring volatile abundances in lower-crustal cumulate minerals, researchers have found that pyroxenes retain the water content of the magmas and record a range of water contents in primitive melts. Subsequent crystal fractionation results in the formation of super-hydrous melts with a water content of approximately 12-20 wt%, which have been rarely observed in previous studies.