Deep hydrous melting of the mantle wedge recorded by Mg isotopes.

Subduction zones govern Earth's deep volatile cycle. The slab-mantle interface is a key locus of volatile exchange between the subducting crust and the overlying mantle wedge, yet whether and how chemical reactions at this interface affect arc magmatism remains poorly constrained. Arc magmas typically lack a geochemical hallmark of garnet fractionation, leading to the prevailing interpretation that they derive predominantly from the shallower, garnet-free mantle wedge. Here, we challenge this view using Mg isotopic data from Nicaragua, the South Sandwich Islands, and the central-eastern Aleutian arc, integrated with a global arc magma compilation. We show that heavy Mg isotopic signatures in mafic arc magmas vary with indicators of volatile enrichment, pointing to a volatile-rich source, most likely the heavily metasomatized base of the mantle wedge. The heaviest Mg isotopic compositions occur in arcs underlain by the most steeply dipping subducting slab, suggesting a focusing effect by the geometry of the downgoing slab. Monte Carlo isotope mixing models indicate that up to 50% of the Mg budget in some arc magma sources derives from deep, water-saturated melting near the slab-mantle interface. These findings highlight the base of the mantle wedge as a volumetrically and geochemically significant contributor to arc magmatism and the water cycle.