Pressure-temperature-time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes, and hence the compositions of liquid fractionates. Here we use La/Yb and Dy/Yb versus SiO₂ relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma. Production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid-lower crust, where amphibole is stable. Because amphibole is rarely a phenocryst phase, we term this “cryptic amphibole fractionation.” If this process is as widespread as our investigation suggests, then (1) amphibole cumulates may act as an effective fi lter for water dissolved in mantle-derived magmas; (2) amphibole cumulates may act as a fertile source of intracrustal melts and fl uids; and (3) recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle.4 page(s