Evidence for pressure-release melting beneath magmatic arcs from basalt at Galunggung, Indonesia

The melting of peridotite in the mantle wedge above subduction zones is generally believed to involve hydrous fluids derived from the subducting slab1. But if mantle peridotite is upwelling within the wedge, melting due to pressure release could also contribute to magma production. Here we present measurements of the volatile content of primitive magmas from Galunggung volcano in the Indonesian arc which indicate that these magmas were derived from the pressure-release melting of hot mantle peridotite. The samples that we have analysed consist of mafic glass inclusions in high-magnesium basalts. The inclusions contain uniformly low H2O concentrations (0.21–0.38 wt%), yet relatively high levels of CO2 (up to 750 p.p.m.) indicating that the low H2O concentrations are primary and not due to degassing of the magma. Results from previous anhydrous melting experiments on a chemically similar Aleutian basalts2 indicate that the Galunggung high-magnesium basalts were last in equilibrium with peridotite at ∼1,320 °C and 1.2 GPa. These high temperatures at shallow sub-crustal levels (about 300–600 °C hotter than predicted by geodynamic models1,3), combined with the production of nearly H2O-free basaltic melts, provide strong evidence that pressure-release melting due to upwelling in the sub-arc mantle has taken place. Regional low-potassium4 and low-H2O (ref. 5) basalts found in the Cascade arc indicate that such upwelling-induced melting can be widespread.