EVIDENCE FOR PRESSURE-RELEASE MELTING BENEATH MAGMATIC ARCS FROM BASALT AT GALUNGGUNG, INDONESIA

The melting of peridotite in the mantle wedge above subduction zones i s generally believed to involve hydrous fluids derived from the subduc ting slab(1). But if mantle peridotite is upwelling within the wedge, melting due to pressure release could also contribute to magma product ion. Here we present measurements of the volatile content of primitive magmas from Galunggung volcano in the Indonesian are which indicate t hat 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 un iformly low H2O concentrations (0.21-0.38 wt%), yet relatively high le vels of CO2 (up to 750 p.p.m.) indicating that the low H2O concentrati ons are primary and not due to degassing of the magma. Results from pr evious anhydrous melting experiments on a chemically similar Aleutian basalts(2) indicate that the Galunggung high-magnesium basalts were la st in equilibrium with peridotite at similar to 1,320 degrees C and 1. 2 GPa. These high temperatures at shallow sub-crustal levels (about 30 0-600 degrees C hotter than predicted by geodynamic models(1-3)), comb ined with the production of nearly H2O-free basaltic melts, provide st rong evidence that pressure-release melting due to upwelling in the su b-are mantle has taken place. Regional low-potassium(4) and low-H2O (r ef. 5) basalts found in the Cascade are indicate that such upwelling-i nduced melting can be widespread.