Control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge

The water released from descending oceanic lithosphere is thought to have an important role in subduction-zone magmatism, as this water might trigger partial melting of the mantle wedge above the subducting plate. If, however, there is incomplete wetting of mineral grain boundaries in the mantle (that is, the dihedral angle at the triple junctions between grains is more than 60°), then the water would not form an interconnected network and might instead be trapped as interstitial fluid in the mantle peridotite. The water would then be transported to deeper parts of the mantle rather than triggering partial melting. Here we use dihedral-angle data to estimate the connectivity of an aqueous fluid phase in a model upper-mantle mineral assemblage (forsterite) at pressures from 3 to 5 GPa (corresponding to depths of ~80-150 km). By combining these data with previous results, we find that the dihedral angle is greater than 60°at low pressure and temperature (<1,000°C at 2 GPa and <800°C at 4 GPa) and lower than 60°at higher pressures and temperatures, suggesting that wetting is incomplete below these conditions. This indicates that the connectivity of water in hydrous upper- mantle peridotite at convergent plate boundaries might control the position of the volcanic front in island arcs.