The influence of H2O on mantle wedge melting

The solidus and near-solidus melting behavior of a primitive undepleted peridotite composition has been determined over a pressure range of 1.2–3.2 GPa at H2O saturated conditions. Vapor-saturated melting (melting in the presence of an H2O-rich supercritical fluid) begins at 940 °C at 1.2 GPa and the solidus temperature decreases continuously to 860 °C at 2 GPa and 800 °C at 3.2 GPa. This solidus is similar to the lower temperature results of previous investigations. The temperature discrepancies found in earlier studies could be a result of short run times used in several studies and the slower kinetics of olivine vs. diopside melting. The solidus phases include olivine, orthopyroxene, high-Ca clinopyroxene and Al-rich phases that change from spinel + amphibole (1.2–1.8 GPa) to spinel + chlorite over the pressure range of 2–2.4 GPa. Above 2.4 GPa garnet + chlorite + ilmenite are present along with olivine + orthopyroxene + clinopyroxene on the solidus. Chlorite may be a stable phase at the base of the mantle wedge and it may play a role in the onset of flux melting. Ilmenite might play a role in the development of HFSE depletions in arc magmas. Flux melting of the mantle wedge above the subducting oceanic lithosphere begins when an H2O-rich component (either fluid or melt) released from the slab ascends within the overlying mantle. As it ascends into the mantle wedge the H2O triggers melting at the vapor-saturated solidus at a depth shallower than the wedge–slab interface. Melting continues as the melt ascends into shallower, hotter overlying mantle. Melting in this part of the wedge occurs at vapor-undersaturated conditions because the H2O content of the melt is continually diluted as the melt ascends through the wedge, dissolving and re-equilibrating with shallower, hotter mantle. Final equilibration with the mantle wedge occurs at shallow depths near the top of the wedge. A model of this process is developed using the vapor-saturated phase relations as a starting point.