Magma Source Evolution Following Subduction Initiation: Evidence From the Element Concentrations, Stable Isotope Ratios, and Water Contents of Volcanic Glasses From the Bonin Forearc (IODP Expedition 352)

International Ocean Discovery Program Expedition 352 to the Bonin forearc drilled sequence of volcanic rocks erupted in immediate aftermath subduction initiation along western margin Pacific Plate. Pristine glasses collected during this expedition were analyzed for major and trace elements, halogens, sulfur, H O isotopes with goals characterizing fluids melts subducted materials that involved generating nascent upper plate crust. Incompatible element compositions oldest lavas (forearc basalts [FAB]) are similar those most depleted mid-ocean ridge globally. Most FAB generated by decompression melting seafloor spreading a near-trench, supra-subduction zone environment only minor involvement diverse generally dilute water-rich from subducting plate. Boninite series enriched incompatible elements mobilized plate, but strongly other such as middle-heavy rare-earth elements. These traits attributed generation boninites largely flux involving first derived leading edge basaltic crust then both sediment. at low pressures shallow, embryonic slab extracted heat hot asthenosphere near trench. The progressive depletion mantle source FAB-through-boninite suggests asthenospheric remained trapped above several million years beneath Philippine Sea