International Ocean Discovery Program Expedition 352 to the Bonin forearc drilled the sequence of volcanic rocks erupted in the immediate aftermath of subduction initiation along the western margin of the Pacific Plate. Pristine volcanic glasses collected during this expedition were analyzed for major and trace elements, halogens, sulfur, and H and O isotopes with goals of characterizing the fluids and melts of subducted materials that were involved in generating the nascent upper plate crust. Incompatible trace element compositions of the oldest lavas (forearc basalts FAB) are similar to those of the most depleted mid‐ocean ridge basalts globally. Most FAB were generated by decompression melting during seafloor spreading in a near‐trench, supra‐subduction zone environment with only minor involvement of diverse and generally dilute water‐rich fluids from the subducting plate. Boninite series glasses are enriched in incompatible trace elements mobilized from the subducting plate, but strongly depleted in other elements, such as the middle‐heavy rare‐earth elements. These traits are attributed to generation of boninites largely by flux melting involving water‐rich melts first derived from the leading edge of subducted basaltic crust and then from both subducted crust and sediment. These melts were generated at low pressures as the shallow, embryonic slab extracted heat from hot asthenosphere near the trench. The progressive depletion of the mantle source for the FAB‐through‐boninite sequence suggests that the asthenospheric mantle remained trapped above the nascent subducting plate for the first several million years of subduction beneath the Philippine Sea Plate. Plain Language Summary The origin of crust created along the leading edge of the Philippine Sea Plate after the Pacific Plate began subducting beneath it about 52 million years ago was investigated by analyzing volcanic glasses recovered by drilling the Bonin forearc during International Ocean Discovery Program Expedition 352. The glasses have not been affected by alteration and thus preserve pristine compositions of the erupted lavas. Two deep water drill sites (U1440 and U1441) recovered basalts and rare andesites produced by crystal removal and crust assimilation. The compositions of the basalt glasses suggest that they were produced by depressurization of hot mantle during near‐trench seafloor spreading in the presence of minor amounts of water‐rich fluid from the newly subducting plate. Younger, more silica‐rich lavas termed “boninites” were recovered from two sites drilled in shallower water (U1439 and U1442). Boninite glass compositions demonstrate that these boninites were generated when shallow hot mantle was invaded by water‐rich melts derived first from the subducting basaltic crust and then this crust plus sediment. Key Points Forearc basalts formed in response to subduction initiation by melting of depleted mantle after minor inputs of variably saline fluids Boninites were generated by melting residual mantle after fluxing by water rich melts from altered oceanic crust then crust and sediment The subducting plate melted at exceedingly shallow depths beneath oceanic crust being generated by seafloor spreading