Magmatic processes during the earliest stage of subduction initiation are still not well understood. We examined peridotites recovered from an exhumed crust-mantle section exposed along the landward slopes of the northern Izu-Bonin Trench using the Japan Agency for Marine-Earth Science and Technology's remotely operated vehicle KAIKO7000II. Based on the Cr# [Cr/(Cr + Al) atomic ratio] of spinel, two distinctive groups, (1) high-Cr# (>0.8) dunite and (2) medium-Cr# (0.4–0.6) dunite, occur close to each other and are associated with refractory harzburgite. Two distinctive melts were in equilibrium with these dunites: a boninitic melt for the high-Cr# dunite and a mid-oceanic ridge basalt (MORB)–like melt for the medium-Cr# dunite. The TiO2 content of the latter melt is lower than typical MORB compositions. We suggest that the medium-Cr# dunite was a melt conduit for a basalt recently reported from the Mariana forearc that was erupted at the inception of subduction. The wide range of variation in the Cr#s of spinels in dunites from the Izu-Bonin-Mariana forearc probably reflects changing melt compositions from MORB-like melts to boninitic melts in the forearc setting due to an increase of slab-derived hydrous fluids and/or melts during subduction initiation.