Subduction initiation is one of the least understood aspects of plate tectonics. In an effort to obtain the first in situ magmatic record of subduction initiation, the International Ocean Discovery Program Expedition 352 drilled at four sites in the inner trench wall of the Bonin Trench to recover 1.22 km of oceanic upper crust accreted within a few m.y. of subduction initiation. The two sites nearer to the trench (U1440 and U1441) yielded axial and off-axis fore-arc basalts (FAB), while those c. 15 km further from the trench (U1439 and U1442) yielded axial low-silica boninites and high-Mg andesites overlain by off-axis high-silica boninites. This study uses Hf–Nd–Sr–Pb isotope analyses from c. 50 stratigraphically representative core samples to trace the evolution of the mantle source during the brief period of FAB-through-boninite magmatism immediately following subduction initiation. Results show that: 1) the FAB have high εHf relative to εNd and were derived from variably depleted mantle of ‘Indian’ provenance with no detectable subduction input; 2) the axial boninites follow mixing trends between a residual FAB mantle source and a subduction component derived from shallow (amphibolite facies) melting of oceanic crust of ‘Pacific’ provenance; and 3) the off-axis boninites define mixing trends between a hybrid mantle wedge (residual mantle + slab melt) and an additional subduction component with lower εNd and higher 207Pb/204Pb that requires a significant contribution from pelagic sediment. This incoming of pelagic sediments may signify a change from an accretionary to non-accretionary margin as subduction evolves. The results thus indicate a rapidly evolving system in terms of geodynamics, magma genesis and crustal accretion immediately following subduction initiation.