The Izu-Bonin-Mariana arc contains a unique group of shoshonitic volcanoes from along the magmatic front of this intraoceanic arc. Shoshonites are greatly enriched in incompatible elements compared to lavas typically found in primitive arc settings but have fractionations of lithophile (LIL) and high-field strength (HFSE) incompatible elements characteristic of convergent margin magmas and thus are characterized by an unusually large subduction component. New geochemical and isotopic data for Izu-Bonin-Mariana shoshonites and related rocks are presented and interpreted to examine the origin of these enrichments. Enrichments are associated with distinctive isotopic compositions, including the most radiogenic Pb(206Pb/204Pb~19.47) and least radiogenic Nd(ϵNd~5.6) from along the magmatic front of the arc. Despite highly elevated concentrations of fluid-mobile lithophile elements in the lavas, the similarity of diagnostic element ratios (e.g., Ba/La, Pb/Ce, and U/Th) to those in mid-ocean ridge basalts and ocean island basalts indicates little role for fluid-induced elemental fractionation in the generation of these shoshonites. Modeling isotopic data allows up to 6% subducted sediments to be involved, but oxygen isotopic evidence limits this to <3%. Low-P fractionation explains most of the chemical variations observed in these shoshonites. Removal of <2% Ti-rich phases could fractionate HFSE from LIL, indicating an important role for low-P fractionation. Although many features of these shoshonites are consistent with a greater role for subducted sediments, such a role is not accompanied by an unequivocal and universal signal in both isotopic compositions and trace element abundances and fractionations. This signifies a large role for both equilibration of these melts with mantle and for low-pressure fractionation. ¿ 2001 American Geophysical Union