Genesis of Mariana shoshonites: Contribution of the subduction component

The Izu-Bonin-Mariana are contains a unique group of shoshonitic volcanoes from along the magmatic front of this intraoceanic are. Shoshonites are gre atly enriched in incompatible elements compared to lavas typically found in primitive are settings but have fractionations of lithophile (LIL) and hig h-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 shoshon ites 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 (Pb-206/Pb-204 similar to 19 .47) and least radiogenic Nd (epsilon Nd similar to5.6) from along the magm atic front of the are. Despite highly elevated concentrations of fluid-mobi le lithophile elements in the lavas, the similarity of diagnostic element r atios (e.g., Ba/La, Pb/Ce, and U/Th) to those in mid-ocean ridge basalts an d ocean island basalts indicates little role for fluid-induced elemental fr actionation in the generation of these shoshonites. Modeling isotopic data allows up to 6% subducted sediments to be involved, but oxygen isotopic evi dence limits this to < 3%. Low-P fractionation explains most of the chemica l variations observed in these shoshonites. Removal of < 2% Ti-rich phases could fractionate HFSE from LIL; indicating an important role for low-P fra ctionation. 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 bot h equilibration of these melts with mantle and for low-pressure fractionati on.