Insights into operation of the subduction factory from the oxygen isotopic values of the southern Izu-Bonin-Mariana Arc

Oxygen is the most abundant element in the earth, and isotopic analysis of this element in island arc lavas potentially provides sensitive constraints on the proportion of oxygen recycled from subducted material, relative to that extracted from the mantle. Here we report on 225 new oxygen isotopic analyses of whole-rock and glass samples, and clinopyroxene separates, from lavas collected from the southernmost 1500 km of the Izu-Bonin-Mariana (IBM) convergent margin. Whole-rock samples clustered around a mean of 6.11 ± 0.4‰, whereas Mariana Trough glasses and mafic melts, calculated to be in equilibrium with mafic phenocrysts, clustered narrowly around a mean of 5.7‰. These data demonstrate that unequivocal identification of magmatic oxygen requires analysis of fresh glass or mafic minerals, and that the source of southern IBM Arc melts is entirely, or almost entirely, in equilibrium with normal mantle oxygen. If the elemental enrichments characteristic of the subduction component originate in subducted materials, these oxygen isotopic data are most consistent with the interaction of a small amount of sediment melt (<4%; mostly less than 1%) with mantle peridotite to yield the hybrid mantle that melts to form IBM Arc magmas.