Variations of oxygen isotope ratios in arc-related lavas can constrain the contributions of subducted crustal igneous rocks, sediments, and fluids to the sub-arc mantle. We have measured oxygen isotope ratios in 72 arc and back-arc lavas from five ocean–ocean subduction zone systems using laser-fluorination analyses of olivine and other phenocrysts and glass. Eighty percent of our samples have {delta}18O values for any given phase (olivine, plagioclase, glass, or biotite) within 0·2{per thousand} of the average value for that phase in upper-mantle peridotites and mid-ocean ridge basalt (MORB); the range for each phase is <=1·0{per thousand}. This result contrasts with previous studies of whole-rock samples (which are significantly more variable even after exclusion of samples believed to be altered or fractionated by magmatic differentiation) and demonstrates that most arc-related lavas contain <=1–2% of 18O-enriched crustal oxygen from any source (i.e. assimilation or subducted contributions). Elevations in {delta}18O that do occur in these basic, arc-derived magmas relative to values most common for mantle-derived lavas are associated both with ‘enriched’ radiogenic isotope signatures and, even more strongly, with chemical indices consistent with high integrated extents of melting of their peridotite sources. We interpret these relationships as evidence that melting in the sources of the high-{delta}18O lavas we have studied was fluxed by addition of high-{delta}18O aqueous fluid (or perhaps a hydrous melt) from the subducted slab, such that sources that contain relatively large components of slab-derived fluid or melt are both relatively 18O enriched and also experienced relatively large amounts of melting. We have developed a quantitative model linking the amount of melting to the extents of 18O, radiogenic isotope, and trace-element enrichment in a mantle source being fluxed by addition of aqueous fluid. Comparison of this model with observed variations in the geochemistry of lavas from the Vanuatu–Fiji–New Caledonia region (the suite of related samples showing the greatest range in {delta}18O observed in this study) constrains the amounts and chemical and isotopic compositions of slab-derived phases in the sources of these arc-related lavas. Assuming a {delta}18O value of 20{per thousand} for the slab-derived fluid, 0·5–1·0 wt % is added to the sources of most mantle-derived arc magmas; the maximum amount of slab-derived flux in the sources of arc magmas according to our results is 2·5 wt %
