THE hypothesis that subducted sediments survive dehydration and/or melting in subduction zones to become long-lived geochemical reservoirs in the mantle has gained support in recent years(1,3). Evidence for such reservoirs is found in the geochemistry of ocean island basalts (OIBs), some of which have isotopic and trace-element characteristics plausibly associated with ancient sedimentary components. In particular, the EM1 mantle end-member has been identified, principally on the basis of strontium, neodymium and lead isotopes, and has been proposed to carry a large sediment fraction(3,5). Oxygen isotopes should be sensitive indicators of subducted sediment in the sources of OIBs because minerals that interact with water at low temperatures near the Earth’s surface (during weathering, for example) become enriched in O-18 relative to O-16 (ref. 6). We report here the O-18:O-16 ratios of phenocrysts from basalts from Pitcairn Island (southeast Pacific Ocean), which, together with the nearby Pitcairn seamounts, contain among the most extreme EM1 signatures known, We find the oxygen isotope ratios of the phenocrysts to be indistinguishable from the average for mantle peridotite, These results show that the end-member EM1 signature can be produced in the absence of substantial (>1-2%) recycled sediment in the mantle.