OXYGEN-ISOTOPE EVIDENCE AGAINST BULK RECYCLED SEDIMENT IN THE MANTLE SOURCES OF PITCAIRN ISLAND LAVAS

THE hypothesis that subducted sediments survive dehydration and/or mel ting in subduction zones to become long-lived geochemical reservoirs i n the mantle has gained support in recent years(1,3). Evidence for suc h reservoirs is found in the geochemistry of ocean island basalts (OIB s), some of which have isotopic and trace-element characteristics plau sibly associated with ancient sedimentary components. In particular, t he EM1 mantle end-member has been identified, principally on the basis of strontium, neodymium and lead isotopes, and has been proposed to c arry a large sediment fraction(3,5). Oxygen isotopes should be sensiti ve indicators of subducted sediment in the sources of OIBs because min erals that interact with water at low temperatures near the Earth's su rface (during weathering, for example) become enriched in O-18 relativ e 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, t ogether with the nearby Pitcairn seamounts, contain among the most ext reme EM1 signatures known, We find the oxygen isotope ratios of the ph enocrysts to be indistinguishable from the average for mantle peridoti te, These results show that the end-member EM1 signature can be produc ed in the absence of substantial (>1-2%) recycled sediment in the mant le.