OXYGEN-ISOTOPE GEOCHEMISTRY OF LAVAS FROM AN OCEANIC TO CONTINENTAL ARC TRANSITION, KERMADEC-HIKURANGI MARGIN, SW PACIFIC

New oxygen isotope data are presented for submarine lavas erupted clos e to the transition between the oceanic Kermadec island are and the co ntinental Taupo Volcanic Zone, New Zealand. Volcanic glasses display d elta(18)O values ranging from +5.65 parts per thousand to +5.83 parts per thousand, clinopyroxenes range from +5.23 parts per thousand to +5 .78 parts per thousand and olivines range from +4.83 parts per thousan d to +5.47 parts per thousand. Coexisting glass and phenocrysts in the lavas are in isotopic equilibrium, with one exception. Oxygen isotope ratios of back-are lavas erupted through oceanic crust are indistingu ishable from mid-ocean ridge basalts or lavas erupted in nearby back-a re settings. Although lavas from the are front display elevated oxygen isotope ratios, the magnitude of O-18-enrichment is too great to resu lt from recycling of subducted material alone. A single back-are lava erupted through continental crust is also relatively O-18-rich suggest ing that the most likely origin for the high delta(18)O signature is l imited amounts of interaction between continental crust and melts deri ved from a mantle wedge that has been variably fluxed by recycled oxyg en, The results of modelling open system behaviour in this volcanic sy stem highlight the need for strong controls on the composition of loca l contaminants, Application of 'average' crustal lithologies, as in ot her volcanic provinces, may lead to erroneous conclusions regarding th e involvement of local basement. (C) 1998 Elsevier Science B.V. All ri ghts reserved.