OXYGEN-ISOTOPE VARIATIONS IN LAU BASIN LAVAS

New laser-fluorination oxygen isotope data are presented for volcanic glasses and phenocrysts from Lau and North Fiji Basin lavas. The low o xygen blank of the technique allows accurate analysis of these often s carce igneous phases. delta(18)O values of volcanic glass range from 5 .53 parts per thousand to 6.06 parts per thousand. Oxygen isotope rati os of ferromagnesian and plagioclase phenocrysts are lower and higher than their host glasses, respectively, and coexisting phases appear to represent oxygen isotope equilibrium at magmatic temperatures. Evolve d lavas from the propagating and dying rift tips in the Central Lau Ba sin display significant O-18-enrichment with magmatic evolution due to FeTi-oxide crystallisation and require that precursor melts had relat ively low delta(18)O values immediately prior to oxide saturation. Dev elopment of these precursor melts from basaltic parents involved assim ilation of hydrothermally altered sheeted dykes ol gabbros in recently accreted oceanic crust accompanied by crystallisation of a plagioclas e-rich assemblage. The stable isotope geochemistry and petrology of Ce ntral Lau Basin lavas, constrain magmatic evolution to depth between 2 .5 and 5 km beneath the seafloor. A dacite from Valu Fa Ridge is sligh tly O-18-enriched relative to Valu Fa basalts, consistent with crystal lisation of a mineral assemblage similar to mid-ocean ridge basalts. O -18/O-16 ratios of Lau Basin basaltic glasses decrease with Mg Number; this observation could be reconciled with the assimilation and fracti onal crystallisation model proposed for initial differentiation of the evolved lavas. However, oxygen isotope data for Central Lau Basin bas alts correlate positively with Ba/La while basaltic glasses from throu ghout the region display a negative correlation between delta(18)O and Na-8.0 suggesting that recycling of subducted oxygen and/or mantle fe rtility may also exert an influence on O-18/O-16 of primary melts. Pre sently, the lack of quantitative control over oxygen isotope fractiona tion in basaltic systems prohibits resolution of these effects. Detail ed investigation of less complex suites of lavas, employing the precis ion offered by laser fluorination, may provide new insights into the b ehaviour of oxygen at magmatic temperatures. (C) 1998 Elsevier Science B.V.