Oxygen isotope systematics of the Banda Arc: Low delta O-18 despite involvement of subducted continental material in magma genesis

This study reports new laser fluorination oxygen isotope data for 60 volcan ic rocks and 15 sediments distributed over the whole length of the Banda Ar e, eastern Indonesia. The melt oxygen isotope values (delta O-18(melt)) wer e calculated from phenocryst delta O-18 data using theoretical and empirica l mineral-melt fractionation factors. The delta O-18(melt) of individual vo lcanic centers within the are varies between 5.57 and 6.54 parts per thousa nd, except for Serua (delta O-18(melt) = 6.13-7.48 parts per thousand) and Ambon (delta O-18(melt) = 8.12-8.38 parts per thousand). These delta O-18(m elt) values are up to 2 parts per thousand lower than new and previously pu blished oxygen isotope data obtained on whole-rock powders by conventional methods. We attribute this discrepancy to post-emplacement low-temperature alteration and/or to a systematic deviation of the bulk analysis. Sediment delta O-18(wr) (calculated from the delta O-18 carbonate and silica fractio ns, both measured conventionally) range between 12.9 and 24.2 parts per tho usand. The low delta O-18(melt) values (excluding Serua and Ambon) overlap with the mantle range, and are in agreement with simple two-component sourc e-mixing models that predict 1-5% addition of subducted continental materia l to a depleted MORE-type source in the sub-are mantle. This percentage is consistent with previous models based on Sr-Nd-Pb-Th-He-Hf isotope data. Ho wever, correlations between incompatible trace-element ratios and oxygen is otope systematics requires involvement of partial melts derived from subduc ted continental material as the major slab component rather than bulk addit ion. The contribution of hydrous fluids, from both subducted altered oceani c crust and continental material is probably of minor importance. Magma-man tle wedge interaction models could account for the observed low delta O-18 signatures, but predicted effects are difficult to distinguish from models without mantle-wedge interaction. Assimilation of arc-crust material is tho ught to be important for the high delta O-18(melt) values of Serua and Ambo n. AFC modelling suggests up to 20% and 80% assimilation at Serua and Ambon , respectively. Inclusions of meta-sedimentary material and whole-rock Sr-N d isotopes point to assimilation processes at Nila, but this probably had l ittle effect on the delta O-18 of phenocrysts, which record original source values. According to radiogenic isotope data, magma sources in the Banda A re are the most heavily influenced by fluxes of subducted continental mater ial among currently active oceanic island area. Hence, the results of this study suggest that high delta O-18 (>6.5 parts per thousand) in are lavas a re difficult to reconcile with addition of subducted components to magma so urces, but must reflect assimilation of are crustal material. Copyright (C) 2001 Elsevier Science Ltd.