Oxygen isotope systematics of the Banda Arc: low δ 18O despite involvement of subducted continental material in magma genesis
This study reports new laser fluorination oxygen isotope data for 60 volcanic rocks and 15 sediments distributed over the whole length of the Banda Arc, eastern Indonesia. The melt oxygen isotope values (δ 18O melt) were calculated from phenocryst δ 18O data using theoretical and empirical mineral-melt fractionation factors. The δ 18O melt of individual volcanic centers within the arc varies between 5.57 and 6.54‰, except for Serua (δ 18O melt = 6.13-7.48‰) and Ambon (δ 18O melt = 8.12-8.38‰). These δ 18O melt values are up to 2‰ lower than new and previously published 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 δ 18O wr (calculated from the δ 18O carbonate and silica fractions, both measured conventionally) range between 12.9 and 24.2‰. The low δ 18O melt values (excluding Serua and Ambon) overlap with the mantle range, and are in agreement with simple two-component source-mixing models that predict 1-5% addition of subducted continental material to a depleted MORB-type source in the sub-arc mantle. This percentage is consistent with previous models based on Sr-Nd-Pb-Th-He-Hf isotope data. However, correlations between incompatible trace-element ratios and oxygen isotope systematics requires involvement of partial melts derived from subducted continental material as the major slab component rather than bulk addition. The contribution of hydrous fluids, from both subducted altered oceanic crust and continental material is probably of minor importance. Magma-mantle wedge interaction models could account for the observed low δ 18O signatures, but predicted effects are difficult to distinguish from models without mantle-wedge interaction. Assimilation of arc-crust material is thought to be important for the high δ 18O melt values of Serua and Ambon. AFC modelling suggests up to 20% and 80% assimilation at Serua and Ambon, respectively. Inclusions of meta-sedimentary material and whole-rock Sr-Nd isotopes point to assimilation processes at Nila, but this probably had little effect on the δ 18O of phenocrysts, which record original source values. According to radiogenic isotope data, magma sources in the Banda Arc are the most heavily influenced by fluxes of subducted continental material among currently active oceanic island arcs. Hence, the results of this study suggest that high δ 18O (>6.5‰) in arc lavas are difficult to reconcile with addition of subducted components to magma sources, but must reflect assimilation of arc crustal material.