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

Vroon, P.Z., Lowry, D., Van Bergen, M.J., Boyce, A.J. and Mattey, D.P. (2001) Oxygen isotope systematics of the Banda Arc: Low delta O-18 despite involvement of subducted continental material in magma genesis. Geochimica et Cosmochimica Acta, 65(4), pp. 589-609. (doi: 10.1016/S0016-7037(00)00554-8)

Abstract

This study reports new laser fluorination oxygen isotope data for 60 volcanic rocks and 15 sediments distributed over the whole length of the Banda Are, eastern Indonesia. The melt oxygen isotope values (delta O-18(melt)) were calculated from phenocryst delta O-18 data using theoretical and empirical mineral-melt fractionation factors. The delta O-18(melt) of individual volcanic centers within the are varies between 5.57 and 6.54 parts per thousand, 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(melt) values are up to 2 parts per thousand 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 delta O-18(wr) (calculated from the delta O-18 carbonate and silica fractions, both measured conventionally) range between 12.9 and 24.2 parts per thousand. The low delta O-18(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 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. 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 delta O-18 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 delta O-18(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 delta O-18 of phenocrysts, which record original source values. According to radiogenic isotope data, magma sources in the Banda Are are the most heavily influenced by fluxes of subducted continental material among currently active oceanic island area. Hence, the results of this study suggest that high delta O-18 (gt 6.5 parts per thousand) in are lavas are difficult to reconcile with addition of subducted components to magma sources, but must reflect assimilation of are crustal material.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Boyce, Professor Adrian
• Authors: Vroon, P.Z., Lowry, D., Van Bergen, M.J., Boyce, A.J., and Mattey, D.P.
• Subjects: Q Science > QE Geology
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Geochimica et Cosmochimica Acta
• ISSN: 0016-7037