CHEMICAL AND ISOTOPIC COMPOSITIONS OF VOLCANIC GASES FROM THE EAST SUNDA AND BANDA ARCS, INDONESIA

The easternmost Sunda Arc and the Banda Arc represent a continent-arc collision zone where magma genesis is influenced by subducted continent-derived material. Chemical and isotopic studies of volcanic gas samples from this environment provide information on the sources of volatiles in arc magmas. These volcanic gases, some of which last equilibrated at magmatic temperatures, are characterized by anomalous low He-3/He-4 values, but by common arc values of C/S almost-equal-to 2-4, delta-C-13 almost-equal-to -3 parts per thousand, and delta-S-34tot almost-equal-to +5 parts per thousand. Abundant helium and high He/Ar ratios are consistent with the subduction of terrigenous components in local sediments (or slivers of continental crust). Although individual concentration and isotope ratios of volatile components may be explicable by complex fractionation in the recycling process, the combined data are in agreement with an important role of subducted sedimentary source components. Comparison of our results with volcanic gas data from other arcs indicates that the carbon and sulphur signals in arc gases are relatively insensitive to the amount and nature of sediment on the subsiding plate. Hence, a contribution to arc volcanic gases from subducted altered oceanic crust cannot be excluded.