HE AND SR ISOTOPIC CONSTRAINTS ON SUBDUCTION CONTRIBUTIONS TO WOODLARK BASIN VOLCANISM, SOLOMON-ISLANDS

In order to assess the nature and spatial extent of subduction contributions to arc volcanism, Sr and He isotopic compositions were measured for dredged volcanic rocks from the Woodlark Basin in the western Pacific. 87Sr 86Sr ratios increase geographically, from ocean ridge values (.7025-.7029) at the Woodlark Spreading Center to island arc ratios (.7035-.7039) in the Solomon Islands forearc, with intermediate values near the triple junction where the Woodlark Spreading Center subducts beneath the Solomon Islands. 3He 4He ratios are also more radiogenic in the forearc (6.9 ±.2 Ra at active Kavachi volcano) than along the spreading center, where values typical of major ocean ridges were found (8.2-9.3 Ra). Very low 3He 4He ratios occur in many triple junction rocks (.1 to 5 Ra), but consideration of He isotopic differences between crushing and melting analyses suggests that the low ratios were caused by atmospheric (1 Ra) and radiogenic (~.02 Ra) helium addition after eruption. Variations in unaltered, magmatic 3He 4He, and 87Sr 86Sr ratios are best explained by subduction-related fluid or silicate melt contributions to the magma source region, perhaps from ancient Pacific lithosphere. However, mantle volatiles dominate the generation of Woodlark Basin rocks despite extensive subduction in the region. © 1990.