Helium isotopic composition of the Tabar-Lihir-Tanga-Feni island arc, Papua New Guinea

Abstract

Helium abundances and isotopic ratios have been measured in samples of geothermal gases, submarine ultramafic xenoliths, and subaerial mafic phenocrysts and xenoliths from the Tabar-LihirTanga-Feni (TLTF) arc in the Bismarck Archipelago of Papua New Guinea. These unusual volcanoes are produced by adiabatic decompression melting of subduction-modified upper mantle and apparently carry an exceptionally large slab-derived component. They, therefore, provide an ideal setting for studying the role of subducted helium in arc volcanoes. Helium isotopic ratios in the geothermal fluids, submarine xenoliths, and most helium-rich subaerial samples indicate the mantle source regions beneath the Tabar Island Group and Lihir Island have 3He/ 4He ratios of about 7.2 times the atmospheric ratio ( R A), whereas the mantle source for Ambitle Island (Feni Group) in the more southern TLTF arc has a lower ratio of about 6.6 RA. These ratios are only slightly lower than typical depleted upper mantle values of about 8.5 R A, indicating that even severely slab-modified mantle wedge carries a relatively minor slab-derived helium component. The systematically lower 3He/ 4He ratios of the Ambitle samples are interpreted to reflect a slightly greater slab-derived helium component. Helium isotopic ratios as low as 2 R A, which are correlated with low total helium abundances, were observed in subaerial augitic clinopyroxene phenocrysts. Coexisting olivine and sodium-rich acmitic clinopyroxene have higher helium abundances and isotopic ratios. This isotopic disequilibrium between coexisting mineral phases is interpreted as crustal-level addition of a low 3He/ 4He component to an actively outgassing magma from which different mineral phases trap helium sequentially.