Other
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.2485p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 61, Issue 12, pp.2485-2496
Other
5
Scientific paper
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 3 He/ 4 He 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 3 He/ 4 He 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 3 He/ 4 He component to an actively outgassing magma from which different mineral phases trap helium sequentially.
Farley Ken A.
McInnes Brent I. A.
Patterson Desmond B.
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