ABSTRACT

We have measured ³He/⁴He and ⁴He/²⁰Ne ratios and ⁴He concentrations in 19 South Pacific water samples with various depths from station SA-6 located in the Solomon Sea (6°38'S, 153°38'E) and SA-7 in the central Coral Sea Basin (14°16'S, 154°19'E) on the KH-92-4 cruise of the Research Vessel, Hakuho Maru of the University of Tokyo. The ³He/⁴He and ⁴He/²⁰Ne ratios vary significantly from 0.988 R_atm to 1.276 R_atm and from 0.254 to 0.292, respectively. An extensive plume of water enriched in ³He has been discovered at a depth of about 2000 meter. Taking ³He/⁴He data of the Geosecs and the South Tow expeditions, 25% excess ³He plume can be traced over 5000 kilometers to the west of the East Pacific Rise. Observed 25% excess ³He plume at SA-6 and 7 may be independent from the ridge, which suggests that there is another source of the mantle derived helium in the area studied.