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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
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Helium isotopes of seawater in adjacent sea of Nansei Islands, Southwest Japan

Naoto Takahata, Tsuyoshi Watanabe, Kotaro Shirai, Manabu Nishizawa, Yuji Sano
Geochemical Journal, Vol. 38, No. , P. 593-600, 2004

ABSTRACT

We have measured helium isotopic ratios of 43 water samples with various depths collected in Eastern East China Sea and Northwestern Philippine Sea, adjacent region of Nansei Islands, Southwestern Japan. The 3He/4He ratios vary significantly from 0.994 Ratm to 1.242 Ratm where Ratm is the atmospheric ratio of 1.39 × 10-6. Mid-depth (750–1500 m) samples in this study would be affected by the source of subduction-type mantle helium in the Okinawa Trough. Noble gas abundances (neon, argon, krypton and xenon) were measured in 24 water samples with various depths collected in Northwestern Pacific Ocean and Northwestern Philippine Sea (Nansei Trench). Neon abundances show slight excess relative to air saturated seawater at ambient temperature and salinity. This may be due to either air bubble effect or contamination during the sampling. When these effects are corrected using the neon anomaly, heavier noble gas abundances (argon, krypton and xenon) of samples with the temperature higher than 5°C (shallower than 500 m) agree well with those of calculated air saturated seawater, while the lower temperature samples (deeper than 500 m) indicate anomaly of −7% to +10%.

KEYWORDS

3He/4He ratio, mantle helium, heavy noble gas, Okinawa Trough, seawater

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