Gas Chemistry of Pore Fluids from Oomine Ridge on the Nankai Accretionary Prism - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 259

where ( 3 He/ 4 He) cor and ( 3 He/ 4 He) obs denote the corrected and observed 3 He/ 4 He
ratios, respectively, and ( 3 He/ 4 He) a is the atmospheric ratio 1.393 × 10 −6 ( R atm ). The
mixing ratio of the mantle and crustal components of the corrected 3 He/ 4 He ratio
can be estimated from the isotopic ratio of mantle He, ( 3 He/ 4 He) m , which is 8 R atm
(Lupton and Craig 1975 ; Craig and Lupton 1976 ; Rison 1980 ), and the radiogenic
crustal ( 3 He/ 4 He) c ratio, which is 0.02 R atm (Morrison and Pine 1955 ). The resulting
equation for helium composed of crustal (C) and upper mantle (M) components is

( He /

3 4

He) = ( He /

3 4

He) ×+ M ( He /

3 4

He) × C (12)

cor m c

where M + C = 1.
The calculated mixing ratios for all of the samples show that the apparent con-
tribution of radiogenic crust-derived He is as large as 95%. Crustal He can be a
mixture of in situ radiogenic He and trapped He from the degassing of the basement
rock beneath the sediments, equivalent to those observed in subduction zones
(Gerling et al. 1971 ; Sano and Wakita 1987 ). The measured samples were collected
at a seepage field, which could reflect discharge of fluid through a splay fault from
the plate boundary (Park et al. 2002 ). However, the small contribution of mantle He
in these samples more likely reflects mixing of crustal He from sediment and crust
surrounding the pathway of the discharging fluid.
The sampling procedure employed here has been shown to be useful for trace
gas chemistry in pore water in sediment, as it can distinguish between mantle and
crustal contributions. It is important to estimate the contribution of components
from magma (abiotic origin) to assess the contribution of sediment and microbes
(biotic origin), particularly in sediment-rich hydrothermal fields. The trace gases in
pore water provide important information on its origin and modification by hydro-
thermal processes; hence, this new technique extends the horizons of studies of
rock-water interactions.

5 Conclusions

1. New techniques were employed for gas sampling, extraction, and measurement.
The results showed that the gas concentrations of the gas samples were higher
than those of fluid ones, suggesting that the effect of degassing on gas samples
during sampling is reduced by rapid gas extraction on board ship. Assessment of
the air contamination by using compositions of noble gases showed that contami-
nation was negligible. Profiles of isotopic compositions of CH 4 and ΣCO 2
showed congruity between fluid and gas samples, suggesting that degassing did
not significantly affect isotopic composition and that the extraction procedure
using the vacuum system worked well.
2. Profiles of Cl, d 18 O, and dD in pore water showed depletion with depth, suggest-
ing that pore water was affected by seeping fluids depleted in Cl, d 18 O, and dD. The
convex-upward Cl concentration profile indicates an advection rate of 3 mm/