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The oxygen isotopic ratio of water was measured on an aliquot of 100 ml, which
was equilibrated with 0.5 mg reagent NaHCO
3
for 12 h at 25°C, and a headspace
gas of 15 ml was injected into a mass spectrometer (Ijiri et al.
2003
). All samples
were measured at least twice, and analytical precision was 0.1‰. Hydrogen isoto-
pic composition of water was measured using an aliquot of 2 ml, which was reduced
to H
2
on zinc pellets at 470°C. The reduced H
2
was desorbed and its isotopic com-
position was measured by a mass spectrometer (Coleman et al.
1982
). Analytical
precision was 1‰. Isotope compositions are reported as d values relative to the
PDB standard for carbon and oxygen (Craig
1957
) and the SMOW standard for
hydrogen (Craig
1961
), where
(
)
δ =
R
/
R
−×
1
1,000
‰
,
(1)
sample
standard
with
R
being the respective isotope ratios
13
C/
12
C,
18
O/
16
O, and D/H.
3
He/
4
He ratios were measured with a high-precision mass spectrometer
(VG5400) at the Ocean Research Institute following the procedure described by
Sano et al. (
2006
). About 0.5 cm
3
STP of gas sample was introduced into a metal-
lic high-vacuum line. He and Ne were purified using hot Ti-Zr getters and acti-
vated charcoal traps held at 77 K. Abundance ratios of
4
He/
20
Ne were determined
by a quadrupole mass spectrometer installed in the purification line. Then, He
was separated from Ne using a cryogenic charcoal trap held at 40 K, and the
3
He/
4
He ratios were measured statically with a branch tube mass spectrometer.
Resolving power of about 550 at 5% peak height was attained for complete sepa-
ration of the
3
He beam from the H
3
and HD beams. Observed
3
He/
4
He ratios were
calibrated using the atmospheric He standard. The standard deviation of the
3
He/
4
He ratio was less than 1%, and that of the
4
He/
20
Ne ratio was estimated to be
about 10%.
4
Results and Discussion
4.1
Quality of Extracted Gas
The analytical results are shown in Figs.
3
and
4
. The novelty in this study is
gas extraction from pore water by an onboard vacuum system. Gas concentra-
tions in gas samples were higher than those in fluid samples (Fig.
4a, b
), sug-
gesting that the gas sampling could minimize degassing. Figure
4d, e
show that
profiles of isotopic composition of CH
4
and SCO
2
in pore water are reasonably
congruent with those of the fluid and gas samples. It suggests that the degassing
process does not critically affect isotopic compositions, and the vacuum extrac-
tion procedure adopted in this study yields results that are compatible with
conventional methods.
Air contamination can be assessed by using compositions of noble gases (Fig.
5
).
The isotopic ratio of upper mantle-derived He and Ne, (
4
He/
20
Ne)
m
, is 1 × 10
3
(Lupton
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