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a
b
c
pH
Alk. (meq.)
NH
4
+
(µmol/kg)
7.4
7.8
8.2
0
10
20
0
200
400
0
0
0
1
1
1
2
2
2
3
3
3
4
4
4
d
e
f
Mg
2+
(mmol/kg)
0
Ca
2+
(mmol/kg)
0
SO
4
2-
(mmol/kg)
0
20
40 0
10
20
20
40
0
0
0
1
1
1
2
2
2
3
3
3
4
4
4
g
h
i
δ
18
O
H2O
(‰)
-0.5
Cl
-
(mmol/kg)
540
δ
D
H2O
(‰)
500
580
0
+0.5
-10
-5
0
0
0
0
1
1
1
2
2
2
3
3
3
4
4
4
Fig. 3
Chemical and isotopic compositions of pore water from Oomine Ridge
and Craig
1975
; Craig and Lupton
1976
; Rison
1980
), as is the radiogenic crustal
ratio, (
4
He/
20
Ne)
c
(Morrison and Pine
1955
). The atmospheric ratio, (
4
He/
20
Ne)
a
, is
0.25. Because the
4
He/
20
Ne ratios of radiogenic and mantle He are much larger than
that of atmospheric He dissolved in seawater, it is possible to correct atmospheric
He components as follows (Craig et al.
1978
):
=
4
20
4
20
r
( He / Ne) /( He / Ne)
,
(2)
a
obs
where (
4
He/
20
Ne)
obs
is the observed
4
He/
20
Ne ratio. The
r
value is useful to evaluate
the air contamination, and in our samples we obtained
r
values from 0.011 to 0.052,
indicating negligible contamination, corresponding to 1.1% and 5.2%.
Therefore, the data for gas samples are sufficiently reliable and closely reflect in
situ chemical composition, thanks to our effort to avoid degassing. We rely on the
data from gas samples in discussing concentrations and isotopic data from fluid
samples as complementary information.
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