6 Isotope Geochemistry of Ctay Minerals 161
(Fig. 6.8; Yeh 1980). A major switch in hydrogen isotopic composition occurs
likewise at about 3000 m depth. The changes in the hydrogen and oxygen isotope
compositions are likely to be related to the transformation of smectite into illite.
Once again we see the confirmation of a very general rule of the oxygen-hydrogen
isotopic system, i.e. that changes in isotopic composition must be related to some
kind of mineralogic transformation or restructuring of a mineral lattice.
With the help of oxygen isotopic analyses on various quartz grain-size
fractions, Yeh and Lavin (1977) could demonstrate that new quartz growth took
place at the same time as the smectite-illite transition (see equations I and III).
This study proves interesting in another regard, too. The authors observed that the
81sO values, calculated for pore water in equilibrium with the <0.1 I.tm fractions
of illite and smectite, rise consistently with increasing depth and temperature. The
oxygen isotopic exchange between smectite, illite and pore water must have
occurred in an almost completely closed diagenetic system. On the basis of good
correlation between the 5~80 of the pore water, the temperature and the bore
profile depth, it can be assumed that vertical pore water circulation must have
been limited in the clay-rich sedimentary, sequence, at least during the period of
illitization.
~ " 3
E
zl
2
"..,
A
I
0
10
5
z~
Z~
9
O
A 0.1-0.5 pm
,O
J~
9 2"101Jm
1
",,,
o >lO pm
e~..
"
/
S / "
,(
,
<
/"
9 , <
I
I
I
I
I
20
30
~0
50
60
Ouortz (%)
Fig. 6.5. Quartz contents of various size fractions as a function of depth. (Hower et al.
t976)
(Fig. 6.8; Yeh 1980). A major switch in hydrogen isotopic composition occurs
likewise at about 3000 m depth. The changes in the hydrogen and oxygen isotope
compositions are likely to be related to the transformation of smectite into illite.
Once again we see the confirmation of a very general rule of the oxygen-hydrogen
isotopic system, i.e. that changes in isotopic composition must be related to some
kind of mineralogic transformation or restructuring of a mineral lattice.
With the help of oxygen isotopic analyses on various quartz grain-size
fractions, Yeh and Lavin (1977) could demonstrate that new quartz growth took
place at the same time as the smectite-illite transition (see equations I and III).
This study proves interesting in another regard, too. The authors observed that the
81sO values, calculated for pore water in equilibrium with the <0.1 I.tm fractions
of illite and smectite, rise consistently with increasing depth and temperature. The
oxygen isotopic exchange between smectite, illite and pore water must have
occurred in an almost completely closed diagenetic system. On the basis of good
correlation between the 5~80 of the pore water, the temperature and the bore
profile depth, it can be assumed that vertical pore water circulation must have
been limited in the clay-rich sedimentary, sequence, at least during the period of
illitization.
~ " 3
E
zl
2
"..,
A
I
0
10
5
z~
Z~
9
O
A 0.1-0.5 pm
,O
J~
9 2"101Jm
1
",,,
o >lO pm
e~..
"
/
S / "
,(
,
<
/"
9 , <
I
I
I
I
I
20
30
~0
50
60
Ouortz (%)
Fig. 6.5. Quartz contents of various size fractions as a function of depth. (Hower et al.
t976)