160 Peter Stille and Graham Shields
The following reactions are likely to take place:
CaCO3 + H , O +COz ....... >Ca *+ + 2HCO3
(II)
2KAISi3Os (K-feldspar) + 2CO 2 +11H_,O ..... >
AI :Si., O.~ (OH).; (smectite) + 2K § + 4I-b, SiO 4 + 2HCO3" (III)
Thus, the decomposition of alkali feldspar does not only lead to the release of
potassium but also to the further formation of smectite. The C O , which is
necessary for these reactions to take place may be provided by the decomposition,
oxidation and maturation of organic carbon (CH 20) at great depths:
1) by the formation of soluble iron through the reduction of iron (lid oxides:
2CHzO + 8 FeOOH + 16H § = 2 C O : + 14 H_,O + 8Fe 2§
(IV)
2) by the reduction of sulphate and formation of hydrogen sulphide:
2 C H , O + SO.~ 2 + H ~ ' = H S - + 2 C O _ , + 2 H z O (V)
Protons which are necessary for these reactions to take place can be produced by
the formation of alumino-silicates (e.g. chlorite) as the following equation shows:
SiO, + clay + cations ---> Al-silicates + water + H"
(VI)
Fig. 6.2. confirms this reaction path and shows that the chlorite formation stage
sets in at just 3000 m depth. The disappearance of K-feldspar and the increasing
amount of illite at the expense of smectite can be explained by diagenetic
processes which according to Hower et al. (1976) took place in a nearly closed
diagenetic system.
Yeh and Savin (1977) carried out oxygen isotope analyses on clay minerals of
various grain sizes. The results of these experiments can be seen displayed in Fig.
6.7 as a function of temperature and depth. The large variation in ~5isO values in
the upper part of the bore profile points to the detrital origin of the micas there.
The isotopic ratios imply that there were various sources for the detrital material
and allow us to disregard the effect of isotopic exchange with pore waters. The
micas begin to show signs of exchange with pore water with increasing
temperature and depth. At temperatures above 85" C and depths of burial greater
than 3000 m the region of variation in isotopic ratios decreases markedly and the
isotopic signatures of the various grains start to merge. Yeh and Savin (1977)
assume this means that significant isotopic exchange has indeed taken place. This
direct isotopic exchange can be related to the dehydration of the clays and
crystallographic alterations in the clay minerals, namely the transformation of
smectite into illite at temperatures of around 80 ~ C. Hydrogen behaves similarly
The following reactions are likely to take place:
CaCO3 + H , O +COz ....... >Ca *+ + 2HCO3
(II)
2KAISi3Os (K-feldspar) + 2CO 2 +11H_,O ..... >
AI :Si., O.~ (OH).; (smectite) + 2K § + 4I-b, SiO 4 + 2HCO3" (III)
Thus, the decomposition of alkali feldspar does not only lead to the release of
potassium but also to the further formation of smectite. The C O , which is
necessary for these reactions to take place may be provided by the decomposition,
oxidation and maturation of organic carbon (CH 20) at great depths:
1) by the formation of soluble iron through the reduction of iron (lid oxides:
2CHzO + 8 FeOOH + 16H § = 2 C O : + 14 H_,O + 8Fe 2§
(IV)
2) by the reduction of sulphate and formation of hydrogen sulphide:
2 C H , O + SO.~ 2 + H ~ ' = H S - + 2 C O _ , + 2 H z O (V)
Protons which are necessary for these reactions to take place can be produced by
the formation of alumino-silicates (e.g. chlorite) as the following equation shows:
SiO, + clay + cations ---> Al-silicates + water + H"
(VI)
Fig. 6.2. confirms this reaction path and shows that the chlorite formation stage
sets in at just 3000 m depth. The disappearance of K-feldspar and the increasing
amount of illite at the expense of smectite can be explained by diagenetic
processes which according to Hower et al. (1976) took place in a nearly closed
diagenetic system.
Yeh and Savin (1977) carried out oxygen isotope analyses on clay minerals of
various grain sizes. The results of these experiments can be seen displayed in Fig.
6.7 as a function of temperature and depth. The large variation in ~5isO values in
the upper part of the bore profile points to the detrital origin of the micas there.
The isotopic ratios imply that there were various sources for the detrital material
and allow us to disregard the effect of isotopic exchange with pore waters. The
micas begin to show signs of exchange with pore water with increasing
temperature and depth. At temperatures above 85" C and depths of burial greater
than 3000 m the region of variation in isotopic ratios decreases markedly and the
isotopic signatures of the various grains start to merge. Yeh and Savin (1977)
assume this means that significant isotopic exchange has indeed taken place. This
direct isotopic exchange can be related to the dehydration of the clays and
crystallographic alterations in the clay minerals, namely the transformation of
smectite into illite at temperatures of around 80 ~ C. Hydrogen behaves similarly