6 Isotope Geochemistry of Clay Minerals 157
only possible to recognize minor oxygen and hydrogen isotope exchange with
seawater in particles smaller than 0.1 gtm in diameter. Yeh and Eslinger (1986)
carried out similar investigations on the <0.3 ~m clay mineral fraction of deep sea
sediments. The measured 5t80 values scatter around 20%c (SMOW) and do not lie
on the theoretical curve, which can be calculated for oxygen isotopic equilibrium
between clay minerals, porewater and seawater. Even the sample from the deepest
borehole shows low 5z80 values, where one might expect some diagenetic
recrystallization and isotopic exchange. Such low values can easily be
differentiated from the higher ones of authigenic clay minerals.
These data indicate that: t) the isotopic composition of the clay minerals has
not been significantly altered, and 2) the detrital minerals have not been
overgrown by authigenic clay minerals. No significant isotopic exchange could
have taken place between clay minerals and seawater throughout this 600 meter
deep bore hole. The <0.3 gtm fraction has thus far been able to retain its purely
detrital isotopic signature for 3 million years.
Clay minerals behave very differently at even greater burial depths under the
influence of diagenesis. This was the case in several mineralogic, geochemical
and isotopic studies of pelitic sediments in bore holes, sunk into the Mississippi
delta of the US Gulf coast of Texas.
2 ~._.~
/01-2~,m
34 ~l)~. >2pro
E ,v"
f-O.
(U
O
!
I
o
5
Chlorite (%)
Fig. 6.2. Depth dependence of chlorite contents. (Hower et al. 1976)
15