156 Peter Stilte and Graham Shields
3
E
t..(>
0.5 - 2tJm i -'~t~
,L
N
"r
I
I
t
I
o
20
40
60
BO
ioo
l
Illire (%)
Fig. 6.1. Depth dependence of the proportion of illite layers in iltite/smectlte. (Hower et al+
1976)
on material which contained not only detrita] clay minerals but also other silicate
minerals such as micas and feldspars with very, high S?Sr/S6Sr ratios.
Many of the detrital weathering products formed at the Earth's surface under
the influence of isotopically light groundwater (low tsO content; see Sect. 2.4).
These detrital micas should contain far lower 61sO values than those of authigenic
clay minerals which would have formed in isotopic equilibrium with the
isotopically heavier seawater (higher lsO content). The investigations o f Savin
and Epstein (1970) demonstrated that detrital micas possessed ~ I sO values of 1618%~ (SMOW) while authigenic clay minera|s showed much higher values of 263t%c. The oxygen isotope method can therefore be used as an important tool in
sedimentary, research, especially for the differentiation of detrital and authigenic
clay minerals.
In the isotope study of Yeh and Savin (1976) and Yeh and Epstein (1978),
carried out on Quaternary, detrital clay minerals from marine sediments, it was