178 Peter Stilte and Graham Shields
(component DRL in Fig. 6.18). Dissolution and crystallization processes must
have run their course during this time without the opening of the glauconite
system to the outside and with seawater. It can therefore be excluded from
consideration that the rise in potassium content reflects any input of potassium. It
is possible that this apparent increase is due to volume changes only, which are
characteristic for this stage.
The 87Sr/86Sr ratios begin to fall (Fig. 6.19) while the 1+3Nd/l'UNd ratios begin
to rise around a K content of about 4.5%. Thus, more mature glauconites start to
move along the D'-SW' mixing line (Fig. 6. t7 and 6.I8). As the Sr and Nd
concentrations continue to fall (Fig. 6.19), the decrease in the STSr/SrSr ratios and
the increase in the 1"~3Nd/t~Nd ratios can only be explained by the preferential
release of radiogenic Sr and non-radiogenic Nd of component D from the
glauconite. Dissolution of the now chemically unstable, detrital clay fraction is the
most likely mechanism responsible for the loss of Sr and Nd. The increasing loss
of these elements from the detrital fraction has the consequence that the influence
of Sr and Nd rich phosphatic components (SW) is greatly increased As the
glauconites start to grow at this stage of their development it can be assumed that
chemical equilibrium and isotopic exchange with the immediate environment aiso
begins at this point. This direct exchange with seawater could possibly have led to
the decrease in the 87Sr/SrSr ratios. As REE concentrations in seawater are
comparatively low, it can be assumed that exchange is not responsible for the rise
in Nd isotope ratios, whilst remembering that both Sr and Nd contents decrease
with increasing glauconite development and K20 content. Stille and Clauer (1994)
suggest that the reestablishment of a seawater Sr and Nd isotope signature in the
glauconite is largely controlled by the amount of Sr and Nd in detrital components
that have been released and through the amount and Sr and Nd contents of
phosphate-rich phases in the glauconite.
These studies make it clear that the isotopic composition of detrital clay
minerals can be altered during recrystallization to suit the environment of
deposition or formation, but that isotopic equilibrium is unlikely to be reached.
The possibilities for dating such minerals are therefore restricted. However,
authigenic gtauconites, which formed in chemical equilibrium with seawater and
have incorporated seawater isotopic composition, can be treated differently.
6.3 D a t i n g o f A u t h i g e n i c C l a y M i n e r a l s
Rb*Sr isotope work on authigenic glauconites from the island of Crete containing
up to 8% potassium were used for fixing the age of the Cenomanian / Turonian
boundary by Odin and Hunziker (1982). The samples define a straight line whose
gradient defines an age of 93.5 4- 1.6 Ma (Fig, 6.20). The age agrees with K-Ar
apparent ages (93.0_+ 1.4 Ma) and also with the assumed stratigraphic age.
(component DRL in Fig. 6.18). Dissolution and crystallization processes must
have run their course during this time without the opening of the glauconite
system to the outside and with seawater. It can therefore be excluded from
consideration that the rise in potassium content reflects any input of potassium. It
is possible that this apparent increase is due to volume changes only, which are
characteristic for this stage.
The 87Sr/86Sr ratios begin to fall (Fig. 6.19) while the 1+3Nd/l'UNd ratios begin
to rise around a K content of about 4.5%. Thus, more mature glauconites start to
move along the D'-SW' mixing line (Fig. 6. t7 and 6.I8). As the Sr and Nd
concentrations continue to fall (Fig. 6.19), the decrease in the STSr/SrSr ratios and
the increase in the 1"~3Nd/t~Nd ratios can only be explained by the preferential
release of radiogenic Sr and non-radiogenic Nd of component D from the
glauconite. Dissolution of the now chemically unstable, detrital clay fraction is the
most likely mechanism responsible for the loss of Sr and Nd. The increasing loss
of these elements from the detrital fraction has the consequence that the influence
of Sr and Nd rich phosphatic components (SW) is greatly increased As the
glauconites start to grow at this stage of their development it can be assumed that
chemical equilibrium and isotopic exchange with the immediate environment aiso
begins at this point. This direct exchange with seawater could possibly have led to
the decrease in the 87Sr/SrSr ratios. As REE concentrations in seawater are
comparatively low, it can be assumed that exchange is not responsible for the rise
in Nd isotope ratios, whilst remembering that both Sr and Nd contents decrease
with increasing glauconite development and K20 content. Stille and Clauer (1994)
suggest that the reestablishment of a seawater Sr and Nd isotope signature in the
glauconite is largely controlled by the amount of Sr and Nd in detrital components
that have been released and through the amount and Sr and Nd contents of
phosphate-rich phases in the glauconite.
These studies make it clear that the isotopic composition of detrital clay
minerals can be altered during recrystallization to suit the environment of
deposition or formation, but that isotopic equilibrium is unlikely to be reached.
The possibilities for dating such minerals are therefore restricted. However,
authigenic gtauconites, which formed in chemical equilibrium with seawater and
have incorporated seawater isotopic composition, can be treated differently.
6.3 D a t i n g o f A u t h i g e n i c C l a y M i n e r a l s
Rb*Sr isotope work on authigenic glauconites from the island of Crete containing
up to 8% potassium were used for fixing the age of the Cenomanian / Turonian
boundary by Odin and Hunziker (1982). The samples define a straight line whose
gradient defines an age of 93.5 4- 1.6 Ma (Fig, 6.20). The age agrees with K-Ar
apparent ages (93.0_+ 1.4 Ma) and also with the assumed stratigraphic age.