166 Peter Stille and Graham Shields
This is made clear in the study of Clauer et al. (1990), who investigated
recrystallized smectite from lower Cretaceous and Paleocene shales from the
North Atlantic, not only isotopically (Rb-Sr, O, D) but also using electron
microscopy, X-ray diffraction and major and trace element analyses. Three
different populations of smectite could be differentiated by morphology :
A) a flake-like type with diffuse and irregular rims,
B) a lath-like type, well crystallized with idiomorphic rims and,
C) an intermediate type with similarites to both the first and second types.
The lath-like type was found to be particularly enriched in the finest fraction (<0.2
p.m), while the flakes were found most abundantly in the coarsest fraction. The
authors suggest that the laths are of authigenic origin and the flakes are more
likely to be detrital smectite. The chemical investigations allow no differences to
be established between the three kinds. Likewise. rare earth elements, normalized
to the standard (average North American shales) are not fractionated and yield the
characteristic patterns for detrital clay. The REE data, as well as the major and
trace element information, imply that the laths were formed directly from the preexisting flakes, without any loss or gain of elements. The process appears to have
taken place according to the rule of dissolution/precipitation.
Various grain-size fractions separated out from the clays were brought into
suspension over 15 minutes in IN HCI and allowed to react with the hydrochloric
acid (leaching). The Sr isotopic compositions of residues and leachates arising
from this experiment were analyzed.
According to mass balance taws, untreated clay fractions as well as leachates
and residues deriving from this sort of experiment should all lie on a mixing line
on a Rb/Sr isochron diagram. If the clay minerals formed in isotopic equilibrium
with the ambient fluid phases, from which other accessory minerals possibly
crystallized, then this straight line must represent an isochron. Its gradient should
reflect the age of crystallization, i.e. of isotopic equilibrium and ofdiagenesis. The
inital Sr isotope ratio of this isochron ought to reflect the isotopic composition of
the environment in which the clay minerals crystallized. Should the clay minerals
also contain old, inherited, detrital parts, then these gradients would represent a
geologically meaningless age and the straight line would reflect mixing between
detritus and diagenetic new formation. The results of the leaching experiments
define straight lines in isochron diagrams whose gradients are directly related to
the grain size (Fig. 6.12). The finer the grain size, the lower are the calculated,
apparent ages. These correlation lines represent mixing lines between the detrital
flake and authigenic lath types.
Therefore, these mixing lines do not record the age of formation. The decrease
in the steepness of the mixing line with decreasing grain size records either the
preferential transfer and transport away of 87Sr into the porewater or may be a
result of an increase in the Rb/Sr ratio in the fine, mostly authigenic, clay fraction.
The Sr isotopic ratios do not reflect the isotopic composition of seawater, and so
This is made clear in the study of Clauer et al. (1990), who investigated
recrystallized smectite from lower Cretaceous and Paleocene shales from the
North Atlantic, not only isotopically (Rb-Sr, O, D) but also using electron
microscopy, X-ray diffraction and major and trace element analyses. Three
different populations of smectite could be differentiated by morphology :
A) a flake-like type with diffuse and irregular rims,
B) a lath-like type, well crystallized with idiomorphic rims and,
C) an intermediate type with similarites to both the first and second types.
The lath-like type was found to be particularly enriched in the finest fraction (<0.2
p.m), while the flakes were found most abundantly in the coarsest fraction. The
authors suggest that the laths are of authigenic origin and the flakes are more
likely to be detrital smectite. The chemical investigations allow no differences to
be established between the three kinds. Likewise. rare earth elements, normalized
to the standard (average North American shales) are not fractionated and yield the
characteristic patterns for detrital clay. The REE data, as well as the major and
trace element information, imply that the laths were formed directly from the preexisting flakes, without any loss or gain of elements. The process appears to have
taken place according to the rule of dissolution/precipitation.
Various grain-size fractions separated out from the clays were brought into
suspension over 15 minutes in IN HCI and allowed to react with the hydrochloric
acid (leaching). The Sr isotopic compositions of residues and leachates arising
from this experiment were analyzed.
According to mass balance taws, untreated clay fractions as well as leachates
and residues deriving from this sort of experiment should all lie on a mixing line
on a Rb/Sr isochron diagram. If the clay minerals formed in isotopic equilibrium
with the ambient fluid phases, from which other accessory minerals possibly
crystallized, then this straight line must represent an isochron. Its gradient should
reflect the age of crystallization, i.e. of isotopic equilibrium and ofdiagenesis. The
inital Sr isotope ratio of this isochron ought to reflect the isotopic composition of
the environment in which the clay minerals crystallized. Should the clay minerals
also contain old, inherited, detrital parts, then these gradients would represent a
geologically meaningless age and the straight line would reflect mixing between
detritus and diagenetic new formation. The results of the leaching experiments
define straight lines in isochron diagrams whose gradients are directly related to
the grain size (Fig. 6.12). The finer the grain size, the lower are the calculated,
apparent ages. These correlation lines represent mixing lines between the detrital
flake and authigenic lath types.
Therefore, these mixing lines do not record the age of formation. The decrease
in the steepness of the mixing line with decreasing grain size records either the
preferential transfer and transport away of 87Sr into the porewater or may be a
result of an increase in the Rb/Sr ratio in the fine, mostly authigenic, clay fraction.
The Sr isotopic ratios do not reflect the isotopic composition of seawater, and so