198 Peter Sfille and Graham Shields
However, this only goes some way to answering all the questions related to
chemical exchange between the newly formed clay minerals, detritus, organic
hydrocarbons and the fluid diagenetic phases. How, for example, would this fluid
phase crystallize, in which the clay minerals are formed? Where did it come from?
Has there been large exchange of rock volume? Has the fluid travelled far? Did
chemical exchange occur between the fluid phase and the oil ?
Some answers to these questions can come from investigations of the isotopic
characteristics of the coarser fractions and the whole rock at the time of
sedimentation and diagenesis as they are predominantly carriers of the detrital
components. In the isochron diagram of Fig. 7.8A, the Nd isotopic data for the
clay fraction and the whole rock have been corrected for a diagenetic and
sedimentary age of 2.06 Ga. This is important to remember when considering
these data!
The finest clay fractions must in this case, according to the definition of the
isochron, possess identical initial Nd isotopic ratios to the bitumen and thus come
to rest on horizontal lines. The coarser clay fractions and the corresponding whole
rocks define, instead, two straight lines with negative gradients. They cut the
horizontal lines on the low Sm/Nd ratio side at a point defined by the coarsest clay
fractions (<0.4 ~tm), which also help to define the corresponding isochrons.
The 14~Sm/taaNd ratios of these junctions are low and vary between 0.085 and
0.09. The Sm/Nd ratios of the whole rocks and the coarser clay fractions rise with
decreasing potassium and organic carbon content (Fig. 7.8 B, C). The sample with
the highest potassium and organic carbon contents lies between two horizontal
lines, as does the bitumen, and yields an identical Nd isotopic composition to that
of the finest clay fractions of the other two whole rocks. This observation allows
us to make some conclusions regarding the origins of the detritus as well as
shedding light on the diagenetic evolution of the oil -bearing clay shale.
Now to the origin of the detritus: The negative correlation between the
14~Nd/laaNd and Sm/Nd isotopic ratios permits us to assume that these sediments
contain two isotopically different and important detrital components A and B that
were mixed together in different proportions at the time of sedimentation of this
rock.
Component A shows a Sm/Nd ratio which is far lower than the mean of all clay
shales or crustal rocks. Scarcely any mineral other than alkali feldspar possesses
such low ratios. This confirms Norm-composition calculations, which show that
the normative contents of alkali feldspar increase with decreasing Sm/Nd ratios
and increasing r'=3Nd/laaNd isotopic ratios. The whole rock, which lies closest to
the junction of the straight lines contains the highest potassium and normative
alkali feldspar contents. The mixing lines AB reflect therefore mixtures of a
potassium-rich alkali feldspar component A with another detrital component B,
which must have far lower 143Nd/J'UNd and higher Sm/Nd isotopic ratios. With
the help of crustal residence time calculations, the origins of the rare earth
elements and the detritus could be established (Stille et al. 1993).
However, this only goes some way to answering all the questions related to
chemical exchange between the newly formed clay minerals, detritus, organic
hydrocarbons and the fluid diagenetic phases. How, for example, would this fluid
phase crystallize, in which the clay minerals are formed? Where did it come from?
Has there been large exchange of rock volume? Has the fluid travelled far? Did
chemical exchange occur between the fluid phase and the oil ?
Some answers to these questions can come from investigations of the isotopic
characteristics of the coarser fractions and the whole rock at the time of
sedimentation and diagenesis as they are predominantly carriers of the detrital
components. In the isochron diagram of Fig. 7.8A, the Nd isotopic data for the
clay fraction and the whole rock have been corrected for a diagenetic and
sedimentary age of 2.06 Ga. This is important to remember when considering
these data!
The finest clay fractions must in this case, according to the definition of the
isochron, possess identical initial Nd isotopic ratios to the bitumen and thus come
to rest on horizontal lines. The coarser clay fractions and the corresponding whole
rocks define, instead, two straight lines with negative gradients. They cut the
horizontal lines on the low Sm/Nd ratio side at a point defined by the coarsest clay
fractions (<0.4 ~tm), which also help to define the corresponding isochrons.
The 14~Sm/taaNd ratios of these junctions are low and vary between 0.085 and
0.09. The Sm/Nd ratios of the whole rocks and the coarser clay fractions rise with
decreasing potassium and organic carbon content (Fig. 7.8 B, C). The sample with
the highest potassium and organic carbon contents lies between two horizontal
lines, as does the bitumen, and yields an identical Nd isotopic composition to that
of the finest clay fractions of the other two whole rocks. This observation allows
us to make some conclusions regarding the origins of the detritus as well as
shedding light on the diagenetic evolution of the oil -bearing clay shale.
Now to the origin of the detritus: The negative correlation between the
14~Nd/laaNd and Sm/Nd isotopic ratios permits us to assume that these sediments
contain two isotopically different and important detrital components A and B that
were mixed together in different proportions at the time of sedimentation of this
rock.
Component A shows a Sm/Nd ratio which is far lower than the mean of all clay
shales or crustal rocks. Scarcely any mineral other than alkali feldspar possesses
such low ratios. This confirms Norm-composition calculations, which show that
the normative contents of alkali feldspar increase with decreasing Sm/Nd ratios
and increasing r'=3Nd/laaNd isotopic ratios. The whole rock, which lies closest to
the junction of the straight lines contains the highest potassium and normative
alkali feldspar contents. The mixing lines AB reflect therefore mixtures of a
potassium-rich alkali feldspar component A with another detrital component B,
which must have far lower 143Nd/J'UNd and higher Sm/Nd isotopic ratios. With
the help of crustal residence time calculations, the origins of the rare earth
elements and the detritus could be established (Stille et al. 1993).