7 The Sm-Nd Isotope System in Detrital and
Authigenic Argillaceous Sediments
As was shown in the previous chapter, classic isotopic methods such as K-Ar and
Rb-Sr allow us to date authigenic mineral components found in detrital sediments
and sedimentary rocks. We can date diagenetic, hydrothermal or metamorphic
events provided we can separate the associated authigenic mineral components
cleanly. The presence of detrital components, carrying various isotopic
compositions, can however render ages geologically irrelevant. Isotopic
homogenization between migrating fluids and authigenic mineral components at
high temperatures can also lead to a reopening of the isotopic systems of
authigenic clay minerals as they adapt to new physical and chemical conditions.
This is especially relevant for the Rb-Sr and K-At isotopic systems and has the
consequence that mineral phases of more ancient systems frequently remain open.
That Rb-Sr and K-At systems hardly ever permit the correct absolute age dating
of early diagenesis in detrital sediments was demonstrated by the first systematic
Sm-Nd isotopic studies on sediments (Stille and Clauer 1986; Schaltegger et al.
1993). The Sm-Nd age can also be thermally and chemically reset after deposition
and dlagenesis (Toulkeridis et al. 1990; 1994). However, in these studies and
others it was shown that the rare earth elements, including the Sm-Nd isotopic
system, are far more robust against fluid migration in sedimentary rocks than
either the Rb-Sr or the K-Ar systems. This can be understood better using the
example of the clay-rich sedimentary rocks of the Proterozoic Gunflint Banded
Iron Formation (BIF).
7.1 The First Attempts to Date Diagenesis Using the Sm-Nd
Isotope Method
The very first Sm-Nd isotopic study, which dealt systematically with the age and
genesis of clay-rich sedimentary rocks was reported by Stille and Clauer (1986).
The authors investigated clay minerals deposited in the iron-rich horizons of the
Authigenic Argillaceous Sediments
As was shown in the previous chapter, classic isotopic methods such as K-Ar and
Rb-Sr allow us to date authigenic mineral components found in detrital sediments
and sedimentary rocks. We can date diagenetic, hydrothermal or metamorphic
events provided we can separate the associated authigenic mineral components
cleanly. The presence of detrital components, carrying various isotopic
compositions, can however render ages geologically irrelevant. Isotopic
homogenization between migrating fluids and authigenic mineral components at
high temperatures can also lead to a reopening of the isotopic systems of
authigenic clay minerals as they adapt to new physical and chemical conditions.
This is especially relevant for the Rb-Sr and K-At isotopic systems and has the
consequence that mineral phases of more ancient systems frequently remain open.
That Rb-Sr and K-At systems hardly ever permit the correct absolute age dating
of early diagenesis in detrital sediments was demonstrated by the first systematic
Sm-Nd isotopic studies on sediments (Stille and Clauer 1986; Schaltegger et al.
1993). The Sm-Nd age can also be thermally and chemically reset after deposition
and dlagenesis (Toulkeridis et al. 1990; 1994). However, in these studies and
others it was shown that the rare earth elements, including the Sm-Nd isotopic
system, are far more robust against fluid migration in sedimentary rocks than
either the Rb-Sr or the K-Ar systems. This can be understood better using the
example of the clay-rich sedimentary rocks of the Proterozoic Gunflint Banded
Iron Formation (BIF).
7.1 The First Attempts to Date Diagenesis Using the Sm-Nd
Isotope Method
The very first Sm-Nd isotopic study, which dealt systematically with the age and
genesis of clay-rich sedimentary rocks was reported by Stille and Clauer (1986).
The authors investigated clay minerals deposited in the iron-rich horizons of the