188 Peter Stille and Graham Shields
In this study, Stille and Clauer were able to observe for the first time that the
behaviour of the Sm-Nd isotopic system can be decoupted from that of the Rb-Sr
and K-Ar isotopic systems. Four whole rock fractions, which were shown on the
basis of X-ray diffraction to consist of authigenic clay minerals (chlorite, itlite),
and a pure chlorite fraction, defined an isochron on the Sm-Nd diagram giving an
age of 2.08 • 0.25 Ga (Fig. 7.1). The relatively high error results from the quite
small variation in the Sm/Nd ratios of these fractions. The Sm/Nd ratios appeared
to be more dependent on the organic carbon content than the illite/chlorite ratios
as the REE tend to form complexes with organic matter that have higher Sm/Nd
ratios than clay minerals. The incorporation of different amounts of organic
carbon into &e clays led to a positive correlation between organic carbon content
and Sm/Nd ratios (Fig. 7.2). The influence of organic carbon on the mineralogic
and chemical development of detrital sediments was already discussed in Sect.
6.1. and will be referred to again in Sect, 7.2. in the light of the Sm-Nd isotopic
system.
Geologic constraints confirm this older age of deposition, i.e. around 2.O8 Ga.
However doubts stiJ[ creep in concerning the absolute correctness of ~his age
interpretation. For example, the possible presence of detritus leaves questions
unanswered especially as we know that their presence can remain undetected
using standard methods such as X-ray diffraction. Undetected minerals, such as
apatite, with high REE concentrations can alter the Sm-Nd isochron age
significantly (see Sect. 7.3). Therefore, in order to date diagenetic processes using
the Sm-Nd isochron system, other approaches must be applied that allow us to
identity small quanmies of REE-rich phases and to analyze them both chemically
and isotopically. Also the question of to what extent Nd isotopic homogenizatton
can be achieved between authigemc and detrital mineral phases could not be
answered for the Gunflint Formation. Nevertheless, this study was of great
importance for the work that was to follow, as the possibility of dating diagenetic
processes using the Sm-Nd isotopic system had been clearly demonstrated. Also
the influence of organic matter on Nd isotopic homogemzauon during diagenesis
could be recognized. In the following sections, we will discuss in more detail the
influence of both. organic carbon and REE-rich accessory minerals on the Sm-Nd
isotopic systems of clay-rich sediments and sedimentary rocks.
7.2 The Process of Nd Isotopic Homogenization in Bituminous
Shales
Isotope studies of Proterozoic black shales from Gabon (Africa) permitted a full
description of the process of Nd isotopic equilibration between authigenic mineral
phases, fluid phases and oil (Bros et al. 1992; Stille etal. 1993). These sediments
derived from 1000 meter thick sedimentary piles of the "Francevillian", which
In this study, Stille and Clauer were able to observe for the first time that the
behaviour of the Sm-Nd isotopic system can be decoupted from that of the Rb-Sr
and K-Ar isotopic systems. Four whole rock fractions, which were shown on the
basis of X-ray diffraction to consist of authigenic clay minerals (chlorite, itlite),
and a pure chlorite fraction, defined an isochron on the Sm-Nd diagram giving an
age of 2.08 • 0.25 Ga (Fig. 7.1). The relatively high error results from the quite
small variation in the Sm/Nd ratios of these fractions. The Sm/Nd ratios appeared
to be more dependent on the organic carbon content than the illite/chlorite ratios
as the REE tend to form complexes with organic matter that have higher Sm/Nd
ratios than clay minerals. The incorporation of different amounts of organic
carbon into &e clays led to a positive correlation between organic carbon content
and Sm/Nd ratios (Fig. 7.2). The influence of organic carbon on the mineralogic
and chemical development of detrital sediments was already discussed in Sect.
6.1. and will be referred to again in Sect, 7.2. in the light of the Sm-Nd isotopic
system.
Geologic constraints confirm this older age of deposition, i.e. around 2.O8 Ga.
However doubts stiJ[ creep in concerning the absolute correctness of ~his age
interpretation. For example, the possible presence of detritus leaves questions
unanswered especially as we know that their presence can remain undetected
using standard methods such as X-ray diffraction. Undetected minerals, such as
apatite, with high REE concentrations can alter the Sm-Nd isochron age
significantly (see Sect. 7.3). Therefore, in order to date diagenetic processes using
the Sm-Nd isochron system, other approaches must be applied that allow us to
identity small quanmies of REE-rich phases and to analyze them both chemically
and isotopically. Also the question of to what extent Nd isotopic homogenizatton
can be achieved between authigemc and detrital mineral phases could not be
answered for the Gunflint Formation. Nevertheless, this study was of great
importance for the work that was to follow, as the possibility of dating diagenetic
processes using the Sm-Nd isotopic system had been clearly demonstrated. Also
the influence of organic matter on Nd isotopic homogemzauon during diagenesis
could be recognized. In the following sections, we will discuss in more detail the
influence of both. organic carbon and REE-rich accessory minerals on the Sm-Nd
isotopic systems of clay-rich sediments and sedimentary rocks.
7.2 The Process of Nd Isotopic Homogenization in Bituminous
Shales
Isotope studies of Proterozoic black shales from Gabon (Africa) permitted a full
description of the process of Nd isotopic equilibration between authigenic mineral
phases, fluid phases and oil (Bros et al. 1992; Stille etal. 1993). These sediments
derived from 1000 meter thick sedimentary piles of the "Francevillian", which