7 Sm-Nd Isotope Geochemistry of Argillaceous Sediments 209
that existed during diagenetic compaction and dewatering, 2) the fluid phases
were rich in strontium but depleted in REE, and 3) the cement minerals are
strongly enriched in REE and would react less sensitively to external neodymium.
The decoupling of the two isotopic systems can also be controlled by mineralogic
effects such as the growth of minerals which carry various amounts of Sm-Nd and
Rb-Sr and which can develop and exchange with each other independently of
various thermal events. The fact that the finest clay fraction (<0.2 ~m) provides an
age of isotopic homogenization of 523 Ma for Nd and around 300 Ma for Sr
demonstrates that the decoupling of both systems from each other is related to the
different bonding of Nd and Sr with the crystal lattice of layered silicates such as
chlorite and illite.
The influence of phosphate-rich phases, in particular apatite, on the
development of the Sm-Nd isotopic system during diagenesis or weak
metamorphism was also discussed by Ohr et al. (199:4). The investigated samples
allowed the authors not only to separate and analyze the clay minerals but also
authigenic apatite. This apatite is characterized by a relatively high I~VSm/1"~Nd
ratio (0.3) and an initial J~3Nd/l"Nd ratio, which when calculated back to the time
of sedimentation or early diagenesis gives an almost identical isotopic
composition to that of the clay minerals. The clay minerals and the apatite appear
to have undergone earl)' isotopic exchange with the same fluid phase at the time of
their formation, thus incorporating Nd of the same isotopic composition. The
Sm/Nd ratios, determined on various clay mineral fractions, show a strong grain
size dependency.
In the stud,, of Ohr et al., the Sm/Nd ratios fall with decreasing grain size. The
Sm/Nd ratios of the bituminous shales (Sect. 7.2) behaved in exactly the opposite
fashion. In the latter case. finer grain-size fractions had higher Sm/Nd ratioss 9
Obviously different mechanisms must be responsible for the fractionation of Sm
with respect to Nd. We assume that progressive crystallization of clay minerals
and the formation of organo-metallic complexes led to REE fractionation in the
case of the bituminous clay shales. Phosphate-bearing detrital sediments behave
differently. The leaching experiments of Ohr et al. (199~) on clays help to
demonstrate this. These authors observed that the amount of leachable Nd and Sm
in the samples increased with increasing grain-size, while the Sm/Nd ratios of the
leachates decreased. Provided the REE ratios and abundances are controlled by
the presence of acid-soluble and REE-rich mineral phases, it can be assumed that
these phases possess lower Sm/Nd ratios but higher REE concentrations in the
coarser fractions than in the finer fractions. This observation was confirmed using
the electron microscope: it was shown that the coarse fraction consisted largely of
monazite and florencite, which are particularly enriched in the lighter REE.
whereas the finer fractions contained more apatite, which tends to be enriched in
the middle REE.
that existed during diagenetic compaction and dewatering, 2) the fluid phases
were rich in strontium but depleted in REE, and 3) the cement minerals are
strongly enriched in REE and would react less sensitively to external neodymium.
The decoupling of the two isotopic systems can also be controlled by mineralogic
effects such as the growth of minerals which carry various amounts of Sm-Nd and
Rb-Sr and which can develop and exchange with each other independently of
various thermal events. The fact that the finest clay fraction (<0.2 ~m) provides an
age of isotopic homogenization of 523 Ma for Nd and around 300 Ma for Sr
demonstrates that the decoupling of both systems from each other is related to the
different bonding of Nd and Sr with the crystal lattice of layered silicates such as
chlorite and illite.
The influence of phosphate-rich phases, in particular apatite, on the
development of the Sm-Nd isotopic system during diagenesis or weak
metamorphism was also discussed by Ohr et al. (199:4). The investigated samples
allowed the authors not only to separate and analyze the clay minerals but also
authigenic apatite. This apatite is characterized by a relatively high I~VSm/1"~Nd
ratio (0.3) and an initial J~3Nd/l"Nd ratio, which when calculated back to the time
of sedimentation or early diagenesis gives an almost identical isotopic
composition to that of the clay minerals. The clay minerals and the apatite appear
to have undergone earl)' isotopic exchange with the same fluid phase at the time of
their formation, thus incorporating Nd of the same isotopic composition. The
Sm/Nd ratios, determined on various clay mineral fractions, show a strong grain
size dependency.
In the stud,, of Ohr et al., the Sm/Nd ratios fall with decreasing grain size. The
Sm/Nd ratios of the bituminous shales (Sect. 7.2) behaved in exactly the opposite
fashion. In the latter case. finer grain-size fractions had higher Sm/Nd ratioss 9
Obviously different mechanisms must be responsible for the fractionation of Sm
with respect to Nd. We assume that progressive crystallization of clay minerals
and the formation of organo-metallic complexes led to REE fractionation in the
case of the bituminous clay shales. Phosphate-bearing detrital sediments behave
differently. The leaching experiments of Ohr et al. (199~) on clays help to
demonstrate this. These authors observed that the amount of leachable Nd and Sm
in the samples increased with increasing grain-size, while the Sm/Nd ratios of the
leachates decreased. Provided the REE ratios and abundances are controlled by
the presence of acid-soluble and REE-rich mineral phases, it can be assumed that
these phases possess lower Sm/Nd ratios but higher REE concentrations in the
coarser fractions than in the finer fractions. This observation was confirmed using
the electron microscope: it was shown that the coarse fraction consisted largely of
monazite and florencite, which are particularly enriched in the lighter REE.
whereas the finer fractions contained more apatite, which tends to be enriched in
the middle REE.