192 Peter Stille and Graham Shields
amounts of partially or totally acid-soluble accessory phases, such as apatite or
iron hydroxides, which are likely to be strongly enriched in the REE. Thus, the
isotopic system of the authigenic minerals will be disturbed should this apatite be
of detrital origin. Precise dating of diagenesis would not be possible in this case.
However, if these phases were cogenetic with the authigenic clay minerals, they
will also have incorporated the same, initial isotopic composition. The possible
presence of phosphatic minerals is always a factor to be considered when
interpreting the Nd isotopic ratios of clay minerals in sedimentary rocks. Chemical
analyses (not less reliabJe petrographic o r minera)ogic analyses) demonstrated that
there was no phosphate in the leachates of these samples.
Not only the untreated samples, but also the leachates and the residues were
investigated for their Sr and Nd isotopic compositions. The granulometric
fractions, as well as the leachates and residues of two samples yielded closely
parallel straight lines on the isochron diagrams. The gradients of these straight
lines are nearly identical and yielded identical ages within error of 2036 + 79 und
2099 + 115 Ma (Fig. 7.4). The leachates show systematically higher SmJNd rauos
than the corresponding residues (R) and the untreated clay fractions (El.
According to the law of mass-balance, the residues, leachates and the untreated
clay-fractions should all lie on a straight line on the isochron diagram.
Provided the clay minerals were in isotopic equilibrium with the coexisting
diagenetic fluid phase during their crystallization, the Sm/Nd isotopic ratios of
this fluid phase should correspond to those of the teachates. The resulting parallel
isochrons and their gradients shvuld correspond to the time of isotopic
equilibrium, crystallization and thus of diagenesis. The initial ratios would thus
reflect the Nd isotopic composition of the environment in which the clay minerals
crystallized. It. however, the clay mineral fractions contained inherited deft,tat
components, then the gradients wtll represent a geologically irrelevant age and the
straight lines will correspond to a mixture between detritus and later diagenetic
phases (see Sect. 6.3).
Interpretation of the data shown here requires a careful mineralogic,
morphologic and chemical characterization of the investigated material. The
following observations allow us to consider that these straight lines are indded
geologically relevant.
1) X-ray diffraction investigations (to determine the illite crystallinity index
and to analyze the composition of the mineral components qualitatively), chemtcal
analyses (in order to investigate the mineral assemblages quantitatively) and
morphologic observations using the electron microscope, show us that the
smallest clay fractions are enriched in IM illite (authigenic). A significant amount
of detrital components could only be found in coarser fractions and in the whole
rock. Fig. 7.5 shows the well crystallized I M illite from the smatlest clay fractions
compared with the detrital illite from the coarser fractions.
2) Two different clay fractions, the associated residues, and leachates of two
whole rocks, which were collected at two different localities, define straight lines
amounts of partially or totally acid-soluble accessory phases, such as apatite or
iron hydroxides, which are likely to be strongly enriched in the REE. Thus, the
isotopic system of the authigenic minerals will be disturbed should this apatite be
of detrital origin. Precise dating of diagenesis would not be possible in this case.
However, if these phases were cogenetic with the authigenic clay minerals, they
will also have incorporated the same, initial isotopic composition. The possible
presence of phosphatic minerals is always a factor to be considered when
interpreting the Nd isotopic ratios of clay minerals in sedimentary rocks. Chemical
analyses (not less reliabJe petrographic o r minera)ogic analyses) demonstrated that
there was no phosphate in the leachates of these samples.
Not only the untreated samples, but also the leachates and the residues were
investigated for their Sr and Nd isotopic compositions. The granulometric
fractions, as well as the leachates and residues of two samples yielded closely
parallel straight lines on the isochron diagrams. The gradients of these straight
lines are nearly identical and yielded identical ages within error of 2036 + 79 und
2099 + 115 Ma (Fig. 7.4). The leachates show systematically higher SmJNd rauos
than the corresponding residues (R) and the untreated clay fractions (El.
According to the law of mass-balance, the residues, leachates and the untreated
clay-fractions should all lie on a straight line on the isochron diagram.
Provided the clay minerals were in isotopic equilibrium with the coexisting
diagenetic fluid phase during their crystallization, the Sm/Nd isotopic ratios of
this fluid phase should correspond to those of the teachates. The resulting parallel
isochrons and their gradients shvuld correspond to the time of isotopic
equilibrium, crystallization and thus of diagenesis. The initial ratios would thus
reflect the Nd isotopic composition of the environment in which the clay minerals
crystallized. It. however, the clay mineral fractions contained inherited deft,tat
components, then the gradients wtll represent a geologically irrelevant age and the
straight lines will correspond to a mixture between detritus and later diagenetic
phases (see Sect. 6.3).
Interpretation of the data shown here requires a careful mineralogic,
morphologic and chemical characterization of the investigated material. The
following observations allow us to consider that these straight lines are indded
geologically relevant.
1) X-ray diffraction investigations (to determine the illite crystallinity index
and to analyze the composition of the mineral components qualitatively), chemtcal
analyses (in order to investigate the mineral assemblages quantitatively) and
morphologic observations using the electron microscope, show us that the
smallest clay fractions are enriched in IM illite (authigenic). A significant amount
of detrital components could only be found in coarser fractions and in the whole
rock. Fig. 7.5 shows the well crystallized I M illite from the smatlest clay fractions
compared with the detrital illite from the coarser fractions.
2) Two different clay fractions, the associated residues, and leachates of two
whole rocks, which were collected at two different localities, define straight lines