204 Peter Stille and Graham Shields
Several possible straight lines may result from mixing according to various
mixing parameters (f; Faure, 1986). The gradients of the resultant straight lines
will decrease with increasing carbonate strontium influence. This effect will serve
to decrease the apparent age of the samples.
As the REE are highly mobile in acidic environments and tend to precipitate
under conditions of high pH (see Sect. 3.2), it can be assumed that the alkaline
carbonate-rich, fluid phase only contained very low REE concentrations.
Likewise, it can be assumed that the REE incorporated into, or adsorbed onto, the
clay minerals were n~: mobilized. REE from other sources axe also unlikely to
have been added to the system under these conditions. This would explain how
the Sr isotopic system can be opened while the Nd isotopic system may remain
closed during late diagenesis. Further possibilities of this decoupling will be
pursued in the next part.
Therefore, the organic matter in elastic sediments, whose de~adation creates
the necessary conditions for carbonate dissolution, plays an important role.not
only for the in situ crystallization of isotopically homogeneous clay minerals after
feldspar dissolution but also for the closed system behavior of REE and Pb. These
findings are important also in the context of nuclear waste disposal. Clay minerals
togther with organic matter may help retain or retard actinides and fission
products released from nuclear v~aste (Bros et al., 1993; Stille and GauthierLafaye 1997; Gauthier-Lafaye et al. 1997).
7.3 N d I s o t o p i c H o m o g e n i z a t i o n in P h o s p h a t i c , D e t r i t a l
S e d i m e n t s
The Sm-Nd isotopic system in detrital sediments, which have only minor amounts
of organic material but detectable quantities of phosphate-rich mineral phases
such as apatite, behaves quite differently than the Sm-Nd isotopic system in
bitumtnous clays. Phosphates preferentially incorporate rare earth elements and
can therefore influence the process of Nd homogenization and the behavior of the
Sm-Nd isotopic system of the whole rock as has been shown by Awwiller (1994),
Ohr et al. (1994) and Schaltegger et al. (1994).
Authigenic apatite forms as a result of early diagenetic processes close to the
sediment-water interface (Jahnke et al., 1983; Glenn und Arthur, 1988; Van
Cappellen and Bemer, 1991) depending on the availability of phosphorus in Ihe
por~waters. The incerporation of rare earth elements into the initially REE
depleted biogenic apatite takes place by the coupling of the Si~'/P 5+ -substitutions
onto the tetrahedral positions REE-~/C,a z§ and Na*/Ca 2§ onto the 2 Ca positions. [t
is assumed that REE can be incorporated as a result of a variety of processes. The
enrichment of REE and dissolved phosphorous in the porewaters of anoxic
sediments is thus brought about mainly by the decay of organic components,
Several possible straight lines may result from mixing according to various
mixing parameters (f; Faure, 1986). The gradients of the resultant straight lines
will decrease with increasing carbonate strontium influence. This effect will serve
to decrease the apparent age of the samples.
As the REE are highly mobile in acidic environments and tend to precipitate
under conditions of high pH (see Sect. 3.2), it can be assumed that the alkaline
carbonate-rich, fluid phase only contained very low REE concentrations.
Likewise, it can be assumed that the REE incorporated into, or adsorbed onto, the
clay minerals were n~: mobilized. REE from other sources axe also unlikely to
have been added to the system under these conditions. This would explain how
the Sr isotopic system can be opened while the Nd isotopic system may remain
closed during late diagenesis. Further possibilities of this decoupling will be
pursued in the next part.
Therefore, the organic matter in elastic sediments, whose de~adation creates
the necessary conditions for carbonate dissolution, plays an important role.not
only for the in situ crystallization of isotopically homogeneous clay minerals after
feldspar dissolution but also for the closed system behavior of REE and Pb. These
findings are important also in the context of nuclear waste disposal. Clay minerals
togther with organic matter may help retain or retard actinides and fission
products released from nuclear v~aste (Bros et al., 1993; Stille and GauthierLafaye 1997; Gauthier-Lafaye et al. 1997).
7.3 N d I s o t o p i c H o m o g e n i z a t i o n in P h o s p h a t i c , D e t r i t a l
S e d i m e n t s
The Sm-Nd isotopic system in detrital sediments, which have only minor amounts
of organic material but detectable quantities of phosphate-rich mineral phases
such as apatite, behaves quite differently than the Sm-Nd isotopic system in
bitumtnous clays. Phosphates preferentially incorporate rare earth elements and
can therefore influence the process of Nd homogenization and the behavior of the
Sm-Nd isotopic system of the whole rock as has been shown by Awwiller (1994),
Ohr et al. (1994) and Schaltegger et al. (1994).
Authigenic apatite forms as a result of early diagenetic processes close to the
sediment-water interface (Jahnke et al., 1983; Glenn und Arthur, 1988; Van
Cappellen and Bemer, 1991) depending on the availability of phosphorus in Ihe
por~waters. The incerporation of rare earth elements into the initially REE
depleted biogenic apatite takes place by the coupling of the Si~'/P 5+ -substitutions
onto the tetrahedral positions REE-~/C,a z§ and Na*/Ca 2§ onto the 2 Ca positions. [t
is assumed that REE can be incorporated as a result of a variety of processes. The
enrichment of REE and dissolved phosphorous in the porewaters of anoxic
sediments is thus brought about mainly by the decay of organic components,