Re: conodont apatite

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Hello Clive:
        I am familiar with the work that Elderfield has done, along with
the studies with Aldridge. My understanding of the differences between fish
and conodont apatite has to do with the degree of "porousity" (not the
right word but good enough) and how "opened" and "closed" the systems are. 
Conodont apatite is a relatively closed system and crystal packing is
tight.  Once the REE (and for my purposes the Nd) gets incorporated into
the lattice (in the Ca sites) it becomes stable against further diagenesis.
 That last statement should be taken with a grain of salt....perhaps mild
degrees of diagenesis is better said.  Fish bits (mostly scales) are
relatively open and porous and interact/exchange with their surroundings
and are more prone to changes due to diagenic affects.  However, in defense
of fish, I still think there is excellent potential for their use in
geochemical studies.  I believe that if samples of ichthyloliths were
prestine and showed no signs of contamination, they would still give
excellent proxy signatures.  As for conodonts, for this study we have a
range of CAI values and have gone as high as CAI 5 to test how far you can
push diagenesis before it disrupts the values and signatures.  We are still
waiting for the results to come in.  We do the work at Harvard University
at with Stein Jacobsen.  I spent quite a few months there but I had to
leave before all the samples were through the mass spec.  Hopefully, the
data will not be too much longer and I can find out what the real story is.

I'm interested in your work as well....let me know what is going on with
you.


Ciao
Cindy


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Cindy Wright
School of Earth and Ocean Sciences
University of Victoria