I agree with Jim Hendry.  Diagenesis is the likely cause of the low oxygen
and carbon isotopic values.  Cathodoluminescence cannot be used as the sole
indicator of preservation.  Is the original texture and mineralogy
preserved?  Evidence for preservation of original chemistry is also a plus.
Furthermore, it helps to analyze other fossils and especially diagenetic
phases to establish diagenetic trends in isotopic composition.

In addition to the papers mentioned by Jim, there are worm tube data in
Gonzalez and Lohmann (1985, Geology 13:811-814).  That paper shows serpulid
worm tubes ("mixed mineralogy") to be similar in O-18 to calcitic
components, whereas half the analyses show C-13 depletion and half do not.



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