Eric,

As John points out, there is a good chance of changing the delta values
with dissolution and reprecipitation - clearly the important issue is
the amount of carbon and oxygen in the rock and in the liquid/fluid.  As
the amount of oxygen is fixed in water (although not the only fluid -
but a common one) and in CaCO3 and the amount of carbon is fixed in the
CaCO3, the real question is how much carbon is in the fluid (and its
d13C value).  There is also the issue of the evolution of fluid
compositions (and rock for that matter) in time and space as water-rock
interactions occur.

There are good examples of such reactions in low temperature systems
involving meteoric water, etc. and metastable marine carbonates--

Lohmann (1987) Paleokarst, (Choquette and James, eds.) Springer-Verlag,
p. 58-80.
Banner and Hanson (1990) GCA 54: 3123-3137.

While there are many examples of wholesale modification of original
isotopic compositions - there are many examples of mineralogic
transformations (dissol-ppt. reactions) that involve preservation of
original "rock" compositions.  For many metastable carbonates subjected
to meteoric waters and basinal brines, etc., d13C values can be
preserved relatively easily (at relatively high apparent W/R ratios)
while preservation of d18O values require signficantly lower W/R
ratios.  Many would argue that such low W/R ratios (where d18O values
are preserved) do not exist in situations where dissol-ppt. reactions
occur - however, several examples of oxygen isotope preservation in such
cases come to mind.  If you would like more information  - contact me
directly.

Hope this helps.

Scott Carpenter



John Valley wrote:

> >Any time you see solution and reprecipitation, there is a real chance
> of a
> >shift of O or C isotope ratio. For oxygen, we have documented shifts
> of up
> >to 10%o over 200 microns in calcite, 14%o over 400 microns along
> cracks in
> >quartz, 25%o across a few microns in quartz overgrowths, and 10%o
> over a
> >few 100 microns in magnetite. I can send reprints.
>
> John Valley
>
>         Dear Isogeochemists,
> >
> >        Can anybody tell if pressure-solution along cleavage in a
> limestone
> >or a marble may shift the carbonate C- and O-isotope compositions,
> not only
> >because of the infiltration of an exotic fluid, but especially
> because of a
> >dissolution mechanism similar to decarbonatation reactions (those
> often
> >modelled by a Rayleigh fractionation model) ?
> >        Any personal statement or bibliographic reference is welcome.
>
> >Cheers,
> >Eric
> >
> >
> --------------------------------------------------------------------------
>
> > Eric Pili
> > Center for Isotope Geochemistry
> > University of California - Berkeley
> > Lawrence Berkeley National Laboratory - Earth Sciences Division
> > MS: 70A-3363 - 1 Cyclotron road , Berkeley, CA 94720, USA
> > Phone: 510-495-2443 (office) or 510-486-4975 (lab) - Fax:
> 510-486-5496
> >
> >email : [log in to unmask]
> >web pages :   http://www.ens-lyon.fr/~pili
> >
> >Formerly and occasionally at :
> > Laboratoire de Sciences de la Terre - Ecole Normale Superieure de
> Lyon
> > 46, Allee d'Italie - F 69364 Lyon Cedex 07 - France
>
> John W. Valley,
>     Professor and Chairman             phone: 608-263-5659
> Dept. of Geology and Geophysics        fax:   608-262-0693
> University of Wisconsin
> 1215 W. Dayton St.
> Madison, WI 53706, USA
>             http://www.geology.wisc.edu/people/valley.html



--
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Scott J. Carpenter
Assistant Professor

Phone:        (972) 883-2481
FAX:          (972) 883-2537
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