> Dirk Sachse,[log in to unmask] wrote
> So I'd like to calculate the initial isotopic composition of the
> lagoonal water (assuming a temperature range). I found several equations
> for the
> fractionation of d18O in water vs. dolomite (Land, 1983). They seem to
> give reasonable values (0 to 4 ‰ VSMOW) for temperatures around 30°C.
> But, since the sediment was deposited as calcite first (assuming the
> brine reflux model) these equations don't give me information about the
> initial water but about the dolomitizing fluids.
For calculating the temperature have also a look on the equation of Fritz
and Smith (1970):
*Fritz, P. & Smith, D.G.W. (1970): The isotopic composition of secondary
dolomites. - Geochimica et Cosmochimica Acta, 34, 1161-1173
Using this eqation results in difference of +2,9%o between calcite and
dolomite at 25°C. This is more or less in agreement with other work on this
*Land, L.S. (1980): The isotopic and trace element geochemistry of
dolomite: The state of the Art. - In: D.H. Zenger, J.B. Dunham & R.L.
Ethington (Hrsg.): Concepts and models of dolomitization, SEPM Special
Publication, 28, 87-110
*McKenzie, J.A. (1981): Holocene dolomitization of calcium carbonate
sediments from the coastal sabkhas of Abu Dhabi, U.A.E.: A stable isotope
study. - Journal of Geology, 89, 185-198
But the dolomite problem "primary or not" still remains. For a brief
discussion of the most popular models and equations you may hav look on the
book of Tucker and Wright (should be in your libary):
*Tucker, M.E. & Wright, V.P. (1990): Carbonate Sedimentology.- Blackwell
Scientific Publications, Oxford.
> So, what exactly happens especially to oxygen isotopes during
> dolomitization of calcite by hypersaline brines? Are there any
Oh, the holy gral of dolomitization :-)
A newer paper of Zheng (1999) suggests that there is nearly _no_
fractionation between dolomite and calcite (only 0.56%o at 25°C) and that
dolomite should behave isotopically like calcite. This in contrast to the
older papers cited above, where a difference of ca. 3%o is observed.
Furthermore it is suggested that "dolomite can be a primary chemical
precipitate with a dolomite-water fractionation close to that of
*Zheng, Y.F. (1999): Oxygen-Isotope Fractionation in Carbonate and Sulfate
Minerals. - Geochemical Journal, 33, 109-126.
Robert van Geldern
Institute of Geology and Mineralogy
University of Erlangen
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