On 11/27/01 7:31 AM, "Jens Radke" <[log in to unmask]> wrote:
> Dear all,
> I've been asked by a student (Dirk Sachse,[log in to unmask]) to
> forward this question to the list,
> Dear list members,
> I analysed d18O and d13C of a dolomite core (lagoonal, southern margin
> the Zechstein basin, E-Germany). The data show quite clearly that the
> values are derived from initial water isotopic composition (controlled
> evaporation and freshwater inflow; correlate with lithology, high d18O
> anhydrit bearing horizons, low d18O in shales; d13C shows opposite
> So I'd like to calculate the initial isotopic composition of the
> water (assuming a temperature range). I found several equations for the
> of d18O in water vs.
> dolomite (Land, 1983). They seem to give reasonable values (0 to 4 ‰
> for temperatures around 30°C.
> But, since the sediment was deposited as calcite first (assuming the
> reflux model) these equations don't give me information about the
> water but about the dolomitizing fluids.
Three approaches to dolomite-water fractionation have been proposed:
experimental, the "Land" equation, and co-existing calcite-dolomite pairs.
There are several equations, including:
Fritz and Smith
Sheppard and Schwarcz
Matthews and Katz
O'Neil and Epstein
Northrup and Clayton
These are based on high-temperature equilibration experiments where the A
and B constants are empirically determined. Note that at 25°C, these
equations give a spread of over 3” for a given water composition. Note also
that, as in any system, fractionation decreases with increasing temperature.
The Land equation:
Land used a 3” difference between dolomite and calcite at 25°C (to which one
would add 0.8” for the difference in phosphoric acid fractionation, again at
25°C). One could then use the dolomite-calcite fractionation and the O'Neil
and others calcite-water fractionation to "derive" the dolomite-water
fractionation. Unfortunately, in my mind anyway, this value of 3.8” has been
more or less universally accepted, with no real a priori basis for that
acceptance. As before, fractionation (for both dolomite-calcite and for the
acid fractionations) decreases with increasing temperature. Not only should
we expect that natural dolomites form under various temperatures, but also
very few people do their acid digestion of dolomite at 25°C anymore.
Co-existing dolomite-calcite pairs:
Studies of Neogene (and more specifically, Quaternary) dolomite that
co-exists with calcite may provide suitable dolomite-calcite fractionation
relations if, and only if, one can demonstrate co-genesis (something that is
hard to do...). A survey of studies shows there to be a range of about 5”
for these naturally occurring examples (+1.5 to about +6.5 dolomite over
None of these cases provides a satisfactory solution to the question of
dolomite-water fractionation. There is likely no unique value for all
dolomites. BTW, I'm nearing completion on a paper that addresses these
issues - e-mail me off list if you want to chat.
Dr. John D. Humphrey [log in to unmask]
Associate Professor [log in to unmask]
Dept. of Geology and Geological Engineering
Colorado School of Mines
Golden, CO 80401 USA
(303) 273-3819; fax (303) 273-3859