Is anyone prepared to offer an opinion on the fractionation of oxygen
isotopes during the reaction between dolomite and phosphoric acid?

I have been happily using the fractionation factor published by Rosenbaum &
Sheppard (1986, GCA 50, 1147), but I've recently had cause to reconsider
the validity of the value published by these authors.

The recent publication of a revised fractionation factor for magnesite (Das
Sharma et al. 2002, GCA 66, 589) suggests to me that the R&S value may be
incorrect. If one assumes that the acid fractionation factor varies
linearly as a function of chemical composition (mole fraction) in a binary
solid-solution, then at a reaction temperature of 50oC the R&S value is too
large by about 0.6 (on a 1000lnalpha scale). This assumes 1000ln alpha
values for calcite and magnesite of 9.25 and 10.79 respectively, and an
'average' dolomite value (using eqn 3 of R&S) of 10.60.

If a similar relationship is established for a reaction temperature of 25oC
then this allows the fractionation factor reported by Sharma & Clayton (as
revised in Friedman & O'Neil, 1977, USGS Prof Pap 440-KK) to be compared.
On this basis the S&C value plots extremely close to the 'predicted' trend
and appears to provide some empirical evidence to support my uneasiness
regarding the R&S fractionation factor for reactions at 50oC.

I'd be very interested to receive views on this subject. I'd also like to
gauge the general feeling regarding the preference for using dolomite-acid
fractionation factors, as opposed to the Lynton Land approach of not using
a dolomite specific fractionation factor.

Regards,

Steve Crowley

University of Liverpool