Coincidentally, I was about to send a similar email to the list.  I tried
the same technique after reading of its use in a Precambrian Research
article (Kimura et al, 2005).  To try it myself, I made a variety of
mixtures of calcite and dolomite standards, reacting them at 25C and then
75C.  I tried collecting CO2 from the calcite anywhere from 30 min after
reaction to almost immediately.  In every case there was cross-
contamination with reacting dolomite.

However, what I was able to do was get the dolomite values by reacting the
mixes at 25C for 1 hour, pumping off the gas from the calcite (and some
dolomite), and then reacting the remainder of the dolomite at a higher
temp, assuming all the calcite had reacted.  There may be some
fractionation due to less reactive crystals, but the values were very close
to what they should have been for the dolomite std.

I have not pursued this farther, but thought if you know the percentages of
each mineral, the overall mixed isotopic value of the sample, and the
dolomite values, should be able to get to the calcite value by a mass
balance equation.

Very interested to hear if you have any success with this.

Dave Mrofka
UC Riverside Earth Sciences