Dear Bill and others,

You're right to question the possibility of isotopic exchange between
carbonate and oxygen flowing through a plasma furnace. It's difficult to
assess the reaction temperature at the surface between carbonate particles
and oxygen in the furnace. When I've removed samples after 4 hours treatment
I estimate that the temperature of the glass container holding the sample
is about 50-60oC. I don't necessarily believe this is the exact temperature
attained by carbonate particles during plasma treatment, but I don't think
the temperature can be much greater. Of course the assumption is that this
temperature is too low to allow isotopic exchange between oxygen and carbonate
to occur- agreement between plasma ashed and bleached samples appears to
support this conclusion. Your test results for roasting fine
grained carbonate samples at 200oC seem particularly informative and
perhaps indicate that roasters should, in future, work at temperatures lower
than those often reported in the literature. As I've mentioned before I can't
think of an obvious reason for using 380oC roasting on calcite when 200oC
roasting appears to be acceptable for aragonite. Perhaps roasting at 200oC
should become the accepted method for those wanting to use this procedure.
This still leaves a question mark against data published for samples roasted
at "high" temperatures. On this basis I think I will conduct a test on a
homogenised sample of brachiopod material to examine the effects of different
roasting temperature and plasma treatment. I'll report the results when
available.

If, by any chance, someone has already conducted this type of test perhaps
they could let us know the outcome. It might also save me running my own
experiments.

Cheers,

Steve Crowley