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Date: | Thu, 17 Jul 1997 16:28:33 +0100 |
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>A roasters reply.
>
>I suspect that 200 deg C would be sufficient.
>
>-----------------------------------------------------
>Bob Fifer
>Benedum Stable Isotope Laboratory
While at U. of Michigan working on aragonite and calcite cement isotopic
differences I picked up on some of the work K.C Lohmann started at Brown
and with the help of D. Dettman I undertook a series of vacuum roasting
trials on calcite (lab grade reagent), aragonite (ground up bivalve), and
HMC (~18 mol % MgCO3 - ground up echinoderm spines) ranging from 470 to 100
C (one hour roasting followed by in vacuum cooling to room temperature).
Above 200 C aragonite is converted to calcite and the transformation is
progressive so that after one hour at 470 you have a complete
transformation and at about 350 C you have over 50% transformation. The
aragonite transformation was accompanied by a significant isotopic shift in
both carbon and oxygen (both getting lighter in the roasted samples). The
magnitude of the isotopic shift was in the range of 0.5 to 1.5 per mil for
carbon and about 0.5 to 0.6 per mil for oxygen.
While there was a significant change in aragonite isotopic compositions
following roasting there was no such change in the calcite and HMC samples.
Numbers for all mineralogies unroasted and roasted at 100 C were noisier
and had slightly larger errors, but no shift in isotopic composition.
After these experiments we (at U. Michigan) started roasting all aragonite
at 200 C and some of us roasted everything at 200.
We also monitored the evolved gas (insignificant for the 200 and 100 C
roasting, i.e. not enough to get an isotopic analysis). The isotopic
composition of the CO2 evolved during roasting showed very heavy oxygen and
carbon. At the time we suspected a decarbonation reaction as a culprit for
the shift and not interactions with the organic matter. We did not follow
up and attempt to identify the source/mechanism of the mineralogy linked
shift.
In very small sample roastings at a temperature greater than 200 is most
likely unnecesary, the vacuum aspect of it might be more important that
heat.
It seems that the reason the paleothermoeter of Epstein et al., and the
fractionation relationships of Friedamn and O'Neil are the same is that
roasting in the Epstein et al. work took care of aragonite vs.calcite
fractionation differences.
***************************************************
Luis A. Gonzalez Tel.: 319-335-0567
Associate Professor Fax: 319-335-1821
Department of Geology e-mail: [log in to unmask]
The University of Iowa [log in to unmask]
Iowa City, Iowa 52240-1379
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