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Dear Hilary and others,

I have not seen a comment on that posting yet either - well, here my two
cents....

First of all, if oxidizing organic matter (combustion) by adding oxygen
might effect the oxygen isotopic composition of the carbonate; however, if
only the carbon isotopes are of interest this can be a valid method. If
pyrolysis is used the gases produced by decomposition of organic matter must
be removed, either in a vacuum system or by a flow of inert gas (N2, He,
..).
Your worry on incapsulated materials is a very correct one - if gases or
other 'decomposition' products are locked in they can start to exchange
isotopes with the hosting material, certainly at elevated temperatures. In
fact, this type of exchange occurs also with fluid inclusions (carbonates
can have fluid inclusions - or locked in fluids or volatile components -
even although fluid inclusions might be less common) up to the moment they
decripitate.
I think there does not exist any 'sound' method removing organic matter
completely from carbonate rocks - all methods I know about have their
problems (pyrolysis in vacuum or under inert gas flow; chemical attack of
the organic matter; plasma ashing). I invite anybody who can give me one
method really removing all organic material without effecting the isotopic
composition of the carbonate to show I am wrong here - and I like to learn
from that method too, then...
Excluded is the solution to dissolve the carbonate totally - then probably
all organic material can be excluded, but then it will be hard not to change
the isotopic composition (the oxygen) of the carbonate when redeposition is
applied.

Hilary, I have sent you 'a proposal' some weeks ago, and I did not hear from
you since - maybe you did not receive the message?
I hope you can inform me on this.

Best wishes,
Pier.
********************************************************************
Dr. Pier A. de Groot
Economic Geology Research Unit
University of the Witwatersrand
Private Bag 3
2050 Johannesburg
South Africa
Tel. +27 11 717 6582  Fax. +27 11 339 1697
E-mail: [log in to unmask]

WEB-site on "Handbook of Stable Isotope Analytical Techniques (2 volumes)":
http://www.icon.co.za/~pagroot/index_1.html
last update: November 8, 2000
********************************************************************


----------
>From: Hilary Stuart-Williams <[log in to unmask]>
>To: [log in to unmask]
>Subject: Re: carbonate roasting in vacuo or under O2?
>Date: Fri, Dec 8, 2000, 11:05 am
>

> Greetings
>
> Was there ever a reply to Tom Trull's query regarding whether we are
> oxidising organics (meaning carbohydrates and hydrocarbons?) or just
> decomposing/pyrolysing them?  The problems of combustion in O2 probably
> hinge on poor temperature control if the organics start to decay
> exothermically, but I wonder whether there might not be other problems
> resulting from reduction by organic matter if heated in vacuo?
>
> Really, the aragonite/calcite conversion problem is perhaps a completely
> separate question.
>
> What changes will occur in organics at 220 degrees C in vacuo?
>
> Either way, what happens to organics trapped inside mineral grains, such as
> humic material in speleothem?
>
> Does anybody have any thoughts on this?
>
> Hilary Stuart-Williams
> Research Officer
> Environmental Sciences
> University of East Anglia
> England
>
> .