Dear Alan,
Just a thought. McKirdy and Powell (1974)[Geology, 2: 591-595] removed
pyrite from sediment by Na-borohydride.
I have no idea if and how this also effects the carbonate, you should have a
try. Any acid attack is obviously excluded. If sulfides are large enough
(probably they are not) and not intergrown with the carbonate you can try
methods such as heavy fluids, centrifugation or mechanical ways of
separation...
Removal of the organic matter is a longer discussion. Methods, such as
roasting (375 - 500 degreeC) under a flow of inert gas (eg dry and O2 or CO2
freeN2, Ar, He) or in vacuo, chemical attack by H2O2, Clorox (=
Na-hypochlorite solution - 5.0 - 5.25 percent), eventually in combination
with an organic solvent (eg acetone, methanol, ethanolchloroform, ...),
attack by hexametaphosphate in deionized water (foraminfera treatment by
D'Hondt and Lindinger (1994)[Palaeogeogr., Palaeoclim., Palaeoecol., 112:
363-378]), or plasma ashing, are used. Added to this, ultrasonic treatment
is used to remove contaminating particles from fossils. Ultrasonic treatment
is also possible with the carbonate in a bath with organic solvent (to
remove more effectively the organic compound).
All these methods suffer from incomplete removal of organic material,
leaving 'reduced-C' in the sample, and causing some (minor?) isotopic
fractionation (whatever the reason). It was stated (I can look this up, if
you like) that the only way to get rid of all included organic matters is
completely dissolving the carbonate.... So, no unique or standard procedure
for removal of organic material from carbonates exist. The method used
depends on 'prefered choise' of the analyst and availability of
tools/chemicals in the lab.
Sorry I cannot be more conclusive on this, but this is where it seems to
stand. Largest problem is to get rid of the sulfide(s) without fractionating
the isotopes in the carbonate. But even the more easy looking problem of
organic matter contamination is not that easy as it looks like after the
first quick view.
If anybody on the list has better ideas, I certainly like to hear about it.
Could always be I missed something somewhere....
Best wishes,
Pier.
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***************
Dr. Pier A. de Groot
European Commission
Joint Research Centre
Institute for Reference Materials and Measurements (IRMM)
Retieseweg
2440 Geel
Belgium
Tel. +32 (0)14 571 628
Fax +32 (0)14 571 685
e-mail: [log in to unmask]
Visit my WEB-site about my "Handbook of Stable Isotope Analytical
Techniques" at:
http://www.geocities.com/padegroot/index.html
last update: 27 September, 2001.
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*********************
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Alan Matthews
Sent: 06 November 2001 16:14
To: [log in to unmask]
Subject: carbonate analyses of sulphide-bearing phases.
Dear Listmembers,
This question has probably come up several times before, but here
goes again. I want to analyse carbonate phases (dolomite and calcite)
either with the Thermoquest Gas Bench II system or using the
conventional Phosphoric acid method. The carbonate samples contain
sulphides as pyrite. This sulphide obviously needs to be removed at
some stage prior to the mass spectrometry. I am aware of the method
of passing the gases over hot silver after the phosphoric acid
extraction. However, what I would like to do is to treat the solid
samples to remove the sulphide prior to the phosphoric acid
extraction. Does anybody know of a method for prior removal of
sulphides without affecting the carbonate phase? The samples I am
planning to run vary from calacareous marls to organic rich shales.
So I would guess that an additional question would be how to remove
the organic phases as well.
Sincerely,
Alan Matthews
--
Alan Matthews, Professor
Institute of Earth Sciences
Edmond Safra Campus, Givat Ram
Hebrew University of Jerusalem
Jerusalem 91904
Israel
Tel: (972) 2 658 4913
Fax: (972) 2 566 2581
e-mail: [log in to unmask]
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