Dear Dave:

With regard to particulate matter contaminating carbonate samples from
which you drill powders (of any size) - I have cut, polished, and sampled
many carbonates (eek ! thousands of them (?) - even in a dusty lab) and
have never had a problem with contamination for C,O or Sr isotopes (where
samples as small as 0.1 mg were taken to exchange columns)...

I can't imagine how this can be a problem
unless there are a large number of clogged pores and you remove these when
you take your samples.  A stalagmite should have relatively large crystals
and small amounts of porosity --therefore few sites for contamination.   I
would even polish the thing with alumina or diamond paste and then clean
it before sampling.  This will help determine where to sample and if you
are sampling powdered carbonate. 

I've even cut and polished  Pleistocene corals (with very high porosity)
and not had contamination - you must NOT use rolling grit with these
however - silicon carbide discs (sandpaper for high speed polishing
wheels) and a few minutes in the ultrasonic bath does the trick ! 

Save the gas- wash your rocks !

So much for the helpful hints...

Scott Carpenter


On Wed, 20 Nov 1996, Dave Wayne wrote:

> ...more on calcite, anti-freeze, etc.
> 
> Scott Carpenter pointed out that...
> >The problem of anti-freeze is one of how easily it is removed from the
> >sample - thus solubility of anti-freeze and the porosity of the sample
> >must be considered.  
> 
> >Also,  can't you drain the anti-freeze, clean the saw (thoroughly) and
> >fill it with water ?
> 
> Yes, Scott, you are absolutely correct on both of the above points. Also, i
> think a comparison between samples cut using water & new anti-freeze
> (followed by cleaning to remove the anti-freeze) might be instructive at the
> outset... just to see if there are really any differences. 
> 
> >Particulate matter from previous cuttings should not be a problem unless
> >the pores (if there are any) fill with debris...repeated ultra-sonic
> >cleaning will help here.  
> 
> The propensity for a process, such as sawing, to cause contamination in a
> sample is going to increase as sample size (or C & O content) decreases. I
> don't know what sort of samples Ethan is working with, but with small
> samples, i don't think even the most scrupulous cleaning measures would be
> good enough if that sample was cross-contaminated with C- & O- rich
> particulates (especially other carbonates!). Maybe i sound TOO cautious, but
> i've had to do too many analyses over again because of stuff just like this!
> 
> Dave Wayne
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