Regarding the roasting of aragonite material, my colleagues and I have embarked upon extensive experimentation in an attempt to address this issue.  We have historically, and will continue to, roast samples at 200C for one hour under vacuum.  Our intention is to publish the results soon, time permitting. 
 
A few preliminary thoughts and observations:
 
1)      Different types of aragonite material (synthetic, mineral, biogenic) appear to respond differently to roasting under vacuum.  Synthetic and mineral aragonite do not appear to change mean oxygen and carbon composition from unroasted values to those generated when samples are roasted up to 300C.  Mineral aragonite does not change even up to 400C.  I do not presently have data for synthetic aragonite at 400C. 
2)      Shifts observed relative to roasting biogenic aragonite are sample-dependent.  Our data are based on roasting the material (or not) for 1 hour at each temperature regime (100, 200, 300, 400C).  In the case of an Acropora coral, we observe shifts of approximately -0.3 per mil in oxygen when comparing unroasted to 200C roasted data (we have not run the statistics on these, but the mean values are based on at least 4 data points at all temperature settings).  This shift in the Acropora oxygen is observed regardless of whether the sample was treated or untreated (with H2O2), drilled or mortared (but we are not necessarily willing to try to address the pre-treatment or drilling debate with these data).  An untreated modern specimen of Mercenaria also displays a shift of approximately –0.2 per mil in oxygen between unroasted and 200C-roasted means.   However, an Eocene specimen of Venericardia, discussed below, does not change at all (in either oxygen or carbon) between unroasted and 300C values (and the shift at 400C is no more than -0.1 permil, perhaps statistically insignificant).  Worth noting is that the values appear to “stabilize” for the coral and the Mercenaria, as mean data for 200C vs. 300C results are essentially indistinguishable. 
3)       In terms of the effects of roasting time, I agree that one hour is sufficient, with the caveat that I imagine this could also be sample-type dependent. However, in the cases of the synthetic and mineral aragonite, we see no isotopic shifts related to not roasting the samples, roasting the samples at 200C for 4 hours, and roasting the samples at 300C for 2.5 hours.  As stated above, the mineral aragonite does not even change at 400C for 1 hour.  For the aforementioned specimen of Venericardia, we also have extensive time and temperature data that, for example, show no differences between unroasted splits, splits roasted at 200C for 4 hours, and splits roasted at 300C for 2.5 hours.  I do not have time/temp. results for the coral and Mercenaria, as they were all roasted for 1 hour. When samples do change relative to roasting, I find the fact that the 200C and 300C mean results are the same a compelling argument that “the job has been done” at 200C for one hour, but perhaps that is a flawed assumption.
4)      So, what is “the job” that we are doing?  Why do some specimens shift in oxygen relative to roasting and others do not?  In regard to the debate about the removal of organic matter, I would note that phosphoric acid is supposedly non-reactive with organic material.  And, theoretically, if there are any volatile organics present, which potentially could result in mass-interference during analysis, vacuum-roasting at 200C should remove those volatile contaminants that could possibly be liberated at lower, common reaction temperatures ( ~75C).  Perhaps I am missing something critical here.  However, a recent paper by Dauphin (Chemical Geology 231­2006, 26-37) notes that the removal of volatile and refractory organics is unpredictable even up to ~800C temperatures, so roasting as a means of removing all organics is not necessarily effective.
5)       If changes in either or both carbon and oxygen occur, this may be suggestive of a volatile, mass interfering, contaminant. If changes occur in both carbon and oxygen, it also may suggest decarboxylation of the carbonate and loss of CO2.  It is important to note that in both natural and synthetic mineral aragonite, no shifts were observed in either carbon or oxygen, suggesting CO2 loss is unlikely.  Extensive XRD work also indicated, as expected, that no inversion to calcite occurred at 200C in any of the samples.
6)      However, as noted, shifts in oxygen are observed in some biogenic samples.  We propose that this might reflect the removal of exchangeable oxygen, conceivably in the form of adsorbed or structural water.  This hypothesis finds support in Dauphin’s work whose FTIR analyses evidence a loss of water in coralline aragonite between 60 and 210C. 
 

Therefore, it is our conclusion that it is most advisable for us to roast samples for 1 hour at 200C.  This appears to remove volatile organics and/or water that could impact the isotopic composition of the carbonate. 
 
Apologies for the rushed, albeit lengthy posting.  We hope to get a more comprehensive handle on all of the data and have them available for publication sooner rather than later.
 
At 01:32 PM 12/5/2006, you wrote:
Hi Stan


I forgot to say that ratios of roasted limestones are only slightly higher
(< 0.5 between triplicates)that those not roasted.  The effect on the record
of the MS ratios (ratio non constant but increasing with time) was only
produced in non roasted limestones and disappeared after roasting.  The
manual of the Micromass 602, that I used during that time (early '80),
explained the line's shape as "changing background at minor mass".
Best regards,

Héctor

   

-----Mensaje original-----
De: Stable Isotope Geochemistry [ mailto:[log in to unmask]] En nombre
de Stan Halas
Enviado el: Martes, 05 de Diciembre de 2006 02:38 p.m.
Para: [log in to unmask]
Asunto: Re: [ISOGEOCHEM] RV: [ISOGEOCHEM] Carbonate roasting temperatures
and other pretreatments

Dear All,

I would like to aware those who roast organics (I mean even impurities in
carbonates) in vacuum.
This may lead to bad background around masses 44 to 46, which in consequence
may result in highly positive delta values, of coarse UNRELIABLE!!!!
Roasting should be done in a stream of He carrier gas which sweeps the
sample to be analyzed.

Cheers,   Stan


At 13:54 06-12-05 -0300, you wrote:
>
>
>
>----------
>Dear all
>
>I have made a lot isotope analyses on gray and black limestones from
>differnt localities of the Precordillera, Argentina.  First without
>roasting and then roasted.   I do not know if in any case obtained the
>actual values.  The only thing I know is that after roasting I coud get
>3 replicates of each sample within 0.1 %o  and witn no roasting
>differences of 1 %o or more.  If you plot the traces of ratios measured
>in the MS they are not parallel, but they increase or decrease linearly
>during the measurement.  I used to roast the samples at 300 ºC under
vacuum.
>regards,
>
>Héctor O. Panarello
>Director
>INSTITUTO DE GEOCRONOLOGÍA Y GEOLOGÍA ISOTÓPICA (INGEIS) Pabellón
>INGEIS - Ciudad Universitaria
>1428 Ciudad de Buenos Aires
>Tel: +54-11-4784 7798
>Fax  +54-11-47833024
>

Dr. Stanislaw Halas
Mass Spectrometry Laboratory
Institute of Physics, UMCS
20-031 Lublin, Poland
phone: +48 81 537-62-75
fax: +48 81 533-36-69

--
Este mensaje ha sido analizado por el servidor AntiSpam de FCEyN.
y esta libre de virus y otros contenidos peligrosos.
Por consultas comuniquese con [log in to unmask]

Lora L. Wingate
University of Michigan
Department of Geological Sciences
Stable Isotope Laboratory
1100 North University
1013 C.C. Little Building
Ann Arbor, MI 48109-1005