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
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