Dear Attila,

yes, you can obtain inversely correlated, linear array of del13C and del18O values for carbonates by degassing CO2 from a H2CO3 dominated aqueous fluid, but for the temperature range of 130-80 deg C you arrive at del13C=+4 (PDB) and del18O= -1(SMOW) for the EQUILIBRIUM FLUID coexisting with calcite with del13C from -3 to -12 and del18O between 10-12.5.  Carbon  and oxygen molar fractions (X) in the degassed phase are 0.49 and 0.1, respectively, i.e. fully unacceptable values. Moreover, the XC value is close to 0.5, i.e. the limiting value at which the Zheng´s model fails.

vratislav
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> From: Attila Demeny <[log in to unmask]>
> To: [log in to unmask]
> Subject: Re: trend in carbonates
> Date: 3. apríl 2000 14:18
>
> Dear Vratislav,
>
>
> I would propose to read the fundamental papers of Yong-Fei Zheng (see
> below). I also observed such trends in basalt- and lamprophyre-related
> carbonates (although formed at much higher temperatures). Possible
> explanations could be found in fractionations during CO2-H2O degassing
> and associated changes in the carbonate-fluid isotope fractionation due
> to XCO2 decrease.
>
> Best regards,
> Attila
>
> Zheng, Y.-F. (1990): Carbon-oxygen isotopic covariation in hydrothermal
> calcite during degassing of CO2. Mineral. Deposita, 25, 246-250.
> Zheng, Y.-F., Hoefs, J. (1993): Carbon and oxygen isotopic covariations
> in hydrothermal calcites. Mineralium Deposita, 28, 79-89.
> Demeny, A., Forizs, I., Molnar, F. (1994): Stable isotope and chemical
> compositions of carbonate ocelli and veins in Mesozoic lamprophyres of
> Hungary. European Journal of Mineralogy, 6, 679-690.
> Demeny, A., Harangi, Sz. (1996): Stable isotope studies on carbonate
> formations in alkaline basalt and lamprophyre series: evolution of
> magmatic fluids and magma-sediment interactions. Lithos, 37, 335-349.
>
>
> --
> *********************************************************************************
>
> Dr. Attila Demeny
> Geokemiai Kutatolaboratorium - Laboratory for Geochemical Research
> Magyar Tudomanyos Akademia - Hungarian Academy for Sciences
> Budapest, Budaorsi ut 45, H-1112, Hungary
> Tel: (361-)309-2600/ ext. 1223 (office), 1394 (lab)
> Tel/fax: (361-)319-3137
> e-mail: [log in to unmask]
> Http://www.core.hu/~demeny
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