Oxygen and carbon isotope fractionation in the system dolomite-water-CO2 to elevated temperatures

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Abstract

An experimental study was conducted to determine oxygen and carbon isotope fractionation factors in the system dolomite-water-CO2 at 80-350 and 100-250°C, respectively, by means of direct precipitation (80°C) and dolomitization of CaCO3 (100-350°C). The products are protodolomite with slight Ca-excess (80-100°C) and well-ordered stoichiometric dolomite (150-350°C). Several experimental artifacts (inheritance, premature reactions, and kinetic effects) were tested, although attainment of isotope equilibrium cannot be proven. 18O/16O fractionation factors of (proto)dolomite-water at 80-350°C can be readily expressed with 1σ error: 103lnαdolomite-water=3.140(±0.022)·106T2-3.14(±0.11).Our experimental study, which is generally consistent with a majority of experimental and theoretical studies in the literature, provides for the first time an accurate equation over a wide range of temperature. In combination of the calcite-water equation (O'Neil et al., 1969; Friedman and O'Neil, 1977), 18O/16O fractionation factors of (proto)dolomite-calcite at 80-350°C can also be expressed with 1σ error: 103lnαdolomite-calcite=0.351(±0.028)·106T2-0.25(±0.13). Dolomite is slightly (0.7-2.6‰) enriched in 18O relative to calcite in this temperature range. Given the very good linearity with a 1/T2 term, the above two equations may be extrapolated beyond the temperature range. Our experimental results of 13C/12C fractionation between CO2 and dolomite at 100-250°C also show a linear function with a 1/T2 term with a cross-over temperature of 200°C, which differs from results of theoretical calculations.

Original languageEnglish
Pages (from-to)111-124
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume129
DOIs
StatePublished - Mar 15 2014

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