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

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 and 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.