Experimental determination of equilibrium CH4–CO2–CO carbon isotope fractionation factors (300–1200 °C)

Carbon isotope fractionation in the CO 2 –CO–CH 4 –C system was investigated at 300–1200 °C at near-atmospheric pressures by thermally decomposing a variety of organic materials in sealed quartz tubes. Measured gas speciations correspond well to the expected range from thermodynamic calculations. We show that chemical and isotopic equilibrium among gas species is obtained when applying a nickel catalyst for CO 2 /CH 4 , CH 4 /CO, and CO 2 /CO at ≤600 °C or without a catalyzing agent for CO 2 /CO at ≥800 °C. The experiments define carbon isotope fractionation factors for the CO 2 /CH 4 , CO 2 /CO and CH 4 /CO pairs as (i) 10 3 ln ⁡ α CO 2 / CH 4 = 8.9 ( ± 0.6 ) ⋅ 10 5 ⋅ ( 1 T 2 ) 0.825 ( ± 0.005 ) (200–1200 °C) (ii) 10 3 ln ⁡ α CO 2 / CO = 1.07 ( ± 0.05 ) ⋅ 10 6 ⋅ ( 1 T 2 ) 0.830 ( ± 0.003 ) (300–1200 °C) (iii) 10 3 ln ⁡ α CH 4 / CO = 1.1 ( ± 0.2 ) ⋅ 10 3 ⋅ ( 1 T 2 ) 0.462 ( ± 0.001 ) (300–1200 °C), which reproduce the experimental values within 0.2‰ for CO 2 /CH 4 and CO 2 /CO and within 0.12‰ for CH 4 /CO ( T in K, 1 σ fit uncertainties in brackets, CO 2 /CH 4 includes the ≤600 °C experimental data of Horita, 2001 ). Carbon isotope fractionation factors at 1000 °C are still large for CO 2 /CH 4 and CO 2 /CO (6.6 and 7.5‰, respectively) but only 1.5‰ for CH 4 /CO. Elemental carbon precipitated through thermal decomposition of the organic starting materials yields δ 13 C values that depend on the X(O) = O/(O + H) of the organic starting material, i.e. the initial oxidation state of carbon in the organics. We further observe a catalytic effect of the quartz walls on chemical and isotopic exchange in the CO 2 /CO system, probably due to the activation of the silicate surface by H + and OH − ions at >650 °C. Our experimental results yield improved calibrations of the CO 2 /CH 4 equilibria and the first experimental calibration of CO 2 /CO and CH 4 /CO carbon isotope fractionation. Applications are in the tracing of magmatic hydrothermal gas emissions, in carbon-precipitating COH-fluids, and in monitoring of coal-seam fires, but our results may also be applied for quality control during steel-making processes.