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

Abstract

Carbon isotope fractionation in the CO2–CO–CH4–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 CO2/CH4, CH4/CO, and CO2/CO at ≤600°C or without a catalyzing agent for CO2/CO at ≥800°C. The experiments define carbon isotope fractionation factors for the CO2/CH4, CO2/CO and CH4/CO pairs as(i)103ln⁡αCO2/CH4=8.9(±0.6)⋅105⋅(1T2)0.825(±0.005) (200–1200°C)(ii)103ln⁡αCO2/CO=1.07(±0.05)⋅106⋅(1T2)0.830(±0.003) (300–1200°C)(iii)103ln⁡αCH4/CO=1.1(±0.2)⋅103⋅(1T2)0.462(±0.001) (300–1200°C), which reproduce the experimental values within 0.2‰ for CO2/CH4 and CO2/CO and within 0.12‰ for CH4/CO (T in K, 1σ fit uncertainties in brackets, CO2/CH4 includes the ≤600°C experimental data of Horita, 2001). Carbon isotope fractionation factors at 1000°C are still large for CO2/CH4 and CO2/CO (6.6 and 7.5‰, respectively) but only 1.5‰ for CH4/CO.Elemental carbon precipitated through thermal decomposition of the organic starting materials yields δ13C 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 CO2/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 CO2/CH4 equilibria and the first experimental calibration of CO2/CO and CH4/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.