A gas-chromatograph, continuous flow-isotope ratio mass-spectrometry method for δ13C and δD measurement of complex fluid inclusion volatiles: Examples from the Khibina alkaline igneous complex, northwest Russia and the south Wales coalfields

A new method is described for on-line extraction of complex mixtures of fluid inclusion volatiles for compound specific carbon- and hydrogen-isotope determination by gas-chromatography–continuous flow mass-spectrometry. Reproducibility for multiple aliquots of gas released from single samples and for duplicate samples is generally ≤ ± 0.7‰ for δ13C measurements of hydrocarbon and CO2 gases to concentrations as low as 20 nmol and ≤ ± 7‰ for δD measurements of hydrocarbon gases to concentrations as low as 100 nmol. This approach significantly improves upon the detection limits for trace gas analysis in fluid inclusions and enables rapid stable isotope measurements of multiple gas species in a single run.Two examples are evaluated and discussed in their geological context. First, stable isotope results for CH4, CO2 and C2–C5 fluid inclusion gases hosted in the undersaturated peralkaline igneous complex of Khibina in northwest Russia indicate that CH4 was the juvenile magmatic gas phase (δ13C = − 13 to − 8‰, δD = − 120 to − 50‰). The δ13C values of the C2–C5 hydrocarbons are lower relative to CH4 (by up to 12‰), confirming their abiogenic origin. The δ13C values of CO2 are consistently lower than CH4 (by up to 9‰) and the δD values of C2H6 are 70–100‰ lower than the δDCH4 values. This pattern indicates that the CO2 and higher hydrocarbons were generated during sub-solidus, kinetic CH4-oxidation fractionation and polymerisation reactions.Second, CH4, CO2 and C2H6 fluid inclusion gases were analysed from quartz veins associated with anthracite from the Nant Helen mine in the south Wales coalfields. The δ13C values of CH4 (− 27.9 ± 0.1‰), CO2 (− 3.1 ± 0.8‰) and C2H6 (− 20.7 ± 0.1‰) are typical of mature thermogenic coal-associated natural gases, as is the δDCH4 values (− 148 ± 1‰). Assuming equilibrium, the Δ13CCO2–CH4 value (+ 24.8‰) suggests that temperatures may have reached ∼ 300 °C in the anthracite zone.