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

Abstract

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 δ 13C measurements of hydrocarbon and CO 2 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 CH 4, CO 2 and C 2–C 5 fluid inclusion gases hosted in the undersaturated peralkaline igneous complex of Khibina in northwest Russia indicate that CH 4 was the juvenile magmatic gas phase ( δ 13C = − 13 to − 8‰, δD = − 120 to − 50‰). The δ 13C values of the C 2–C 5 hydrocarbons are lower relative to CH 4 (by up to 12‰), confirming their abiogenic origin. The δ 13C values of CO 2 are consistently lower than CH 4 (by up to 9‰) and the δD values of C 2H 6 are 70–100‰ lower than the δD CH 4 values. This pattern indicates that the CO 2 and higher hydrocarbons were generated during sub-solidus, kinetic CH 4-oxidation fractionation and polymerisation reactions. Second, CH 4, CO 2 and C 2H 6 fluid inclusion gases were analysed from quartz veins associated with anthracite from the Nant Helen mine in the south Wales coalfields. The δ 13C values of CH 4 (− 27.9 ± 0.1‰), CO 2 (− 3.1 ± 0.8‰) and C 2H 6 (− 20.7 ± 0.1‰) are typical of mature thermogenic coal-associated natural gases, as is the δD CH 4 values (− 148 ± 1‰). Assuming equilibrium, the Δ 13C CO 2–CH 4 value (+ 24.8‰) suggests that temperatures may have reached ∼ 300 °C in the anthracite zone.