Abiogenic hydrocarbon isotopic signatures in granitic rocks: Identifying pathways of formation

Abstract The stability and isotopic composition of hydrocarbons formed in the mantle are controversial subjects. Knowing the range in isotopic compositions of abiogenically-derived hydrocarbons is important for recognising biological signatures in ancient and extra-terrestrial materials. In an effort to enhance this database, stable isotope results are reported here for hydrocarbon-bearing fluid inclusions hosted in two peralkaline igneous complexes from Lovozero, Russia and Strange Lake, Canada. Based on the distribution of isotopic compositions, we propose three pathways for abiogenically-generated hydrocarbons. Type-1 (δ13CCH4 > δ13CCO2 and δ13CC2 +) represents mantle-derived hydrocarbons generated in equilibrium with hyperagpaitic magmas in the upper mantle, and suggests that mantle CH4 and C2H6 δ13C and δ2H could have values of ~ − 5.3 and ~ − 112‰, and ~ − 10.8 and − 162‰, respectively. Type-2 (δ13CCO2 > δ13CCH4 > δ13CC2 +) represents Fischer–Tropsch-type hydrocarbon generation in quartz-bearing peralkaline rocks with CO2 as the initial magmatic gas phase. For Type-2, δ13CCO2 values are ~ − 2‰, whereas δ13CCH4 and δ2HCH4 values range from − 32 to − 20‰ and − 170 and − 160‰, respectively. The δ13CC2H6 values are slightly lower than associated δ13CCH4 values. Type-3 (δ13CCO2 > δ13CCH4 < δ13CC2 +) represents an overprint of Type-2, formed during low temperature alteration of quartz-bearing peralkaline rocks. Here, δ13CCO2 values are variable, − 14.0 to − 9.5‰, δ13CCH4 values range from − 30.8 to − 20.8‰ while δ13CC2H6 values are higher than associated δ13CCH4 values. Both Type-2 and Type-3 isotopic compositions mimic patterns normally considered to be thermogenic in origin, thus demonstrating that isotopic data alone cannot be used reliably to distinguish between hydrocarbons of abiogenic versus biogenic origin.