Micrometer scale carbon isotopic study of bitumen associated with Athabasca uranium deposits: Constraints on the genetic relationship with petroleum source-rocks and the abiogenic origin hypothesis

In situ analytical techniques – Fourier transform infrared microspectroscopy (μFTIR) and ion microprobe – have been used to unravel the origin of solid bitumen associated with the uranium deposits of Athabasca (Saskatchewan, Canada). Both aliphaticity and carbon isotopic compositions within the samples are heterogeneous but spatially organized in concentric zonations at the micrometer scale. Finally, the δ13C values are positively correlated to the aliphatic contents over an extremely large isotopic range from ∼ − 49‰ to ∼ − 31‰. We infer that this positive correlation may be related to the carbon isotopic fractionations associated with the synthesis of bitumen through the catalytic hydrogenation of CO2, rather than the result of pre-existing petroleum product precipitation and/or alteration (such as radiolysis). This explanation is consistent with (i) published results of abiogenic synthesis experiments, in which the differences in δ13C values between saturated and unsaturated hydrocarbons range from + 2 and + 19‰, in contrast to the differences systematically observed in conventional bitumen and petroleum ranging from 0‰ to − 4‰; (ii) the absence of a similar positive correlation between aliphatic contents and δ13C values in the other bitumen analyzed in the present study, for which a biogenic origin has been unequivocally established (samples from Oklo, Gabon, and Lodève, France, uranium deposits); (iii) the presence of CO2 and H2 in the gas-phase of fluid inclusions in the Athabasca uranium deposits, H2 resulting from water radiolysis.The present results suggest that the δ13C vs. aliphaticity correlation could be used as a criterion to discriminate between abiogenic vs. biogenic origin of macromolecular organic matter.