Xenon isotopes in Archean and Proterozoic insoluble organic matter: a robust indicator of syngenecity?

Abstract

Insoluble organic materials (kerogens) isolated from ancient sedimentary rocks provide unique insights into the evolution of early life. However, establishing whether these kerogens are indeed syngenetic with the deposition of associated sedimentary host rocks, or contain contribution from episodes of secondary deposition, is not straightforward. Novel geochemical criterions are therefore required to test the syngenetic origin of Archean organic materials. On one hand, the occurrence of mass-independent fractionation of sulphur isotopes (MIF-S) provides a tool to test the Archean origin of ancient sedimentary rocks. Determining the isotope composition of sulphur within kerogens whilst limiting the contribution from associated minerals (e.g., nano-pyrites) is however challenging. On the other end, the Xe isotope composition of the Archean atmosphere has been shown to present enrichments in the light isotopes relative to its modern composition, together with a mono-isotopic deficit in ¹²⁹Xe. Given that the isotopic composition of atmospheric Xe evolved through time by mass dependent fractionation (MDF) until ∼2.5-2.0 Ga, the degree of MDF of Xe isotopes trapped in kerogens could provide a time stamp for the last chemical equilibration between organic matter and the atmosphere. However, the extent to which geological processes could affect the signature of Xe trapped in ancient kerogen remains unclear. In this contribution, we present new Ar, Kr and Xe isotopic data for four kerogens isolated from 3.4 to 1.8 Gy-old cherts and confirm that Xe isotopes from the Archean atmosphere can be retained within kerogens. However, new Xe-derived model ages are lower than expected from the ages of host rocks, indicating that initially trapped Xe components were at least partially lost and/or mixed together with some Xe carried out by younger generations of organic materials, therefore complicating the Xe-based dating method. Whilst non-null Δ³³S values and ¹²⁹Xe deficits relative to modern atmosphere constitute reliable imprints from the Archean atmosphere, using Xe isotopes to provide information on the syngenetic origin of ancient organic matter appears to be a promising - but not unequivocal - tool that calls for further analytical development.

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