Geobiological significance of organic matter preserved in 3.5 Ga hydrothermal barites from the Dresser Formation (Pilbara Craton, Australia)

3.5 billion-year-old bedded barite of the Dresser Formation (Pilbara Craton, Australia) contains abundant primary fluid and solid inclusions. The fluid inclusions contain volatiles like H2S, COS, and CS2, as well as simple organic compounds (e.g., acetic acid, organic polysulfanes, thiols) [1]. As the bedded barite may have been formed from hydrothermal fluid discharge into an evaporitic caldera [2], these compounds could have acted as fertile substrates for early life forms thriving in this environment [1]. The solid inclusions consist of minerals (metal sulfides, carbonates, silicates) and insoluble organic matter (kerogen) [1,3; this study]. The latter are analysed in greater detail here. High-resolution Raman mapping and microscopy revealed three populations of kerogen inclusions based on their location in the barite deposit: (i) inclusions at the edges of single growth zones of barite crystals, (ii) inclusions within barite crystals, and (iii) inclusions in secondary quartz veins. Peak metamorphic temperatures for these three kerogen populations were determined using Raman spectroscopy. Near edge X-ray absorption fine structure (NEXAFS) measurements revealed a highly aromatic nature of the kerogen. Taken together, the combination of various analytical techniques provides valuable insights into the nature and origin of organic matter in the 3.5 Ga old barites, which significantly advances our understanding of some of Earth´s most ancient habitats.

[1] Mißbach, H. et al. (2021), Nat. Commun. 12, 1101

[2] Van Kranendonk, M.J. (2006), Earth Sci. Rev. 74, 197–240

[3] Philippot, P. et al. (2007), Science 317, 1534–1537