Integration of Observational and Analytical Methodologies to Characterize Organic Matter in Early Archaean Rocks: Distinguishing Biological from Abiotically Synthesized Carbonaceous Matter Structures

Transmission Electron Microscopy (TEM) was applied to observe and characterize carbonaceous materials (CM) extracted from black cherts and argillite in drill core from the Warrawoona Group of the Pilbara Craton, Western Australia. The black chert came from a ‘white smoker type’ seafloor deposit in the ca. 3.49 Ga Dresser Formation, whereas the black argillites were obtained from the 3.46 Ga Apex Basalt. The samples were observed and analyzed in TEM combined with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and C-isotope geochemistry. The TEM and HRTEM observations revealed significant morphological and structural differences between the carbonaceous materials of the Dresser and Apex samples enabling interpretations in terms of primary and secondary origins, as well as metamorphic history.

Organic petrology using reflected light microscopy was used on whole rock ­samples to observe mineral – organic matter relationship and CM structure relative to host rock texture. The results support an in situ, syn-depositional origin for the Dresser Formation CM. Reflectance % (Ro) of CM determined on polished whole rock ­samples and polished resin-embedded CM-concentrates enabled the reconstruction of thermal history. Several Ro populations were identified in the Dresser Formation samples: probable microbial cells preserved in fluid inclusions within quartz crystals, thermally degraded CM originally belonging to microbial cells, CM coating mineral grains and reworked CM particles. In contrast, the Apex Basalt samples yielded ­consistently very high Ro values corresponding to graphite stage organic ­metamorphism. The weak optical anisotropy of the Apex graphite is inconsistent with formation during regional metamorphism. Two main graphite forms were identified, namely platy and tubular varieties. In HRTEM the tubular form showed nano-tubes and fullerenes within mono-layered spheres. Furthermore, TEM and HRTEM show that the void enclosed mono-layered carbon nano-spheres are more often detached from tubular graphite, forming clusters outside the nano-tubes. These forms are a key to the distinction between biologically and abiotically synthesized CM bodies, both by their small size, perfect outline and especially their resistance to thermal degradation.

Dresser Formation samples are isotopically light in the range of −32.1‰ to −38.2‰ consistent with a biological source. Although TEM indicated four distinct types of CM, C-isotope analysis was undertaken on mixed CM concentrates. Nevertheless, the isotopically lighter samples contain a notable input from less thermally degraded low Ro material. On the other hand the isotopically heavier samples contain ­predominantly thermally degraded high Ro CM. C-isotope compositions of the Apex CM are generally heavier than Dresser samples, between −22.5‰ and −28.6‰ consistent with high thermal stress. The samples show a C-isotope trend in which CM at 143 m depth is isotopically lighter, whereas above and below this level CM becomes increasingly and consistently isotopically heavier. The upper part of the section is dominated by platy graphite with rare nano-tubes. Predominantly tubular graphite and fullerenes characterize sample SAL-13 at 142 m depth coinciding with the isotopically lightest values. Below 143 m CM is less well preserved and predominantly of the fragmented platy graphite type, becoming increasingly isotopically heavier signifying close association with peridotite intrusion.