Multiscale textural assessment of the morphogenesis of lenticular microstructures from Archean cherts of the Pilbara Craton (Western Australia)

Criteria used to distinguish genuine Precambrian microfossils from biomorphs have frequently been re-assessed and sometimes led to reconsider the inference of biogenecity for the microstructure of Archean carbonaceous specimens1–3. In particular, mineral growths associated with carbonaceous matter displacement through diagenesis and/or metasomatism may induce biomorphs mimicking simple microfossils shapes1,4,5. The Strelley Pool Formation in the Pilbara craton (Western Australia) hosts the oldest stromatolites for which an origin as microbially-induced sedimentary rocks6 remains undisputed2,3. Chert rocks from the 3.4 Ga Strelley Pool Formation (SPF) and the 3.0 Ga Farrel Quartzite (FQ) display occurrences of lenticular carbonaceous microstructures that are interpreted as probable microfossils7. These remain enigmatic. Firstly, they display complex, atypical morphologies that make them difficult to relate to living or known fossil taxa7. Secondly, heterogenous carbon isotopic ratios measured within these lenticular microstructures argue against a single hydrocarbon intrusion as the origin of the carbonaceous matter, and may represent original biomass-derived heterogeneities8. These sedimentary deposits experienced metamorphism up to greenschist facies (~350°C) and hydrothermal activity that have altered the composition and–most likely–the texture of the carbonaceous matter. Moreover, diagenesis and/or hydrothermalism may have displaced indigenous and/or exogenous (if any) carbonaceous matter (biogenic and/or abiogenic) onto primary carbonaceous structures or mineral biomorphs.
Here we carry detailed petrography of lenticular and associated spheroid microstructures in the SPF and FQ cherts to examine the possible links between carbonaceous matter textures and their host mineral matrix to further assess biogenicity and syngenicity. We used advanced techniques in optical microscopy including confocal laser scanning microscopy (CLSM) coupled to high-resolution scanning transmitted electron microscopy (STEM) of ultrathin sections. Petrography of the crystal matrix has been detailed at the submicrometer scale in Electron backscattered diffraction (EBSD) mapping (in the scanning electron microscope) and with nanodiffraction mapping (in the transmission electron microscope).
Results led to a rigorous documentation of features and characteristics of each types and subtypes of lenticular specimens previously reported7. Lenses display sub-continuous carbonaceous walls and/or fillings of reticulated and granular carbonaceous material. All these carbonaceous textures are solely localized at quartz crystal boundaries. Carbonaceous textures, in particular the occasional walls, that define these lenticular structures appear flexible. Their morphogenesis will be discussed as the possible results of taphonomic and/or diagenetic processes. Comparison will be made with similar observations carried on a range of spheroid microstructures, including abiotic forms and dubiofossils, previously studied from the SPF4 and from the FQ as part of the present study.