CARBONACEOUS CHERTS OF THE EARLY ARCHEAN; THE USE OF IN SITU TECHNIQUES FOR TRACING EARLY LIFE

In the progressively metamorphosed rock record of the Early Archean biosignatures have often been found to be ambiguous. Strain has altered the original morphology of microfossil structures and thermal degradation has turned organic compounds (including molecular biomarkers) into kerogen or graphite. In addition, certain abiologic hydrothermal reactions could have produced organic compounds. Finally, it is often difficult to establish whether a trace of life is syngenetic with and indigenous to the rock. Fluid infiltration, petroleum migration, anthropogenic or microbial contamination are among the non-indigenous artifacts. The combination of these issues has made the search for traces of early life a great challenge, and has led to several ongoing controversies. One of the most important developments in this field of research is the use of in situ techniques to describe chemical, isotopic, and structural characteristics of microstructures within their proper petrographic and geologic context. Laser Raman spectroscopy is now used routinely to determine the degree of structure of carbonaceous material. Since the degree of structure is a function of peak metamorphic temperature, Raman spectroscopy enables to distinguish between metamorphosed indigenous features and post metamorphic artifacts within a rock sample. Secondary ion mass spectrometry (SIMS) is now used routinely to determine isotopic ratio and/or chemical composition of individual microstructures. The combination of Raman and SIMS techniques applied to well-described rock slabs and thin sections therefore has great potential for early life studies. Examples are discussed here of such combined analyses reporting degree of structure, N/C-ratio, and d13C of individual carbonaceous particles that occur in capping cherts within the 3.4-3.2 Ga Barberton Greenstone Belt, South Africa. Several lines of evidence are discussed here to show that abiologic hydrothermal reactions were an unlikely source for this carbonaceous material. It is argued that a uniform degree of structural order (alteration at T range 200-320°C), preserved nitrogen content (N/C-ratio ca. 0.002 to 0.006), and low d13C (ca. -35 to -25 per mil) represents indigenous remnants of Archean biosphere.