POSSIBLE FOSSILS PRESERVED IN ARCHEAN AMPHIBOLITE-FACIES METAPELITES?

The Archean fossil record is currently represented by biomarkers, stromatolites, and silicified microfossils, many of which are surrounded by debate. At present, no convincing body fossils have been recovered from Archean siliciclastic rocks, although Proterozoic and younger shales have been shown to be key paleontological libraries. One of the main reasons that Archean siliciclastics have not been a target of paleontological search is the general belief that they are highly metamorphosed and any fossils therein must have been destroyed. To test this belief, we investigated amphibolite-facies metapelites of the Archean Jinganku Formation of the Wutai Group in North China. Numerous U-Pb ages constrain the Wutai Group to be >2.5 Ga. We studied the Wutai metapelites using thin section and palynological techniques. Our preliminary investigation shows that the Wutai metapelites contain abundant graphitic disks (20–220 µm diameter), whose thermal maturity is similar to bulk carbonaceous material in the metapelites. Raman microprobe analysis confirms that the disks are carbonaceous and suggests that they experienced metamorphic temperature of approximately 500 °C, consistent with amphibolite metamorphic facies, thus verifying the indigenicity of these graphitic disks.

The carbon isotopic value of bulk carbonaceous material in the Wutai metapelites, determined by combustion experiments, is -21.3 per mil PDB. Carbon isotope values of individual graphitic disks, determined by ion probe analysis, show significant heterogeneity ranging from -7.3 to -35.8 per mil. The cause of the heterogeneity is unclear, but the carbon isotope values appear to suggest biogenicity with metamorphic overprinting. As observed through scanning electron microscopy, many graphitic disks show apparent evidence of marginal folding and surface texturing. Transmission electron microscopy shows that some disks appear to have two distinct layers compressed against each other. We interpret these Archean graphitic disks as compressed and graphitized vesicles similar to acritarchs that are abundant in Proterozoic shales. This study may open a new window onto the Archean biosphere.