CHARACTERIZATION OF THE CARBONACEOUS MATERIAL IN THE 3.4 GA STRELLEY POOL CHERT, PILBARA CRATON, WESTERN AUSTRALIA

Carbonaceous microstructures in Archaean cherts from the Warrawoona Group, Pilbara Craton, Western Australia, are proposed to be the oldest microfossils on earth (3.46 Ga) and have led to considerable debate and research interest. Recent studies indicate the need for the development of reliable biomarkers and molecular characterization in Archaean carbonaceous materials (CM).

In this study, CM was isolated from five samples in the basal strata of the Strelley Pool Chert, comprising bedded green and black cherts and chert breccia/conglomerates, and grade upward to laminated and stromatolitic carbonates. Characterization of CM was performed using Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Solid state ¹³C Nuclear Magnetic Resonance (NMR) spectroscopy, and High resolution transmission electron microscopy (HRTEM).

The Raman carbon first-order spectra reveals the presence of three bands, two bands of Lorentzian shape, the D and G bands at 1340 and 1600 cm^-1 respectively, and a minor broad Gaussian band between 1500 and 1550 cm^-1 assigned to amorphous carbon (a-C). The Raman spectra acquired from these CM are similar to those of polycrystalline carbons composed of a structurally disordered polynuclear aromatic (PNA) networks. Spectral analysis reveals that the average dimensions of structural units range from a L_a of 24.2 Å to 26.4 Å. This indicates that the aromatic clusters are very large. HRTEM analysis of the CM shows a low level of structural organization in the carbon network with a minor component of amorphous carbon. Solid state ¹³C NMR and FTIR shows that the CM from the Strelley Pool Chert is highly aromatic and contains little to no aliphatic C-H or C-O functionality.

The CM has a thermally mature signal shown by the Raman, FTIR, ¹³C NMR analyses and HRTEM observations. For disordered and amorphous carbons the development of the Raman D band indicates ordering, which is exactly opposite to what is found in graphite. This indicates that the CM is undergoing carbonization and not graphitization. In addition, the combination of FTIR, ¹³C NMR, and HRTEM observations also show that the carbonaceous material is not graphitic carbon. However, these analyses cannot provide definite evidence for excluding an abiotic origin for these cherts.