[MIS07-P03] Morphological and spectroscopic characteristics of organic matter in Banded Iron Formation, Barberton Greenstone Belt, South Africa
Keywords:Archean, Microfossils, Barberton Greenstone Belt, Raman spectroscopy, Infrared spectroscopy
Organic matter was collected by two different methods: extraction by acid treatment of powder samples and pick up with a micro-manipulator from thin sections. When Raman spectra were obtained using 532 nm wavelength laser, acid-treated organic matter showed two peaks ~1350 and ~1580 cm-1 corresponding to disordered and ordered carbon structures, whereas micro-sampled organic matter shows no detectable peak with a high background. Because yellow-green fluorescence was observed in micro-sampled organic matter using a Fluorescence microscope, the fluorescence likely causes a high background in the Raman spectra. Therefore, 785 nm wavelength laser was applied for micro-sampled organic matter. Calculated metamorphic temperature from the Raman spectra obtained by acid-treated and micro-sampled organic matter was from 400 to 540°C, which is consist with greenschist facies of the Barberton Greenstone Belt. This indicates that the organic matter is not a later contamination. Chemical mapping with an Electron Probe Micro Analyzer shows that micro-sampled organic matter is predominately comprised of carbon with slight contribution of nitrogen (< 0.1wt%). Infrared spectra of the organic matter show absorption peaks corresponding to N-O, C-O, and C=O bonds, suggesting that organic matter remains their functional groups in the structure. Based on the morphology and microstructure observed by Field Emission- Scanning Electron Microscope, the organic matter may have been derived from biofilm because of similarity in the morphology as well as the large size (20 – 50 µm). Furthermore, the micro-sampled organic matter has a rough surface while the acid-treated organic matter has a smooth one, which may be due to damages by acid-treatment. Our results suggest that micro-sampling is suitable to observe the detailed morphological and structural features of organic matter preserved in Archean sedimentary rocks.