CHEMICAL CHARACTERIZATION OF TORBANITES BY TRANSMISSION MICRO-FTIR SPECTROSCOPY - ORIGIN AND EXTENT OF COMPOSITIONAL HETEROGENEITIES

Four Permian to Carboniferous torbanites of various geographical origi ns were examined by transmission micro-FTIR spectroscopy on doubly pol ished thin sections (10-25 mum). Several types of heterogeneities (dif ferent types of organic matrix; yellow and orange Botryococcus braunii colonies) were identified and chemically characterized. Important dif ferences were noted between the organic constituents of the matrix and the algal bodies, regarding the intensity of OH, C=O, and aromatic C= C absorptions. The previous IR studies of torbanites on bulk samples t herefore afforded substantially biased information on the composition of B. braunii fossil colonies, on their oil potential, and on the matu rity of such kerogens. Micro-FTIR spectra indicate that the organic ma trix corresponds neither to an extensive breaking up of colonies nor t o humic substances. This matrix is highly heterogeneous; two types wer e identified in the Autun sample (chiefly corresponding to degraded al gal and bacterial constituents, respectively). A precise characterizat ion of the organic matrix was made difficult, however, in Pumpherston torbanite, due to intimate mixing with minerals. The co-occurrence of yellow and orange colonies, with contrasted micro-FTIR features, in Au tun torbanite neither reflects radiolysis processes nor differences in maturation and/or source algae. A specific spatial relation was obser ved between these two types of algal bodies and the organo-mineral mat rix, thus revealing differences in colony microenvironment after depos ition. The orange colonies are likely derived, in agreement with their micro-FTIR spectra and their spatial correlation with the matrix, fro m sedimentological and/or matrix-catalysed diagenetic transformations of some yellow colonies. This first application of micro-FTIR to kerog ens confirmed the utility of this nondestructive, in situ pin-point me thod. Although torbanites have been extensively studied, all the analy tical methods so far used only provided bulk information. Further insi ght into torbanite composition, origin and evolution can be obtained v ia micro-FTIR spectroscopy.