SM-ND ISOTOPIC DATING OF PROTEROZOIC CLAY MATERIAL - AN EXAMPLE FROM THE FRANCEVILLIAN SEDIMENTARY SERIES, GABON

Clay fractions of < 0.4 mum and < 0.2 mum and acid leachates of these clay fractions were separated from two different kerogen-rich black shale samples from the Francevillian sedimentary series in Gabon. They yielded identical Sm-Nd isochron ages of 2099 +/- 115 Ma and 2036 +/- 79 Ma, with initial epsilon(Nd) values of -6.5 +/- 2.0 and -9.3 +/- 1.5, respectively. These ages define multi-episode illitization during early diagenesis, as suggested by the different Sm/Nd ratios of the clay fractions. The Sm-Nd isotope data of the kerogen source rock (up to 13% organic carbon) plot between the two isochrons but close to the clay fractions, suggesting that this organic matter reached Nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids. Analyses of fracture-filling bitumen showed that it had very elevated Nd-143/Nd-144 and Sm-147/Nd-144 ratios. These values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons. This indicates that the bitumen reached Nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals. Secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between Sm and Nd with an attendant increase in the Sm/Nd ratio. The results suggest that the Sm-Nd isotope method has potential for both dating diagenetically related illitization using leachate-residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil-producing sedimentary basins.