Pb-Pb isotope systematics on diagenetic clays: An example from Proterozoic black shales of the Franceville basin (Gabon)

This study presents the first dating attempt of diagenetic clay material by the Pb-Pb isotope method. Pb-Pb isotopic measurements were performed on authigenic < 0.4-mu m clay fractions (illite 1M, Fe-Mg-chlorite) extracted from black shales highly enriched in organic matter of the Francevillian (lower Proterozoic) sedimentary series in Gabon. The clay residues and acid leachates (1 N HCl) yield a Pb-Pb isochron age of 2328 +/- 130 Ma (MSWD = 1.24). Even if this age is within error limits identical with two Sm-Nd isochron ages previously reported for the same material, it is not considered as a true isochron age. The Pb isotopic data of the residues indicate that the clay minerals are not in isotopic equilibrium. However, the isotope data of the leachates separately define an isochron age of 2265 +/- 150 Ma (MSWD = 0.36), which is closer to the Sm-Nd age. It is suggested that the leachates are the most appropriate phases for dating diagenetic processes with Pb isotopes. The U-Th-Pb fractionation in the clay fractions is related to the occurrence of organic matter in the sediments. The U/Pb and U/Th ratios of the clays correlate with their organic carbon contents. This fractionation is probably due to differential complexation of U, Th and Pb by organic ligands during the dissolution of detrital mineral phases and clay mineral authigenesis.