Dating of weathering profiles by radioactive disequilibria: contribution of the study of authigenic mineral fractions

Soil isotopic dating is important for understanding mass transfers related to rock weathering. Here we evaluate a method including TIMS analyses of (U -234/U-238) activity ratios on < 0.2 mu m fractions of a Cameroon laterite. Variations of the (U-234/U-238) ratios with depth show (1) significant (U- 234/U-238) disequilibrium at the bottom of the pedological units, and (2) p rogressive returns towards secular equilibrium above. These results suggest that U fractionation occurs at the base of the units and that the analysed fractions probably evolve as closed/restricted systems within each unit. F rom these data, an overall age of 1.5 Ma may be proposed for the developmen t of this laterite. ((C) Academie des sciences / Elsevier, Paris.).