Tracing and dating recent chemical transfers in weathering profiles by trace-element geochemistry and 138U-234U-230Th disequilibria: the example of the Kaya lateritic topo-sequence (Burkina-Faso)

The paper presents a synthesis of the approach recently developed to constrain the nature and the age of recent element mobility in weathering profiles. The approach relies on a comparison of variations of trace elements and U-series disequilibria in weathering profiles. It is exemplified in the case of old lateritic profiles from the Kaya topo-sequence, Burkina Faso. Trace elements and 238U-234U-230Th disequilibria have been analysed in whole rock samples from two pits located in very contrasted topographical positions. Trace element data show that the whole topo-sequence is marked by an intense chemical remobilization, including uranium, from the cap to the lower part of the profiles. 238U-234U-230Th disequilibrium data outline that all the levels of the profiles are affected by recent U-Th fractionations, and that each level of the topo-sequence is marked by U gains and losses. The chronological approach developed in this paper leads to an age of about 400 kyr for the dismantling of the iron cap, and ages ranging from 0 to 400 kyr for U accumulation in the pink clay horizon of the profiles. The depth repartition of U accumulation in the profile implies that the remobilization processes in this topo-sequence varied through time, on time constants similar to those of climatic variations. (author)