40Ar/39Ar Dating of African Manganese Pisolites: Implication for Cenozoic Climatic Changes

Abstract

Chemical and mechanical weathering-derived processes are among the most efficient factors in modifying the Earth surface. Weathering gradients are strongly dependent upon temperature, rainfall and tectonic stability over geological time. In this way, the upper part of tropical and sub-tropical cratonic lithosphere has been transformed into thick supergene lateritic mantles with frequent metal accumulations. In West Africa, the lateritic weathering episodes have developed from early Tertiary successive crusts that have controlled the local geomorphology. These indurated formations are evidence of the successive weathering and erosion cycles. In this way, laterites that constitutes the rain forest substratum, may trace the palaeoclimate evolution. One of the best opportunities to date weathering events is to apply the 4~ method on isotopically closed supergene K-bearing minerals such as K-Mn oxides from the hollandite group (AxBsO16, nH20 , where A represents K +, Ba 2+, pb2+or Na + and B represents Mn 3§ Mn 4+, Fe3+,). Initiated by Chukhrov et aI. (1966), using the K-Ar dating method, direct dating of potassium-rich manganese oxides was widely facilitated by the recent development of the 4~ laser probe method as shown by the studies of Vasconcelos et al. (1992 and 1994) and Ruffet et al. (1996). Cryptomelane is well represented in numerous lateritic manganese weathering profiles over the tropical belt. Then radiochronological study may help to understand the genesis of the Mn-lateritic systems and to correlate them to the development of Fe-laterites. This work presents analytical performances of the 4~ argon-ion laser probe method in 'dating' K-rich samples of the pisolitic manganese formation and the underlying manganese crust of the Tambao deposit (northern Burkina Faso).