40Ar/39Ar dating of West African lateritic cryptomelanes

Dating weathering processes is an innovative approach to understanding climate-dependant modifications of the Earth's surface. Potassium-bearing manganese oxides are a ubiquitous feature of weathering mantles developed from Mn-bearing parent rocks. Recent studies have demonstrated the possibility of dating K-rich manganese oxides from the hollandite group, such as cryptomelane (K1-2Mn8O16, nH(2)O), using 40Ar/39Ar argon-ion laser probe method. This work applies that approach to cryptomelanes formed in the lateritic manganese ore deposit of Tambao (northern Burkina Faso), a present unexploited deposit among the cryptomelane-richest in the world. Two samples of massive Mn-rich concretions (10mm and 4mm in thickness) were collected from the massive 80-m high Mn-rich hill at Tambao. A detailed petrographical study shows that these cortical formations are composed of successive Na-rich cryptomelane layers, with Na/(Na + K + Ba) up to 20%. Seven and five subsamples from these two concretions were studied with a 40Ar/39Ar laser probe. The age spectra indicate ages that are coherent with the general organization of the concretions, showing a regular decrease from cores towards rims. Within the two concretions, ages range from 49.8 +/- 0.4 Ma to 48.1 +/- 0.1 Ma, and from 47.5 +/- 0.3 Ma to 44.5 +/- 0.4 Ma, respectively, the youngest ages being obtained for the most external layers. The integrated rates of cryptomelane precipitation are estimated at 1 and 5 mm/Ma. These ages clusters show that weathering at Tambao was functioning during the early Eocene and thus suggest that there is a relationship between the formation of the bauxites of the African Surface and the Mn-Fe laterites. Copyright (C) 1998 Elsevier Science Ltd.