Hénocque O., Ruffet G., Colin Fabrice, Féraud G. (1998). 40Ar/39Ar dating of West African lateritic cryptomelanes. Geochimica et Cosmochimica Acta, 62 (16), p. 2739-2756. ISSN 0016-7037.
Titre du document
40Ar/39Ar dating of West African lateritic cryptomelanes
Année de publication
1998
Type de document
Article
Auteurs
Hénocque O., Ruffet G., Colin Fabrice, Féraud G.
Source
Geochimica et Cosmochimica Acta, 1998,
62 (16), p. 2739-2756 ISSN 0016-7037
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, nH2O), using 40Ar/39Ar argon-ion laser probe method. This work applies that approach to cryptomelanes formed in the lateritic manganese ore deposits of Tambao (northern Burkina Faso), a present unexploited deposit among the cryptomelane-richest in the world. Two samples of massive Mn-rich concretions (10 mm and 4 mm 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 + or - 0.4 Ma to 48.1 + or - 0.1 Ma, and from 47.5 + or - 0.3 Ma to 44.5 + or - 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. (Résumé d'auteur)
