%0 Journal Article
%9 ACL : Articles dans des revues avec comité de lecture non répertoriées par l'AERES
%A Hénocque, O.
%A Ruffet, G.
%A Colin, Fabrice
%A Féraud, G.
%T 40Ar/39Ar dating of West African lateritic cryptomelanes
%D 1998
%L fdi:010016201
%G ENG
%J Geochimica et Cosmochimica Acta
%@ 0016-7037
%K GEOCHIMIE ; ALTERATION CHIMIQUE ; ALTERATION PHYSIQUE ; LATERITE ; MINERALOGIE ; PETROGRAPHIE ; DATATION ABSOLUE ; ISOTOPE
%K ARGON
%K BURKINA FASO
%K TAMBAO REGION
%N 16
%P 2739-2756
%R 10.1016/S0016-7037(98)00185-9
%U https://www.documentation.ird.fr/hor/fdi:010016201
%> https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_53-54/010016201.pdf
%V 62
%W Horizon (IRD)
%X 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)
%$ 064GEOCHI