Formation and evolution of lateritic profiles in the middle Amazon basin: Insights from radiation-induced defects in kaolinite. - Université Pierre et Marie Curie Accéder directement au contenu
Article Dans Une Revue Geochimica et Cosmochimica Acta Année : 2006

Formation and evolution of lateritic profiles in the middle Amazon basin: Insights from radiation-induced defects in kaolinite.

Résumé

The content of radiation-induced defects (RIDs) in kaolinite samples originating from lateritic soils and continental detritic sediments of the middle Amazon Basin (Brazil) is investigated using electron paramagnetic resonance. The paleodose registered by kaolinites ranges from 80 to 900 kGy. Present-day dose rates of radiation, determined from the whole-rock U and Th content, range between 4000 and 40,000 mGy/ka. In most samples, U and Th concentrations are correlated, suggesting that U has not been remobilized by lateritization. This observation is consistent with the fact that 80% of the total U content is incorporated in resistant minerals, such as zircon and Ti oxides. The heterogeneous distribution of U, observed by induced fission tracks mapping, makes it possible to neglect the -radiation contribution of the U decay chains in the dose-rate calculation. The interpretation of the measured content of RIDs in kaolinite is then performed using the calculated present-day dose rate and assuming equilibrium in the radioactive decay chains. For the sedimentary samples, the amount of RIDs is broadly correlated to the dose rate and provides apparent absolute ages older than 20 Ma. The RID contents in kaolinites from the lateritic soils provide apparent ages ranging from 10 to 6 Ma. The high RID content of these lateritic kaolinites shows that their chemical, isotopic, and crystallographic properties are not representative of present-day weathering conditions. Models assuming the “dynamical equilibrium” of kaolinites with local physical-chemical conditions prevailing in lateritic soils are thus questionable. Alternatively, our findings bring strong support for the use of the isotopic composition of kaolinites to decipher continental paleo-climates.

Dates et versions

hal-00079585 , version 1 (13-06-2006)

Identifiants

Citer

Etienne Balan, Thierry Allard, Emmanuel Fritsch, Madeleine Sélo, Christophe Falguères, et al.. Formation and evolution of lateritic profiles in the middle Amazon basin: Insights from radiation-induced defects in kaolinite.. Geochimica et Cosmochimica Acta, 2006, 69, pp.2193-2204. ⟨10.1016/j.gca.2004.10.028⟩. ⟨hal-00079585⟩
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