HR: 10:20h
AN: H42C-01    [Abstracts]
TI: Modeling of Recent Chemical Mobility in Weathering Profile Through U-Th Disequilibria: Example of the Kaya Lateritic Profiles.
AU: * Chabaux, F J
EM: fchabaux@illite.u-strasbg.fr
AF: CGS-EOST Strasbourg, 1 rue Blessig, Strasbourg, 67084 France
AU: Dequincey, O
AF: CGS-EOST Strasbourg, 1 rue Blessig, Strasbourg, 67084 France
AU: Leveque, J
EM: JeanJacques.Leveque@eots.u-strasbg.fr
AF: IPGS-EOST Strasbourg, 5 rue Ren‚ Descartes, Strasbourg, 67084 France
AU: Pelt, E
AF: CGS-EOST Strasbourg, 1 rue Blessig, Strasbourg, 67084 France
AB: Determination and dating of recent mobility in weathering profiles is important to constrain their steps of formation and evolution but also to recover the presently active processes that control the chemical fluxes carried by rivers from continents to ocean. Recent studies (1-2) have shown the interest of comparing variations of trace elements and U-series disequilibria in weathering profiles to constrain the nature and the age of recent element mobility in weathering profiles. We pursue this approach and propose a simple modelling of U series disequilibria, using a Monte Carlo inversion approach, to assess the time scale of U mobility in weathering profiles. This approach is exemplified in the case of an old lateritic profile, the Kaya toposequence, in Burkina Faso. It relies on a comparison of U-Th data in both the isochron (230Th/232Th) vs. (238U/232Th) diagram, and the concordia (234U/238U) vs. (230Th/238U) diagram. The method consists in finding, for a given data point, a set of mobility parameters, that provide the same age in the two above diagram. Application of the method to the Kaya lateritic toposequence underscores that the recent chemical mobility in the profile are mainly controlled by the iron-cap dismantling, and would vary with time possibly in response to climatic variations. 1 Dequincey et al. (2002), GCA, 1197-1210; 2 Chabaux et al.(2003), CR Geoscience 335, 1219-1231.
DE: 9305 Africa
DE: 1030 Geochemical cycles (0330)
DE: 1035 Geochronology
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting