BRAZILIAN LATERITE DYNAMICS USING IN SITU-PRODUCED BE-10

Concentrations of in situ-produced cosmogenic Be-10 (T-1/2 = 1.5 Myr) were measured using accelerator mass spectrometry (AMS) in quartz samp les from three Brazilian lateritic systems in order to characterize an d quantify processes involved in their development. Results from a bed rock-embedded quartz vein outcropping at Gentio do Ouro lead to a calc ulated mean erosion rate of 2.5 m Myr(-1) for a muon participation to the total Be-10 production set at 1.5%. At Itaberaba, the Be-10 conten t of the surficial sample yields a mean erosion rate of 9.0 +/- 0.5 m Myr(-1). In addition, the systematic increase in Be-10 concentrations of 'stone-line' quartz with distance from the quartz vein strongly sug gests an autochthonous development. The data constrain a model of 'sto ne-line' emplacement and imply a rate of lateral displacement on the o rder of 70 m Myr(-1). By contrast, the constant Be-10 concentration me asured within the rounded quartz cobbles all along the Cuiaba 'stone-l ine' suggests that it most likely results from almost contemporaneous rapid events that occurred at least 500 Kyr ago. The proposed scenario involves sheet washing at the surface of the lateritic paleolandscape leading to surficial dispersion of allochthonous quartz cobbles immed iately followed by colluvial deposition of weathered lateritic materia l. (C) 1998 Elsevier Science B.V. All rights reserved.