Brazilian laterite dynamics using in situ-produced 10Be

Concentrations of in situ-produced cosmogenic Be-10 (T-1/2 = 1.5 Myr) were measured using accelerator mass spectrometry (AMS) in quartz samples from three Brazilian lateritic systems in order to characterize and quantify processes involved in their development. Results from a bedrock-embedded quartz vein outcropping at Gentio do Ouro lead to a calculated 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 content 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 suggests an autochthonous development. The data constrain a model of 'stone-line' emplacement and imply a rate of lateral displacement on the order of 70 m Myr(-1). By contrast, the constant Be-10 concentration measured within the rounded quartz cobbles all along the Cuiaba 'stone-line' 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 immediately followed by colluvial deposition of weathered lateritic material. (C) 1998 Elsevier Science B.V. All rights reserved.