Sr isotopic evidence for ion-exchange buffering in tropical laterites from the Parana, Brazil

Sr isotopes have been measured in laterite sequences and in associated waters from the region of Ribeirao Preto (Parana basin, Brasil), in order to characterize the water-rock interactions corresponding to the successive stages of weathering of these continental flood basalts, We analyzed detailed sections of concentric rims of alteration in ball-shaped spheroids developed around cores of fresh basalts. Waters were also analyzed on the different types of facies in this region: unaltered basalts, thick weathered profiles of lateritic material, and surrounding sandstones, The results show that a major part of the initial Sr is removed from the basalt during the earliest stages of weathering, within the narrow grey rims surrounding the unweathered core. Radiogenic Sr is also leached from hydrothermal minerals located in basalt fractures or disseminated in the basalt groundmass at this early stage. Very high water/rock ratios (of at least 100,000) are calculated for this process, from the Sr-87/Sr-86 in the related waters. A general trend of increasing Sr-87/Sr-86 ratios is observed in the increasingly weathered facies (inner yellow cortexes, red kaolinitic soils and matrices) and in their related waters. The very high Sr-87/Sr-86 ratios of the red kaolinitic horizons probably result from isotopic exchange with radiogenic rainwaters or sandstone waters, and also from the presence of residual or neoformed radiogenic minerals. Budget calculations, as well as experimental leaching of kaolinite samples, suggest that significant amounts of radiogenic Sr can be leached from these facies during weathering, Thus, the kaolinitic facies of tropical laterite profiles are able first to trap strontium from surficial waters, then to provide radiogenic Sr to the waters that weather the fresh basalts at the weathering front. Therefore, the thick weathered horizons must be considered as a natural cation-exchange layer, that play an active role in the weathering process.