Climatic and morphological control of rare earth element distribution in weathering mantles on alkaline rocks

Abstract

In humid coastal regions and in areas of accentuated relief of SE-Bahia, Brazil, weathering of alkaline rocks resulted in the formation of kaolinitic and lateritic weathering mantles. Under subhumid conditions in the inland region of Bahia, less mature sandy-clayey soil have developed. Rare Earth Element (REE) distribution in the weathering profiles has been investigated in order to establish relationships between climatic and morphological parameters and the enrichment and fractionation of REE. Under tropical humid climate and intensive drainage conditions, REE fractionation is enhanced. Ce is absolutely enriched in laterite layers while the Heavy REE are retained in the saprolite. Moderate drainage due to high groundwater tables lead to minor fractionation and enrichment of REE. Restricted drainage conditions in subhumid climatic zones hamper liberation of REE from primary minerals and impede enrichment and fractionation of these elements. REE distribution is also controlled by polyphase evolution of the weathering mantle: the positive Ce anomalies in laterite layers are related to stable mineral phases like Mn oxides, which have developed in roots of weathering during former, less humid weathering cycles. The REE fractionation patterns correspond to the drainage conditions which prevailed during the formation of the weathering products. Thus they may represent a valuable tool for the reconstruction of landscape evolution and the investigation of the paleoenvironment.