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.

A review on the potentiality of Rare Earth Elements to trace pedogenetic processes

“…The fractionation between LREEs, MREEs and HREEs in Fe-and Mn-oxides is subjected to debates (Land et al, 1999). Some are enriched in HREEs (Elderfield and Greaves, 1981;Marker and de Oliveira, 1994;Huang and Wang, 2004), as expected by coordination numbers and electronegativity (Tables 1 and 2; Henderson, 1984), while others are enriched in LREEs (Courtois, 1974;Bernat, 1975;Koeppenkastrop and Decarlo, 1993). A MREE-enrichment is even observed in some cases.…”

“…Nesbitt, 1979;Duddy, 1980;Topp et al, 1984;Braun et al, 1990;Walter, 1991;Braun et al, 1993;Boulangé and Colin, 1994;Marker and de Oliveira, 1994;Condie et al, 1995;Öhlander et al, 1996;Braun et al, 1998;Taunton et al, 2000;and references therein). The mobility of REEs through weathering varies between those of Ti and Fe (Bonnot- Courtois, 1981).…”

“…its mineralogical composition and relative mineral stability (Nesbitt, 1979;Braun et al, 1990;Walter, 1991;Marker and de Oliveira, 1994;Condie et al, 1995;Öhlander et al, 1996;Braun et al, 1998;Panahi et al, 2000;Tyler, 2004a); • the properties of the different REEs in the solution (Nesbitt, 1979;Braun et al, 1990;Walter, 1991;Marker and de Oliveira, 1994;Condie et al, 1995;Braun et al, 1998;Panahi et al, 2000;Tyler, 2004a); as well as • the degree of weathering (Walter, 1991;Huang and Gong, 2001).…”

“…Since REE fractionation is related to drainage conditions (Marker and de Oliveira, 1994), they are of peculiar interest to trace redox conditions, especially Ce and Eu (Henderson, 1984). However, the reduction of Eu requires strong reducing conditions (Ronov et al, 1967;Courtois, 1974;Bonnot-Courtois, 1981;Henderson, 1984;Panahi et al, 2000) rarely encountered in soils.…”

REE Comparison Between Muncung Granite Samples and their Weathering Products, Lingga Regency, Riau Islands

“…The anomaly relates to the formation of tetravalent Ce under oxidizing condition near the surface and forms CeO 2 which is a stable compound in laterite/soil layers. The removal of REE, especially LREE, except Ce in an oxidizing condition near the surface develops a positive anomaly, whilst deposition of the removed REE in less oxidizing condition generates a negative anomaly (Marker and Oliveira, 1994;Sanematsu et al, 2009). The cerium enrichment of weathered Muncung Granite is detected in all saprolitelaterite layers (C-B horizons, respectively) in all location and most of the soils (A horizons).…”