Rare earth geochemistry in sediments of the Upper Manso River Basin, Río Negro, Argentina

Abstract

The abundance, distribution and fractionation of rare earth elements was studied in solid material transported by the Upper Manso River and in the sediments deposited in Lake Mascardi, Argentina. The pristine condition of the area offers an ideal environment for assessing geochemical processes, and the seasonal cycles to which the basin is subjected permit us to study seasonal variations that are mainly influenced by glacial pulses. The most remarkable characteristics observed in the REE-normalized patterns are a strong HREE enrichment and a positive Eu anomaly. These normalized patterns are atypical as compared to those considered representative of the world's major rivers and are caused by drainage basin geology. Due to the dissected topography and the small size of the basin, intense mixing phenomena are not favoured and the weathering reactions are inhibited by the fast-flowing river. These physicochemical conditions and near-neutral pH values have an important influence on the REE patterns of the sediments, which mainly originated by glacial erosion, that are transported by the river. The feldspars and their secondary products, which are both enriched in Eu, might be the cause of the Eu anomaly. HREE enrichment, which is mainly associated with high-pH systems or with the presence of accessory phases, shows a clear dependence on source rocks. The calculated(La/Yb) N ratios oscillate between 0.5 and 0.6, contrasting with the values reported in the literature as average values for suspended load material. Nevertheless, these values are consistent with the dependence of(La/Yb) N values on the age of drainage area rocks as was observed by Goldstein and Jacobsen [3]. Seasonal variations are not manifested in REE concentrations, and additional elemental determinations showed coherent behaviour during weathering for all elements except Sr and Ba, which exhibited selective elemental mobilization during periods of minimum discharge.