RARE-EARTH GEOCHEMISTRY IN SEDIMENTS OF THE UPPER MANSO RIVER BASIN, RIO-NEGRO, ARGENTINA

The abundance, distribution and fractionation of rare earth elements w as studied in solid material transported by the Upper Manso River and in the sediments deposited in Lake Mascardi, Argentina. The pristine c ondition of the area offers an ideal environment for assessing geochem ical processes, and the seasonal cycles to which the basin is subjecte d permit us to study seasonal variations that are mainly influenced by glacial pulses. The most remarkable characteristics observed in the R EE-normalized patterns are a strong HREE enrichment and a positive Eu anomaly. These normalized patterns are atypical as compared to those c onsidered 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 ph ysicochemical conditions and near-neutral pH values have an important influence on the REE patterns of the sediments, which mainly originate d 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 associa ted with high-pH systems or with the presence of accessory phases, sho ws a clear dependence on source rocks. The calculated (La/Yb)(N) ratio s oscillate between 0.5 and 0.6, contrasting with the values reported in the literature as average values for suspended load material. Never theless, 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 conce ntrations, and additional elemental determinations showed coherent beh aviour during weathering for all elements except Sr and Ba, which exhi bited selective elemental mobilization during periods of minimum disch arge.