Rare earth elements and Sr and Nd isotopic compositions of dissolved and suspended loads from small river systems in the Vosges mountains (France), the river Rhine and groundwater

The aim of this study is to characterize the evolution of the rare earth el ements (REE) in non-mature streams from small catchment areas in the Vosges mountains downstream to more mature plain rivers including the river Rhine . The dissolved load REE distribution patterns of the low-pH Vosges streams are very different from those of high-pH plain rivers indicating that diff erent physico-chemical parameters control REE transport in these different water systems. The plain rivers and groundwater show similar REE distributi on patterns with a strong negative Ce anomaly and heavy rare earth element (HREE) enrichment. In addition, the river Rhine has a positive Gd anomaly w hich is of anthropogenic origin. Similar to the world's major rivers the li ght rare earth element (LREE) enrichment in the plain rivers is mainly pH c ontrolled. Their Sm/Nd and Nd-143/Nd-144 isotope ratios are close to averag e continental crust values. This is not the case for the less evolved, non- mature and low pH Vosges streams. Their high Sm/Nd and Nd-143/(14)4Nd ratio s but low Sr-87/Sr-86 ratios suggest that chemical alteration of accessory middle rare earth element (MREE) enriched minerals such as apatites from ro cks in the catchment area control the REE abundances of these waters. A com parison of the dissolved load REE distribution patterns with those of the p rincipal lithologies in the corresponding drainage basins illustrates that especially the Eu anomalies of the Vosges streamlets are strongly lithology dependent. Leaching experiments indicate that the suspended load is isotop ically very inhomogeneous. The REE distribution patterns of the suspended l oad leachates have no similarities with those of the corresponding dissolve d load indicating that the leachable reservoir not only contains adsorbed R EE but also REE from leachable mineral phases. Their (14)3Nd/Nd-144, Sm/Nd and Sr-87/Sr-86 ratios support this suggestion being always higher and lowe r, respectively, than those of their corresponding dissolved loads. They ra ther point to the presence of relic primary apatite in the leachable portio n of the suspended load. The flat PAAS normalized REE distribution patterns , the high Sr-87/Sr-86 and low Nd-143/Nd-144 isotopic ratios of the corresp onding residues suggest secondary silicate phases such as clay minerals in the residual phase of the suspended load. Leachates and corresponding resid ual phases define alignments in the Sm/Nd isochron diagram whose slopes def ine ages ranging between 200 and 390 Ma indicating that the mineral phases in the suspended load retain some memory of their primary precursor mineral s in the Hercynian granitic source rocks. (C) 1999 Elsevier Science B.V. Al l rights reserved.