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

摘要

The aim of this study is to characterize the evolution of the rare earth elements (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 different physico-chemical parameters control REE transport in these different water systems. The plain rivers and groundwater show similar REE distribution patterns with a strong negative Ce anomaly and heavy rare earth element (HREE) enrichment. In addition, the river Rhine has a positive Gd anomaly which is of anthropogenic origin. Similar to the world's major rivers the light rare earth element (LREE) enrichment in the plain rivers is mainly pH controlled. Their Sm/Nd and ~(143)Nd/~(144)Nd isotope ratios are close to average continental crust values. This is not the case for the less evolved, non-mature and low pH Vosges streams. Their high Sm/Nd and ~(143)Nd/~(144)Nd ratios but low ~(87)Sr/~(86)Sr ratios suggest that chemical alteration of accessory middle rare earth element (MREE) enriched minerals such as apatites from rocks in the catchment area control the REE abundances of these waters. A comparison of the dissolved load REE distribution patterns with those of the principal 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 isotopically very inhomogeneous. The REE distribution patterns of the suspended load leachates have no similarities with those of the corresponding dissolved load indicating that the leachable reservoir not only contains adsorbed REE but also REE from leachable mineral phases. Their ~(143)Nd/~(144)Nd, Sm/Nd and ~(87)Sr/~(86)Sr ratios support this suggestion being always higher and lower, respectively, than those of their corresponding dissolved loads. They rather point to the presence of relic primary apatite in the leachable portion of the suspended load. The flat PAAS normalized REE distribution patterns, the high ~(87)Sr/~(86)Sr and low ~(143)Nd/~(144)Nd isotopic ratios of the corresponding residues suggest secondary silicate phases such as clay minerals in the residual phase of the suspended load. Leachates and corresponding residual phases define alignments in the Sm/Nd isochron diagram whose slopes define ages ranging between 200 and 390 Ma indicating that the mineral phases in the suspended load retain some memory of their primary precursor minerals in the Hercynian granitic source rocks.