Controls on transport and fractionation of the rare earth elements in stream water of a mixed basaltic-granitic catchment basin (Massif Central, France)
Résumé
We present rare earth element (REE) patterns of small streams from a catchment basin in the Massif Central (France) in order to characterize the individual fractionation stages for the dissolved REE from the source to the catchment outlet. The upper part of the catchment is located on a basalt plateau, followed downstream by deep and narrow valleys within granitic and orthogneissic bedrock. Basalt-normalized 0.45 [mu]m filtered stream water has REE patterns slightly depleted in the light REE (La-Sm, LREE) on the basalt plateau close to the source, followed by a continuous amplification of this LREE depletion downstream. At the same time also a negative Ce anomaly develops in the adsorbed fraction of > 0.45 [mu]m particles, which has been isolated by leaching with 1 M HCl. Strontium and Neodymium isotope ratios of stream water demonstrate that the dissolved REE are essentially of basaltic origin, even in the lower, granitic and gneissic part of the catchment. Mixing with gneiss or granite derived REE thus cannot explain the observed evolution of the REE patterns. Our data suggest that the REE of the < 0.45 [mu]m fraction are associated to Fe colloids, which grow during transport downstream to Fe oxyhydroxide particles exceeding 0.45 [mu]m in size. Precipitation of these oxyhydroxide particles leads to selective removal of LREE from 0.45 [mu]m filtered stream water, and of Ce adsorbed on > 0.45 [mu]m particles. In contrast to earlier studies, we found no link between REE behavior and organic colloids or organic complexes. This scenario is confirmed by a detailed analysis of the REE patterns at 3 stream confluences. The results show that the REE do not behave conservatively during mixing at stream confluences, but that the LREE and Ce are preferentially removed similarly to what has been observed at catchment scale. Our study thus underlines the importance of Fe colloids for the transport and fractionation of the REE in stream water.