『Abstract
　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 0.45μm 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 ＞0.45μm 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 ＜0.45μm fraction are associated to Fe colloids, which grow during transport downstream to Fe oxyhydroxide particles exceeding 0.45μm in size. Precipitation of these oxyhydroxide particles leads to selective removal of LREE from 0.45μm filtered stream water, and of Ce adsorption on ＞0.45μm 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.

Keywords: REE fractionation; Fe colloids; Nd-Sr isotopes; Stream water

1. Introduction
2. Setting and methods
　2.1. Regional setting
　2.2. Field sampling and analytical methods
3. Results and discussion
　3.1. The REE patterns of the bedrock
　3.2. Major element composition, dissolved organic carbon, and physicochemical parameters of stream water
　3.3. Nd-Sr isotope data of the bedrock and 0.45μm filtered stream water
　3.4. The REE of the suspended particle load
　3.5. The REE of the ＜0.45μm fraction
　3.6. The fractionation of the REE of the ＜0.45μm fraction and the link with the colloidal and suspended particle load
　3.7. REE mixing at stream confluences
4. Summary and conclusions
Acknowledgments
References