CHEMICAL EVOLUTION OF RARE-EARTH ELEMENTS - FRACTIONATION BETWEEN COLLOIDAL AND SOLUTION PHASES OF FILTERED RIVER WATER

Within the dissolved fraction of Connecticut River water, there is large scale fractionation of rare earth elements between colloidal and solution phases. Colloidal particles carry a large part of the dissolved REE pool. Filtration through filters with progressively smaller pore size (0.45, 0.22 and 0.025 mum) results in filtrates with lower absolute concentrations of REE and more fractionated compositions. With decreasing pore size, the filtrates have shale-normalized abundances which become systematically more depleted in the LREE relative to the HREE. Hence, both the solution and colloidal phases are significantly fractionated with respect to the dissolved fraction ( < 0.45 mum) of the river water. The solution and colloidal phases are HREE and LREE enriched, respectively, relative to the continental crust. In addition, river colloids are preferentially enriched in the redox-active REE Ce, leaving the solution phase with a more negative Ce anomaly than the filtered river water. These conclusions are consistent with previous studies of REE in rivers, estuaries and soils and with surface-solution speciation models. Colloids play an important role in the aquatic geochemistry of rare earth elements. They are responsible for the fractionation of REE during the estuarine removal of REE. The removal of suspended and colloidal particles from river water leaves the solution phase of river water with compositions of REE as evolved as those of surface seawater. Hence, the evolution of the composition of REE in river water and seawater is significantly influenced by colloid formation during chemical weathering on the continents.