GEOCHEMISTRY OF RARE-EARTH ELEMENTS IN THE MAINSTREAM OF THE YANGTZE-RIVER, CHINA

Water, suspended matter, and sediment samples were taken from 8 locati ons along the Yangtze River in 1992. The concentration and speciation (exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to or ganic matter, and residual forms) of rare earth elements (La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu) were determined by instrumental neutron activa tion analysis (INAA). The contents of the soluble fraction of REEs in the river are low, and REEs mainly reside in particulate form. In the particles, the chondrite-normalized distribution patterns show signifi cant LREE enrichment and Eu-depletion. While normalized to shales, bot h sediments and suspended matter samples show relative LREE enrichment and HREE depletion. REEs are relatively enriched in fine-grained frac tions of the sediments. The speciation characteristics of REEs in the sediments and suspended matter are very similar. The amount of the fiv e forms follows the order: residual > > bound to organic matter simila r to bound to Fe-Mn oxides > bound to carbonates > > exchangeable. Abo ut 65 to 85% of REEs in the particles exist in the residual form, and the exchangeable Form is very low. High proportions of residual REEs r eveal that REEs in sediments and suspended matter are controlled by th eir abundances in the earth's crust. Carbonate, Fe-Mn oxide and organi c fractions of REEs in sediments account for 2.4-6.9%, 5.2-11.1%, and 7.3-14.0% of the total contents respectively. They are similar to thos e in the suspended matter. This shows that carbonates, Fe-Mn oxides an d organic matter play important roles during the particle-water intera ction processes. By normalization to shales, the 3 forms of REEs follo w convex shapes according to atomic number with middle REE (Sm, Eu, an d Tb) enrichment, while light REE and heavy REE are depleted. (C) 1998 Elsevier Science Ltd. All rights reserved.