Rare earth elements in the surface sediments of the Yarlung Tsangbo (Upper Brahmaputra River) sediments, southern Tibetan Plateau

摘要

The Yarlung Tsangbo (YT) is the Tibetan portion of the Brahmaputra River. It transports a large amount of sediment annually to the Indo-Gangetic Plain and thus contributes significantly to the Quaternary evolution of the Plain. Eighteen surface sediment samples were collected from the YT, and the compositions of rare earth elements (REEs) were analyzed for the bulk sediments, <63 urn and <20 μm fractions. Results show that the YT sediments originated mainly from the Lhasa Terrane. The REE compositions in the YT sediments are similar along the river, with the light and middle REEs enriched relative to the upper continental crust, but the REE concentrations are higher than its lower reaches - the Brahmaputra River. The average grain size of the sediments is in the coarse range (123.8 urn). Grain size influences the REE concentrations of the YT sediment significantly. The <63 μm fraction of the sediment has higher REE concentrations and different REE patterns when compared with those in the bulk sample and <20 μm fraction, due to the presence of REE-enriched heavy minerals of zircon, monazite and allanite in this fraction. The relatively low REE concentrations in acid-leachable fractions of the sediments suggest most REEs in the sediments occur in the stable minerals due to the dominance of physical weathering in this region.