干旱区生态环境知识资源中心

Chemical compositional variations of desert sands and loess, loess-like deposits, river deposits and saline lacustrine deposits collected mostly from the southern margin of the Taklimakan Desert, China, provide strong constraints on the chemical weathering processes of continental crust, and also on the provenance compositions of these Quaternary deposits. Constant but lower concentrations of Al relative to the average upper continental crust (UCC) in these Quaternary deposits could be attributed to chemical weathering of Al-rich minerals such as feldspar and mica, and removal of clay minerals by wind and water. Large variations of mobile Na, Ca, Mg and Sr compared to relatively immobile Al, K and Ba resulted from the chemical weathering of plagioclase and carbonate minerals, salinization and carbonation of the arid lands. Most trace elements are not significantly fractionated. Their variations in absolute concentration are caused by the dilution of quartz, evaporates and carbonate minerals. Variations in Sn concentrations probably depend on relative contents of some heavy minerals. High Sr/Ba, low Th/U, K/Cs ratios of the saline lake deposits suggest differences in aqueous chemistry of these elements. The occurrence of the M-type tetrad effect (a REE pattern with four convex curves) of REE compositions, found mainly in the river deposits, is considered the result of water-particle interaction during chemical weathering of crustal materials. Y/Ho ratios of these samples are significantly higher than those of chondrite and the UCC. This suggests different chemical behavior of these two elements, and possibly the enrichment of carbonate materials.

The low LREE/HREE feature of the saline lake deposits can be interpreted as due to the different aqueous chemical behavior of the REE’s. However, it can also be reasonably considered as the result of inheritance from their provenance, since LREE and LREE/HREE variations may be related to the geological setting of the source. According to some elements that are not significantly fractionated, the average provenance composition of these Quaternary deposits are probably characterized by enrichments of Ni, Cr and V over Fe, higher La/Th, lower La/Sc and Th/Sc, and lower Ta/W and Nb/W ratios, compared with the UCC composition.