Element and Sr-Nd Isotopic Mobility during Weathering Process of the Nanhuaian-Cambrian Sedimentary Strata in the Eastern Three Gorges and Its Geochemical Implication

Element and Nd-Sr isotopic mobility during the weathering process of stratotypes for the Nanhuaian, Sinian and Cambrian sedimentary strata have been studied in eastern Three Gorges.Stratigraphic bedrocks of mudstone, glacial mudstone, sandstone, limestone and dolomitic limestone coupled with their correspondent soil layers by chemical weathering were analyzed.It demonstrates that (1) the intensity of chemical weathering of bedrocks is dominated by their lithology; (2) Due to its stability, Ti is regarded as immobile during chemical weathering.Thus, the variations of Ti concentrations in soil samples relative to their bedrocks are used to correct volume changes of soil layers during the weathering and accordingly to recalculate relative contents of other elements in the layers.It shows that obviously content decreasing for most elements in soil layers relative to their bedrocks is found for limestone and dolomitic limestone profiles; in contrast, soil samples from the mudstone profiles exhibit a relative stability for most elements.It, thus, suggests that the elemental mobility during chemical weathering of sedimentary rocks is dominated by their rock-forming minerals.Meanwhile, different classical elements show distinct mobility during the chemical weathering.Contents of chalcophile (Cu, Zn, Pb and Mo) and LILE (Rb, K, Sr and Ba) elements varied obviously, whereas those of the HFSE element (Zr, Hf, Nb and Ta) kept relatively constant; (3) Relative to their bedrocks, the variation of REE content in soil layers by mudstone is indistinctively, whereas that in soil layers by carbonate rocks changed evidently characterized both by enrichment in LREE and HREE relative to MREE and negative Eu and positive Ce anomalies; (4) Both Sr isotopic composition and Rb/Sr ratio changed remarkably during the weathering process of both carbonatic and argillaceous rocks, indicating an opening of their Rb-Sr isotopic system.It shows that the variation in Sr isotopic compositions of the soil layers relative to their bedrocks is dominated both by the lithology of their parental rocks and the Sr isotopic composition input during weathering.Accordingly, the Sr isotopic compositions of the soil layers are not indicative of their bedrocks; (5) The Sm/Nd ratio and Nd isotopic compositions of the soil layers weathered from carbonate and mudstone rocks essentially inherited the characters of their bedrocks, and their depleted mantle model ages (tDM) are indicative of their parental rocks.However, variable degree variations in Nd isotopic compositions are recognized for the soil samples weathered by sandstone and glacial mudstone.