Mineralogical variation of Sr-Nd isotopic and elemental compositions in loess and desert sand from the central Loess Plateau in China as a provenance tracer of wet and dry deposition in the northwestern Pacific

摘要

We determined Sr and Nd isotopic compositions and major, trace, and rare-earth element (REE) compositions of evaporite and detrital minerals in loess and desert sand from the central Loess Plateau (CLP), where 80% of the loess deposits in China are found. The loess exhibited a narrow range of variation in Sr-87/Sr-86 (0.715 +/- 0.001). A sequential extraction experiment on the loess, however, showed that water- and acetic acid (HOAc)-soluble minerals (mainly halite and calcite), hydrochloric acid (HCl)-soluble minerals (chlorite and phosphate), and HCl-insoluble minerals (illite, kaolinite, smectite, vermiculite, feldspar, and quartz) have distinct Sr-87/Sr-86 ratios, approximately 0.7111 +/- 0.0004, 0.7141 +/- 0.0004, and 0.7195 +/- 0.0010, respectively. The water- and HOAc-soluble ratios were regionally uniform and were close to the Sr-87/Sr-86 value of river water from the Huanghe (Yellow River) and the maximum Sr-87/Sr-86 value of spring rainwater in Japan, suggesting selective solution of eolian halite and calcite into the Chinese river water and into the Japanese rainwater. The mineralogical, elemental, and Sr-Nd isotopic compositions of HCl-soluble and -insoluble minerals in the CLP loess were different from those of the northern desert sand but were indistinguishable from those of its fine-grained fraction, supporting the view that the loess is composed of the fine-grained fraction of desert sand that was transported to the southeast in the atmosphere. The Sr-87/Sr-86, and Gd/Yb values of HCl-insoluble minerals in the CLP loess were almost identical to those of airborne dust, soil minerals, and deep-sea sediments in the northwestern Pacific considered to be of eolian origin, suggesting dissolution of the loess evaporite minerals during atmospheric transportation. These relationships demonstrate the usefulness of Sr isotopes of water- and HOAc-soluble minerals and Sr and Nd isotope ratios and REE compositions of acid-insoluble minerals in loess as provenance markers for wet and dry deposition, respectively. (C) 2003 Elsevier B.V. All rights reserved. [References: 62]