| 其他摘要 | The relative greater mass difference of two stable isotope of lithium (6Li and 7Li) results in distinct isotopic fractionation, even up to 35‰ in the surface earth environment. So as a good tracer, lithium isotopes have been used to study some important geological or geochemical issues, such as cosmochemistry, alteration of oceanic crust, modern submarine hydrothermal activity, crust-mantle evolution, subduction-related processes, origin and evolution of brine, et al. Recently the study of lithium isotope in the study of continental weathering has come to be one of the most fast developed fields. To be brief, they include the research of lithium isotopic geochemistry in riverine erosion system, in weathering profile system, and in laboratory experiments. But some results of the above three fields conflict from each other.
So this paper fouses on the lithuim geochemistry characterics of continental weathering. The Changjiang River system has been selected as the object of study. Based on the study of analytical method for lithium isotopic composition of low content samples, the sources of the dissolved lithium, spatial variations of lithium and lithium in the river system, and the main controlling factor of lithium isotopic fractionation are discussed and investigted.
The main conclusion have been summarized as follows:
1、Lithium can be seprated by the procedure of one step exchange treatment with cation exchanger resin, and determined by MC-ICP-MS. The accuracy and precision is up to the advanced level of the world. The δ7Li value determined for sea water is +31.8‰, which agrees well with those of previously reported datum. The precision of this method is conservatively estimated to be 0.5‰ (2σ), which is similar to the precision of other method by TIMS.
2、The Changjiang River water is characterized by high Ca2+ and HCO3-, which account for 50% of the total cations and 70% of the total anions. And a visible change which decreases from the upper stream to the lower stream can be observed in the concentrations of Na+ and Cl-. So the water chemistry of the Changjiang River system is controlled by rock weathering where the weathering of carbonate is the most significant, and partially by evaporation/crystallization.
3、The lithium concentrations and δ7Li values of the suspended matter in the Changjiang River system are stable, which are in the range of 41µg/g~92µg/g and in the range of -4.7‰~+0.7‰ respectively. On the contrary, the sediments are charactorized by lower lithium content (13.26 µg/g~46.32 µg/g) and higher δ7Li values (+0.9‰～+9.1‰). And the lithium concentration of the suspended matter and sediments is positively related with Al2O3/SiO2、Fe2O3/SiO2、MnO/SiO2、K2O/SiO2, and negatively related with the δ7Li value. The relativity may be result from the absorption of clay minerals（especially for 6Li）.
4、The concentration of dissolved lithium in the Changjiang River ranges from 1.04 µg/L to 31.72 µg/L with the average value of 8.87 µg/L, and decrease from the upper stream to the lower stream. The δ7Li values of the dissolved load of the Changjiang River also varies greatly from +7.6‰ to +28.1‰. The change of the δ7Li values increasing from the upper stream to the lower stream is different from that of the suspended matter and dissolved lithium.
5、The positive relationship between 7Li values and 1/Li for the dissolved load shows lithium in the Changjiang River can be regarded as a two-component system. The study of rainwater, human pollution and carbonate indicates that dissolved lithium in the Changjiang main channel is mainly controlled by evaporates and silicate. Lithium from these two sources account for 78% ～ 99% of the total lithium in the main channel of the Changjiang River.
6、The 7Li values of the suspended matter always are lighter than those of dissolved load in the Changjiang River, so the isotopic fractionation between them is strong. Isotopic fractionation factors of the dissolved and suspended loads（αsuspended-dissolved） range from 0.977 to 0.992, which in the range of the absorption of clay minerals. αsuspended-dissolved of the upper stream is similar to the fractionation between gibbsite, smectite and lithium solution, but that of the lower stream is an analogy between kaolinite, vermiculite and seawater. And the same time, αsuspended-dissolved are negatively corrected with the concentration of the suspended matter before the three Gorges of the Changjiang main channel. This relationship also results from the absorption of clay minerals. |
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