地表环境过程的硼同位素地球化学研究

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	地表环境过程的硼同位素地球化学研究
其他题名	BORON ISOTOPIC GEOCHEMISTRY OF SUPERGENIC ENVIRONMENTS
	赵志琦
	2002
学位授予单位	中国科学院地球化学研究所
学位授予地点	中国科学院地球化学研究所
学位名称	博士
学位专业	地球化学
关键词	硼同位素地表环境风化作用流域侵蚀
摘要	硼同位素作为灵敏的地质示踪剂,被成功地用于解决一些地球化学问题.如壳一幔演化和板块俯冲过程的研究、判别沉积环境和物质来源、研究成矿作用和矿床成因、古海洋和古气候研究、水环境污染研究等.但地表风过程的硼同位素地球化学研究非常少,其分馏机理和分馏程度还很不清楚.为此该论文着眼于大陆风化过程的硼同位素地球化学特征的研究.在硼同位素分析方法研究的基础上,成功地测定了洛川黄土剖面酸溶相样品和西江河水样品的硼同位素组成.对典型陆相条件风化过程和流域侵蚀过程的硼同位素地球化学特征进行了探讨.
其他摘要	Boron isotope system, as an extremely sensitive geochemical tracer, has been successfully used to study many geochemical processes, suqh as the crust-mantle evolution and subduction-related processes, sedimentary environments and the source of sediments, ore-forming processes and ore-genesis, paleocean pH estimation and paleoclimatology, and water contamination. But little research work has been conducted on the boron isotopic geochemistry of supergentic environments. As a result, the characteristics of boron isotopic geochemistry during the weathering process within supergentic environments remains largely unknown. This paper primarily focuses on the boron geochemical characteristics of continental weathering. Based on the study of method for determination of boron isotopic composition of low-boron content samples, theδ ~(11)B values of acid-soluble fraction of loess-paleosol samples from the Luochuan Section and river water samples from Xijiang River are successfully measured. And the boron isotope geochemical characteristics of typical continental weathering and river erosion processes are discussed. The main conclusions have been summarized as follows: 1. The procedure of two-step ion exchange treatment, using Amberlite IRA-743 and a mixture of cation-exchange resin and anion-exchange resin, is a good way to treat the samples of acid-soluble fraction in loess-paleosol sequence and river, water with the precision of 0.4-0.6%a, which can satisfy the need for boron isotopic geochemistry research. 2. The boron contents of acid-soluble phases in the Luochuan loess section (S_0-S_2) vary from 0.8 x.10~(-6) to 2.7><10~(-6)) with the difference.of less than 6% between loess layers and paleosol layers, suggesting that B hasn't transferred significantly during the process of weathering. 3. Theδ~(11)B values of acid- soluble fraction in loess-paleosol sequence, which can characterize the changes of climate during the process of weathering, vary from -1.8%o to +18.6%o. Theδ~(11)B values of paleosol layers are higher than those of loess layers, especially so in the loess layer S_1 During the evolution from loess to paleosol, the variation of boron isotopic composition in acid-soluble phase is mainly resulted from that of adsorption phase. The change of silicates weathering intensity results to the change of δ~(11)B values of boron that is adsorptive, and then the oron isotopic composition of adsorption phase changes. The influence of silicate weathering on the boron isotopic composition of acid-soluble fraction is related to the boron content of silicates and the chemical weathering intensity. The higher content of silicates, the stronger influence on the boron isotopic composition of acid-soluble fraction. The higher content of weathering, the higher δ~(11)B value of the acid-soluble fraction in loess-paleosol. Under the condition that the 5 B values of rain and carbonate keep invariable, when silicates, with 64*10~6 boron-content (the average level of loess-paleosol), weathered 0.9%, the δ~(11)B value of acid-soluble fraction will increase 10 %0 . Consequently, the variation of the 6 nB value of acid-soluble fraction really reflects the variation of silicate weathering intensity. In supergentic environments, the variation of silicate weathering intensity is mainly controlled by the climatic conditions (such as temperature and precipitation). As a result, the δ~(11)B values of acid-soluble fraction can act as a sensitive proxy to trace the climatic evolution of weathering period. Just because of the significant fractionation of boron isotope during weathering, little change of weathering intensity may be recorded by 8 nB values. That is, small environmental change can be viewed in a bigger scale, so the boron isotopic proxy has a higher resolution and sensitivity. The Xijiang River water is characterized by high alkalinity, [Ca~(2+)] and [Mg~(2+)], which is controlled by rock weathering, precipitation, and acid deposition, et al. The sequence of contributions is: rock weathering (carbonate>>silicate) > precipitation (part from ocean)>acid deposition>other factors. The boron contents of the Xijiang river water vary from 0.7 u g/L to 22.7 u g/L, with the average value of 9.1 ±4.3 u g/L. The boron flux from the Zhaoqing River cross section into the ocean approaches to 2.61 X 10~9g/yr. The δ~(11)B values of the Xijiang River water vary from -11.2%o to +17%0. 80% of theδ~(11)B values of the samples vary from -2%o to +10%o, and the average is 4.4±5.0%o. As a result, the study on boron and its isotopic composition in Xijiang water catchment reveals that the influence of anthropogenetic contamination to the Xijiang River should not be neglected. About 10 percent of total dissolved boron in river water is from the polluted sites at the upper reaches of the Nanpanjiang River, and the down-stream region of the Xijiang River. Boron isotopic composition of river water is a useful tool to trace the sources of the dissolved material in river, and to identify anthropogenitic influence, especially for rivers that have relatively lower content of contamination. Data fromδ~(11)B and B/Cl research indicate that boron concentration of the Xijiang river is mainly controlled by carbonate weathering and precipitation. Boron from these two sources accounts for 83% of that of the Xijiang River water, and the contribution of silicate weathering is less than 13%. The adsorption of clay minerals and the precipitation of CaCO_3 have no influence on boron isotopic composition of Xijiang River water, and the variation of δ~(11)B values of the Xijiang River water mainly reflects the change of boron source. The variation of δ~(11)B values of the Xijiang River water, which is changed in a small range, is controlled by the boron isotopic composition of rain water and that of carbonates. The change of other boron sources (polluted sites and the weathering of silicates) results to the steep change of the δ~(11)B value.
页数	152
语种	中文
文献类型	学位论文
条目标识符	http://ir.gyig.ac.cn/handle/352002/3602
专题	研究生_研究生_学位论文环境地球化学国家重点实验室
推荐引用方式 GB/T 7714	赵志琦. 地表环境过程的硼同位素地球化学研究[D]. 中国科学院地球化学研究所. 中国科学院地球化学研究所,2002.

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