Element and Sr-Nd Isotopic Mobility during Weathering Process of the Nanhuaian-Cambrian Sedimentary Strata in the Eastern Three Gorges and Its Geochemical Implication
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摘要: 以峡东地区南华纪、震旦纪和寒武纪标准地层泥岩、冰碛泥岩、砂岩、灰岩和白云质灰岩及对应的风化土壤为研究对象, 分析了地层风化成土过程中不同元素的迁移行为, 根据剖面样品的Sr-Nd同位素组成变化, 探讨了其同位素体系的封闭性特征与应用意义.结果表明: (1) 不同岩性基岩在成土过程中的蚀变强度有明显的差异, 在相似地表条件下, 碳酸岩风化剖面的风化程度高于泥质岩和砂岩; (2) 通过对比稳定高场强Ti元素在基岩和风化剖面中的含量变化, 计算出土壤样品在风化过程中体积相对基岩发生的改变量, 进而计算出不同岩性基岩在风化过程中微量元素的绝对含量变化以探讨这些元素的活动规律.结果表明, 灰岩和白云质灰岩的风化剖面元素含量变化明显, 而在泥质岩风化过程中大多数元素保持了相对稳定, 说明沉积岩风化过程中元素的活动性特征明显地受到了原岩矿物组成的制约.风化过程中, 不同性质的元素的活动性差异明显, 其中亲硫元素(Cu、Zn、Pb、Mo) 和大离子亲石元素(Rb、K、Sr、Ba) 在不同岩性的风化剖面中均表现出明显的元素含量变化, 而高场强元素(Zr、Hf、Nb、Ta) 含量则相对稳定; (3) 泥质岩风化形成的土壤层REE含量变化较小, 而碳酸岩风化土壤层REE含量发生了明显下降, 且其风化形成的土壤表现出LREE和HREE相对于MREE的富集.无论是碳酸岩或泥质岩风化形成的土壤, 均出现了明显的Eu负异常和Ce的正异常, 但在其原岩中这些异常并不存在或不明显; (4) 基岩与土壤剖面间Sr同位素组成和Rb/Sr比值存在明显差异, Rb-Sr同位素组成发生了明显的开放.所形成土壤层的Sr同位素组成受到2种因素的约束: 原岩性质和外来组成的Sr同位素比值.因此在总体上, 风化土壤的Sr同位素组成已不能代表基岩的Sr同位素组成; (5) 沉积岩风化过程中, 碳酸岩和泥质岩形成的风化土壤基本保持了原岩的Sm-Nd同位素组成特点, 由其组成所获得的Nd同位素亏损地幔模式年龄等能反映其原岩信息, 而近源沉积形成的砂岩和含砾冰碛泥岩所形成的土壤, 其Nd同位素组成则存在不同程度的改变.
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关键词:
- 南华系-寒武系 /
- 沉积岩 /
- 化学风化 /
- 元素和Sr-Nd同位素 /
- 迁移行为
Abstract: 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.-
Key words:
- Nanhuaian-Cambrian /
- sedimentary rock /
- chemical weathering /
- element and Sr-Nd isotope /
- mobility
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图 1 峡东地区南华纪—寒武纪地层分布简图及采样位置(据Yin et al., 2003修改)
1.英云闪长岩; 2.花岗岩; 3.寒武纪; 4.南华和震旦纪; 5.采样位置
Fig. 1. Simplified geological map of the eastern Three Gorges with sampling location
图 2 峡东地区南华纪—寒武纪地层风化剖面多元素的Ti含量标准化图
A、B和C分别为风化剖面的A、B和C层土壤
Fig. 2. Ti normalized multi-elemental diagrams of the weathering profiles of the Nanhuaian, Sinian and Cambrian strata from the eastern Three Gorges
图 3 峡东地区风化剖面风化剖面稀土元素的Ti含量标准化图
1A和1B分别为灯影组灰岩风化剖面(1) A和B层土壤; 5A、5B和5C分别为南沱组冰碛泥岩风化剖面(5) 的A、B和C层土壤
Fig. 3. Ti normalized REE diagrams of the weathering profiles of the Nanhuaian, Sinian and Cambrian strata from the eastern Three Gorges
图 4 峡东地区南华纪-寒武纪地层风化剖面风化过程中Sr同位素特征图解
a.灯影组灰岩风化剖面; b.灯影组白云质灰岩风化剖面; c.牛蹄塘组灰岩风化剖面; d.牛蹄塘组泥岩风化剖面; e.南沱组冰碛泥岩风化剖面; f.莲沱组砂岩风化剖面; R和A、B、C分别为风化剖面基岩和对应的A、B、C层土壤
Fig. 4. Comparison plots of Sr isotopic ratio of the Nanhuaian, Sinian and the Cambrian strata weathering profiles from eastern Three Gorges
图 5 峡东地区南华纪-寒武纪地层风化剖面风化过程中Nd同位素特征图解
R和A、B、C分别为风化剖面的基岩和对应的A、B、C层土壤; a、b、c、d、e和f表示意义同图 4
Fig. 5. Nd isotopic variation diagrams of the Nanhuaian, Sinian and Cambrian weathering profiles from the eastern Three Gorges
表 1 峡东地区南华纪-寒武纪风化剖面主量-微量元素和Sr-Nd同位素组成
Table 1. Elements and Sr-Nd isotopic compositions of the Nanhuaian, Sinian and Cambrian weathering profiles from the eastern Three Gorges
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