粤北石灰岩化学风化过程中的Sr-Nd同位素
Variation of Sr and Nd isotopes during weathering of limestone: A case study in northern Guangdong, South China
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文摘
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石灰岩化学风化显著控制着其所在流域盆地的河水Sr同位素组成,进而制约全球Sr循环。然而,石灰岩化学风化过程中Sr同位素组成的变化特征及其控制机制的研究报道依旧很少。本文通过报道粤北地区一个典型石灰岩风化剖面的Sr同位素组成变化特征,试图探讨石灰岩化学风化过程中Sr同位素变化的控制机制。结果显示:该剖面上, ~(87)Sr/~(86)Sr比值介于0.70979~0.72216之间;自剖面底部到顶部, ~(87)Sr/~(86)Sr比值呈现出逐渐增大的趋势; Nd同位素组成比较均一(ε_(Nd)(t) = –15.0±0.5)。研究区的潜在源区(如大气降水、地表水),其~(87)Sr/~(86)Sr比值明显低于该剖面上部的最大值,加之,该剖面的风化产物与典型的中国黄土剖面具有截然不同的Sr-Nd同位素组成,暗示了这些潜在源区的输入不大可能造成该剖面上Sr同位素组成发生显著的变化。该剖面上~(87)Sr/~(86)Sr比值与Sr含量之间呈现显著的负相关性(r = –0.95),暗示粤北石灰岩化学风化过程中Sr同位素的演化受控于碳酸盐矿物与硅酸盐矿物在抗风化强度以及Sr同位素组成方面的显著差异:相对硅酸盐矿物,碳酸盐矿物(如方解石)抗风化强度较弱,于是随着风化作用的进行,原岩中具有较低~(87)Sr/~(86)Sr比值的方解石中的Sr大量迁出剖面,而具有较高~(87)Sr/~(86)Sr比值的硅酸盐矿物中的Sr残留在风化产物中,因此,随着风化强度的增加,风化产物中的~(87)Sr/~(86)Sr比值呈现出逐渐增大的趋势。 |
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其他语种文摘
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Limestones are broadly outcropped on Earth's surface. They are very easily weathered and contribute most of the dissolved alkaline earth elements such as Ca and Sr to rivers, and hence to the oceans. The budget of Sr isotopes, an important isotope system in tracing variable surficial processes, is largely controlled by weathering of limestones. In addition to the amount of weathered limestones, variation of Sr isotopes during limestone weathering likely influences the Sr isotopic composition of river water and regulates the global Sr isotope budget. However, consecutive variations of the ~(87)Sr/~(86)Sr ratio in weathering profiles have not been investigated, making it difficult to understand the mechanism for the variation of Sr isotopic composition during limestone weathering. We herein report the Sr and Nd isotopic compositions of a weathering profile developed on limestone in northern Guangdong province, South China. The results show that the Sr isotopes exhibited a large variation range, with the ~(87)Sr/~(86)Sr ratio varying from 0.70979 at the bottom of the profile to 0.72216 at the top, whereas the Nd isotopes are relatively homogeneous, with ε_(Nd)(t) values ranging from –15.53 to –14.46, with an average of –15.0 ± 0.5. Sr from river water and meteoric water are two potential extraneous inputs, and their ~(87)Sr/~(86)Sr ratios vary from 0.70741 to 0.71583, which are obviously lower than the maximum in the upper weathering profile (~0.72216). This implies that river water and rainwater do not contribute much extraneous Sr to the saprolites in the profile. In addition, the studied profile has quite a different distribution from that of China Loess in the ~(87)Sr/~(86)Sr–ε_(Nd)(t) diagram, suggesting that dust deposits from China Loess also contribute very little to the large variation of Sr isotopes in this profile. It is worth noting that a significant negative correlation between the ~(87)Sr/~(86)Sr ratios and Sr concentrations appears in the profile, implying a potential scenario that the variation of Sr isotope composition can be largely attributed to incongruent weathering of minerals during limestone weathering, as there are dramatic differences in the resistance to weathering and ~(87)Sr/~(86)Sr ratios of silicate and carbonate minerals. As a result, dissolved Sr sourced from the weathering of calcite was obviously removed from the weathering profile before Sr occurred in silicate minerals. Finally, the ~(87)Sr/~(86)Sr ratios of the saprolites move toward values that are similar to those of silicate minerals as the chemical weathering becomes stronger. |
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来源
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地球化学
,2018,47(4):415-422 【核心库】
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DOI
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10.19700/j.0379-1726.2018.04.008
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关键词
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Sr同位素
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Nd同位素
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石灰岩
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化学风化
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地址
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1.
中国科学院广州地球化学研究所, 同位素地球化学国家重点实验室, 广东, 广州, 510640
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中国科学院大学, 北京, 100049
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中国科学院广州地球化学研究所, 中国科学院边缘海地质重点实验室, 广东, 广州, 510640
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语种
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中文 |
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文献类型
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研究性论文 |
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ISSN
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0379-1726 |
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学科
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地质学 |
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基金
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国家自然科学基金
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文献收藏号
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CSCD:6291287
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