粤北石灰岩化学风化过程中的元素和Sr-Nd同位素体系：变化特征及其控制机制

发布时间：2018-05-08 00:13

本文选题：石灰岩 + 化学风化　；参考：《中国科学院大学(中国科学院广州地球化学研究所)》2017年硕士论文

【摘要】：石灰岩是广布地表的一类重要岩石,其化学风化过程制约着水圈的化学演化和生物地球化学循环。首先,地表出露的石灰岩快速溶解后,向水体释放大量的可溶性离子,一定程度上控制着其所在流域的水化学组成,进而制约着海水的化学演化。其次,石灰岩化学风化释放的元素,如Mg、Ca、Sr、Ba广泛参与生物地球化学循环,因此,石灰岩化学风化还制约生物地球化学过程。此外,石灰岩化学风化是全球Sr循环的一个重要源区,对地史时期的全球Sr循环具有重要影响。然而,目前鲜有研究报道碳酸盐岩化学风化过程中的元素迁移与再分配以及Sr同位素变化的控制机制。为了更好地认识这一重要科学问题,本文以粤北地区的典型石灰岩风化剖面作为研究对象,基于高分辨率的矿物学、元素地球化学和Sr-Nd同位素的综合研究,得出以下几点认识:1.原岩中方解石的溶解造成了碱土金属元素Ca、Sr大量迁出剖面;而其它碱金属-碱土金属元素以及过渡金属元素则发生了不同程度的富集,这可能是由黏土矿物的吸附作用所致。2.粤北石灰岩化学风化过程中,REE发生了显著的富集与分异:在近中性的介质中,碳酸根/碳酸氢根离子与REE形成稳定络合物,显著提高了REE的迁移能力,促使风化产物中的REE发生了显著富集。然而,随着原子序数的递增,REE的离子半径逐渐减小,造成HREE与碳酸根/碳酸氢根离子形成络合物的稳定性显著大于LREE与碳酸根离子形成的络合物,最终导致HREE的富集程度显著大于LREE。3.该剖面上~(87)Sr/~(86)Sr比值呈现出较大的变化范围(0.70979-0.72216),这与外来源区输入没有直接联系。~(87)Sr/~(86)Sr比值与Sr含量的显著负相关性说明,Sr同位素的演化主要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: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,从而造成了~(87)Sr/~(86)Sr比值从剖面下部到上部呈现出逐渐增大的趋势。
[Abstract]:Limestone is one of the most important rocks on the surface. Its chemical weathering process restricts the chemical evolution and biogeochemical cycle of hydrosphere. Firstly, after the limestone is dissolved rapidly, a large amount of soluble ions are released into the water body, which to some extent controls the hydrochemical composition of the basin, and then restricts the chemical evolution of the sea water. Secondly, the elements released by chemical weathering of limestone, such as MgCa-Sr-Ba, are widely involved in the biogeochemical cycle, so chemical weathering of limestone also restricts the biogeochemical process. In addition, the chemical weathering of limestone is an important source of the global Sr cycle, which has an important influence on the global Sr cycle during the geological history. However, there are few studies on the mechanism of element migration and redistribution during chemical weathering and the control mechanism of Sr isotopic variation in carbonate rocks. In order to better understand this important scientific problem, the typical limestone weathering profile in northern Guangdong is taken as the research object. Based on the comprehensive study of high resolution mineralogy, element geochemistry and Sr-Nd isotopes, the following points are obtained: 1. The dissolution of calcite in the original rock resulted in the mass migration of alkali earth metal element CaOSr out of the profile, while other alkali metal-alkali earth metal elements and transition metal elements were enriched to varying degrees, which may be due to the adsorption of clay minerals. In the chemical weathering process of limestone in northern Guangdong Province, there are significant enrichment and differentiation of REE: in near-neutral medium, carbonate / bicarbonate ions form a stable complex with REE, which significantly improves the migration ability of REE. The REE in the weathering products was enriched significantly. However, with the increase of atomic number, the ionic radius of HREE decreases gradually, which results in the stability of the formation complex of HREE with carbonate / bicarbonate ion is significantly higher than that of the complex formed by LREE and carbonate ion. As a result, the enrichment of HREE was significantly higher than that of LREE.3. In this section, there is a large range of variation in the ratio of Sr ~ (86) ~ + ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ (th), which has no direct relation with the input from the source region. The significant negative correlation between Sr ~ (86) ~ (Sr) ratio and Sr content indicates that the evolution of Sr isotopes is mainly controlled by carbonate minerals and silicate deposits. Significant differences in weathering strength and Sr isotopic composition: as weathering progresses, A large number of Sr migration profiles are found in calcite with a lower Sr ~ (87) ~ (- /) ~ + ~ (86) ~ (-1) Sr ratio in the original rock, while Sr residue in silicate minerals with a high Sr ~ (87) ~ (- /) ~ + ~ (86) Sr ratio is found in weathering products, which results in a gradual increase in the Sr ~ (86) ~ (Sr) ratio from the lower to the upper part of the profile.
【学位授予单位】：中国科学院大学(中国科学院广州地球化学研究所)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P512.1;P59

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