华南碳酸盐岩上覆红土风化壳中硅稳定同位素的研究

Lateritic weathered crusts were widely distributed upon carbonate rocks in southern China, which provides the perfect study stuff for weathering. On basis of extensive theory and practice works about lateritic weathered crust in southern China, the project intend to study the silicon isotope composition characteristic and fractionation rule of lateritic weathered crust. On the basis of studies about the silicon isotope composition characteristic of various weathering end members in weathering process, the project attempt to reveal evolution rules of the weathered crust overlying carbonate rocks in southern China, from the silicon isotope perspective. In addition, the mechanism and process of desilication are quantitatively described, based on the investigation of fractionation of silicon isotope among different mineral components in the lateritic weathered crust. After having a certain understanding of silicon isotope composition in weathered crusts overlying carbonate rocks, the project intend to find the critical factor that influences the silicon isotope fractionation of supergene weathering process by contrasting silicon composition characteristics of weathered crusts developed on different parent rocks/in different climate/covered by different vegetation. We are striving to have a new vision of the evolution process of lateritic weathered crust overlying carbonate rocks in southern China, and to do some contribution to the development of global silicon cycle.

我国南方碳酸盐岩之上广泛发育着红土风化壳，它为风化作用的研究提供了良好的素材。本项目在前期大量有关华南红土风化壳理论和实践工作的基础上，拟对红土风化壳中硅同位素组成特征和分馏规律进行研究；通过对风化过程中各风化端元硅同位素组成特点的研究，进而在硅同位素的视角上揭示华南碳酸盐岩风化壳的演化规律；基于对红土风化壳中不同矿物组分间硅同位素分馏情况的调查，来定量化表征脱硅作用的机制和过程；在对碳酸盐岩风化壳硅同位素有一定研究的基础上，对比不同母岩、气候类型和植被覆盖的风化壳中硅同位素的组成特点，探索表生风化作用中控制硅同位素分馏的主导因素。本项目力争在硅同位素分馏特征研究的基础上，对华南碳酸盐岩红土风化壳的演化过程有新的认识，并为全球硅循环体系完善作用一定的科学贡献。

中国南方是世界著名的喀斯特分布区。区内碳酸盐岩之上广泛覆盖着一层厚度不均的红色风化壳。然而，有关华南碳酸盐岩上覆风化壳的成因和物源等问题争论了近百年尚无定论。主要原因是在华南湿热的气候下，强烈的生物、化学风化彻底改变了风化产物中能够反映其物质来源和成因的地质信号。风化壳所记录的元素地球化学信息很大程度上是气候、生物、地貌、时间等多因素对原始矿物综合改造的结果。在这种研究背景下，本项目选择抗风化能力强的石英作为研究载体，通过硅、氧同位素地球化学的研究手段深度挖掘风化壳中次生石英矿物所记录的真实地质信息，旨在揭示华南碳酸盐岩风化壳的形成和演化机制。通过项目的实施，我们得到以下研究成果：(1)根据石英矿物同位素和形貌特征，将风化壳和基岩中石英矿物进行了系统分类。(2)结合多重证据弄清了风化壳中不同种类石英的形成机理。更正了前人长期以来认为华南碳酸盐岩风化壳中次生石英是在表生土壤环境中形成的错误观点。(3)探讨了次生石英结晶生长的硅质来源。(4)厘定了次生石英生长历史与构造运动演化的耦合关系，这就为利用岩石中次生石英生长历史来揭示区域构造运动的形成时间和发展演化过程提供了新的思路。(5)为华南碳酸盐岩上覆风化壳为原地风化残积物的理论找到了更为有利的证据。同时，提出了“准原地风化”的概念，为揭示华南碳酸盐岩上覆风化壳的物质来源研究做出了一定贡献。