华南新元古代盖帽白云岩沉积微相的镁硅同位素研究

The cap dolostones overlying the diamictites of Snowball Earth connected the extreme climate and the biological evolution. Thus, it is significant to account for the reason of the end of “Snowball Earth” and the environmental changes at the beginning of life. At present, the petrographic and geochemical signatures of high-temperature fluids and methane seeps have been observed on several sections. The source of high-temperature fluids and origin of methane seeps, however, are still strongly debated and more attention should be deserved on them. In this study, the microfacies of the cap dolostones, in particularly the calcite cements with extremely negative δ13C values, will be selected. The sources of depositional fluids, the changes of paleoenvironment and the formation mechanisms of enigmatic structures will be distinguished based on the combined geochemical characteristics of the different microfacies, especially the magnesium and silicon isotope compositions. In addition, the source of high-temperature fluids, the origin of methane seeps and the relationship between high-temperature fluids and methane seeps will be traced. It will shed new light on the evolution of ocean and sources of fluid after Snowball Earth and the role of environment to biologic evolution. Moreover, the fractionation mechanism of magnesium and silicon isotopes during diagenetic alteration will be established.

新元古代“雪球地球”事件之后的盖帽白云岩是联系极端气候与生物演化之间的纽带，对其研究在揭示全球性冰期终结机制和生命初始阶段环境演变方面具有重要意义。目前已在数个盖帽白云岩剖面上发现了高温热液和甲烷渗漏的岩石学和地球化学记录。然而，对于高温热液的来源和甲烷渗漏的成因还存在很大争议，亟待进一步研究。本项目拟以盖帽白云岩中不同沉积微相、尤其是发育极端低δ13C值的方解石胶结物为研究对象，在综合对比不同沉积微相地球化学特征、特别是指示意义较强的镁硅同位素组成的基础上，分析盖帽白云岩不同沉积微相的流体来源、环境变化、特殊沉积组构的成因，追踪高温热液来源、甲烷渗漏成因以及二者之间的关系及分布范围，为全球新元古代“雪球地球”事件之后海洋环境演变、流体来源变化及其与生物演化之间的联系提供新的依据，并建立蚀变过程中硅镁同位素分馏机制。

新元古代全球性“雪球地球(Snowball Earth)”事件(~ 635 Ma)之后，广泛沉积了约几米到十几米厚主要由白云岩组成的碳酸盐岩——盖帽白云岩(cap dolostones)。本项目的主要对盖帽白云岩进行详细的野外和室内岩石学研究，尤其是具有成因意义的特殊沉积组构，要对其中发育的胶结物和成岩世代进行详细的沉积微相分析；对碳酸盐岩不同沉积微相进行原位的主微量元素分析，并进行微区取样测试碳氧镁同位素组成，并根据地球化学特征进行示踪，确定不同世代碳酸盐胶结物的特征、流体来源和形成时间；对硅质岩不同微相进行微区取样，并进行氧和硅同位素分析，根据地球化学特征进行示踪，确定不同世代硅质胶结物的特征、流体来源和形成时间；将不同的地球化学信息综合，探讨碳酸盐岩成岩蚀变过程中的镁硅同位素分馏机制；根据沉积微相特征、流体来源及其变化规律，探讨“雪球地球”事件结束之后，沉积环境变化和主要控制因素及其对生物演化的影响。我们对影响碳酸盐岩原始地球化学记录的因素进行分析，并对排除这些影响因素的手段进行讨论，识别出不同的沉积水体和沉积相下形成的碳酸盐岩具有不同的稀土元素配分特征。