华南新元古代-寒武纪早期黑色岩系的铊同位素组成及古海洋环境示踪

A variety tracer methods of isotope and geochemistry have been used to study the paleo-ocean environments during the key period from Neoproterozoic to early Cambrian transition, among which developing new methods such as applying new isotope technique is very important. A high precision analytical method of Tl isotopes using MC-ICP-MS is going to establish in this study. We will select several well-studied sections from Doushantuo Formation of Neoproterozoic and Niutitang Formation of early Cambrian in south China (Hubei, Hunan, Guizhou provinces) and obtain Tl concentrations and Tl isotopic compositions of black shales from these sections. We will examine the Tl distributions and Tl isotope fractionations in black shales and using the Tl-isotope to indicate the redox conditions of the Neoproterozoic-early Cambrian oceans. Applying this new isotopic method and combining the evidences from sedimentology, redox sensitive trace elements and Mo isotopes, we will not only establish the paleoceangraphic environments, but also provide a new isotopic constrain for the genesis of Ni-Mo multiple metal-enriched sulfide bed in the black shales of Niutitang Formation, which has been hotly debated for decades.

人们已采用了多种不同的地球化学和同位素方法来示踪前寒武纪/寒武纪之交这一关键地史转折期间的古海洋环境变迁。其中，能否开发和利用新的同位素方法来示踪古海洋环境演变，己成为国内外研究者正在努力追求和不懈努力的一项重要任务。本项目拟在国内建立高精度的铊同位素MC-ICP-MS分析方法，并选取华南(湖北、湖南、贵州)几个典型的新元古代陡山沱组和寒武纪早期牛蹄塘组黑色岩系剖面为研究对象，开展黑色岩系铊含量和铊同位素分析，总结铊同位素在黑色岩系中的分馏规律，示踪氧化还原环境。通过运用新的铊同位素示踪方法，结合原有的沉积岩石学、氧化还原敏感元素地球化学和钼同位素数据，系统研究华南新元古代-寒武纪早期黑色岩系的沉积环境，并为进一步厘定颇具争议的牛蹄塘组黑色岩系中的Ni-Mo多金属硫化物矿层的成因提供新的同位素制约。

本项目对扬子地台寒武纪早期地层和新元古代陡山沱组地层的沉积环境开展了深入的地球化学研究工作，包括主量元素、微量和稀土元素、碳、氧、锶、钕同位素和铊同位素研究等, 提出的主要认识有：1)利用稀土元素配分曲线，可区分海水信息或成岩流体信息，一般遭受成岩流体影响的样品均具有帽型的稀土配分曲线。 2)利用Ce异常，表明寒武纪早期海洋中表层海水是氧化的, 尽管当时底层海水及沉积环境为缺氧还原的。牛蹄塘组黑色岩系下段为缺氧硫化还原环境，多金属元素在地层中高度富集；而上段为非硫化的弱还原环境。3)锶-碳-氧同位素可评估成岩作用的影响，微量元素页岩标准化模式可示踪淡水-海水混合过程，钕同位素组成可推断扬子地台埃迪卡拉海洋中稀土元素的来源及沉积物源。4)牛蹄塘组黑色岩系铊同位素组成显示与Mo/TOC, Ni/TOC, U/TOC较好的正相关关系, 反映受氧化还原环境的控制。铊同位素研究进一步支持镍钼矿层的海底热液成因。