塔里木盆地中寒武统叠层石纹层微石英成因及其古海洋硅循环路径

Silica sources are various in sedimentary rocks, and the pristine sedimentary silicon is the key to reflect the silica cycle of paleo-ocean. Studies have shown that there is a widespread presence of diatoms in modern stromatolites, and a large amount of micro-quartz crystals are found in some ancient stromatolites.The relationship between these micro-quartz and siliceous organisms is also controversial, and the process of micro-quartz formation is likely to record the circular path of the paleo-ocean silicon. Aiming at the stromatolites in the Middle Cambrian in the Tarim Basin, the relationship of the silicon-rich organisms, micro-quartz, with bright and dark lamina distribution were identified, based on the multi-scale analyses of the macro-, meso-, and micro-scales of stromatolites. The scanning electron microscopy-cathode luminescence image analysis was used to determine the development stage of micro-quartz in different lamina, and its in-situ ion probe silicon and oxygen isotopes were analyzed to clarify the silica source of micro-quartz. Based on a comprehensive analysis of stromatolite petrology, diagenetic characteristics, micro-quartz distribution and siliceous origin, the genetic mechanism of micro-quartz in different stromatolite lamina was explored.Identify and eliminate the influence of later silicon sources, focusing on the tracking and inversion of the formation of microquartz from the original source of deposited silicon, and explore the path of silicon circulation in the paleo-ocean.The influence of the late silicon source is identified and eliminated, and the trace and inversion of the microquartz formation process from the original sedimentary silicon source is focused on, and the circular path of the paleo-ocean silicon is discussed.The tracking and inversion of the formation process of micro-quartz can not only deepen the understanding of the deposition and diagenesis mechanism of the ancient stromatolites,but also have significance in the research of the paleo-ocean environment, and biological evolution.

沉积岩中硅质来源多样，而寻找原始沉积硅的信息，是研究古海洋硅循环的关键。已有研究表明，现今叠层石中普遍含有硅藻，部分古老叠层石中发现了大量微石英。这些微石英和硅质生物的关系还有争议，微石英形成过程很可能记录了古海洋硅循环路径。该项目以塔里木盆地中寒武统叠层石为研究对象，分析叠层石的宏观、中观、微观等特征，确定叠层石中硅质生物、微石英和亮暗纹层的分布关系。通过扫描电镜-阴极发光分析，确定不同纹层微石英的发育期次，并分别对其开展硅、氧同位素离子探针微区原位分析，明确不同纹层、期次微石英的硅质来源及差异。综合分析叠层石岩石学、成岩序列、微石英分布及硅质来源的基础上，探讨叠层石不同纹层微石英成因机理。识别并排除后期硅源的影响，重点对来自原始沉积硅源的微石英形成过程进行追踪、反演，探讨古海洋硅循环路径。不仅可以深化对古老叠层石亮暗纹层沉积、成岩过程的认识，而且对古海洋环境、生物演化等也具有指示意义。

寒武纪海洋硅循坏和微生物作用关系密切。塔里木盆地寒武系广泛发育微生物岩和硅质岩，为揭示该时期海洋环境提供了良好条件。该项目分析了塔里木盆地寒武系叠层石白云岩的丰度变化及影响因素，开展了叠层石白云岩和含球粒硅质岩的岩石学和硅、氧同位素等地球化学研究。结果表明：（1）塔里木地区中晚寒武世微生物岩复苏可能和风化作用造成的营养物质增多相关。（2）叠层石白云岩中微石英的形成过程主要有两种，一种是早期微生物作用下的低温黏土矿物转化形成的微石英（氧同位素值范围为21.71‰~26.68‰）,该过程中释放的硅、铁等可能为微生物的发育提供了营养物质;另一种为埋藏环境中一定温压条件下的黏土矿物转化产物。（3）硅质结核含有大量微生物成因的似球粒，硅氧同位素值范围分别为2.75‰~3.46‰和23.91‰~27.67‰。结合扫描电镜观察、微量元素测试等多种证据表明了微生物作用下的诱导硅沉淀过程。寒武纪高风化强度为浅海环境提供了大量硅，较高的海洋硅浓度和微生物作用诱导了硅的沉淀。该项研究深化了对古老叠层石亮暗纹层沉积、成岩过程的认识，并对古海洋环境、生物演化等也具有指示意义。