晚奥陶世—早志留世火山活动的地球化学记录及其对页岩有机质富集的影响

The frequent volcanic activity during Late Ordovician and Early Silurian time was considered as an important factor in raising climatic and paleoenviromental changes and has a significant controlling effect on the formation of organic-rich shale. However, the shortage of geochemical records of volcanic activity in the marine shales hampers to study the relationships and influence mechanism between volcanic activity, climatic and environmental change, and organic matter accumulation. Hg concentration and Hg isotopic compositions of marine shale provide effective geochemical means for the recognition of volcanic activity. This project intends to obtain the high-resolution geochemical records of volcanic activity by using Hg concentration and Hg isotopic compositions of Wufeng—Longmaxi Formation shale and to study the variation rules of volcanic activity within biostratigraphic and chemostratigraphic framework. Combined with analyses of total organic carbon, carbon isotopes, mineral composition, as well as major and trace elements, the relationships between volcanic activity and paleoclimate and paleoenvironment changes will be discussed, which can provide important scientific evidence for the paleoenvironmental and climatic changes during Late Ordovician and Early Silurian time. On this basis, the effect of volcanic activity and its induced climatic and oceanic environmental changes on the organic matter accumulation will be studied and the influence mechanism of volcanic activity on the organic matter accumulation will be established. The results will provide scientific basis for prediction and assessment of “sweet spot” and shale-gas exploration.

晚奥陶世—早志留世频繁的火山活动被认为可能是造成当时气候环境变化的重要原因，并且对有机质的富集具有明显的控制作用。然而，由于缺乏火山活动在海相地层中的地球化学记录，制约着对其关系以及作用机制的研究。海相页岩中Hg含量及同位素特征提供了识别火山活动的有效地球化学手段。本项目拟在古生物和化学地层学年代格架约束下，通过对上扬子区五峰组—龙马溪组页岩中Hg含量及其同位素特征进行分析，获取火山活动在页岩中的高分辨地球化学记录，研究这一时期多期次的火山活动及变化规律；并结合有机碳含量、碳同位素、主量、微量元素和矿物组成等分析，探讨火山活动与古气候和海洋环境演化之间的内在关系，为这一时期古气候与海洋环境的演变提供重要科学依据。在此基础上，进一步研究火山活动及其引发的气候和海洋环境变化对页岩有机质富集的控制作用，建立火山活动对有机质富集的影响机制，为页岩气的勘探和“甜点区”的预测和评估提供理依据。

火山活动被认为是奥陶纪—志留纪转折期气候环境变化的主要诱因，并且对对富有机质页岩形成具有明显的控制作用。然而，由于缺乏火山活动在海相地层中的高分辨率地球化学记录，制约着对其关系以及作用机制的研究。本项目通过对上扬子区五峰组—龙马溪组页岩中Hg含量及其同位素特征进行分析，获取了晚奥陶世—早志留世火山活动在海相页岩中的高分辨率地球化学记录，结合有机碳含量、碳同位素、主量、微量元素和矿物组成等分析，初步探讨了奥陶纪—志留纪转折期火山活动与古气候和海洋环境演化之间的耦合关系以及对有机质富集的影响。研究结果表明，在五峰组—龙马溪组底部的富有机质页岩层段存在明显的Hg异常富集现象，Δ199Hg值接近于0或者轻微正偏 (0‰至+0.10‰)，表明这些富集的Hg主要来源于火山活动，为奥陶纪—志留纪转折期火山活动在海相页岩中的记录提供了高分辨率地球化学证据。在Hg富集的层段，TOC含量较高，有机碳同位素和黄铁矿硫同位素明显负偏，水体变为缺氧环境，同时对应于温暖的气候环境，表明火山活动可能是导致当时气候变暖和水体缺氧等环境波动的主要驱动因素，并最终导致有机质的大量富集。本项目的顺利实施为无机金属元素同位素在页岩油气研究方面开拓了新的领域，也为研究晚奥陶世—早志留世气候环境演化提供了新的手段。