西藏南部白垩纪龟裂状冷泉碳酸盐岩结核的形成模式及其古气候、古海洋意义

Gas hydrates are generally considered to be the most potential new energy sources in the 21st century. Cold seep carbonates are increasingly concerned by the scientific community, owing to their relationship with gas hydrates release. In recent years, septarian cold seep carbonate concretion, which is special in morphology, have been reported increasingly, and is considered as a medium for the study of gas hydrate and gas hydrate stable zone, and can be used to illustrate the architecture of the gas hydrate stable zone, and evaluate the processes that acted within this zone in the sedimentary column. In addition, Current theories entertain several possible mechanisms on the cracking of the septarian concretions, and none of these explanations is satisfactory. In the pre-study, the applicant first discovered that there was also a set of septarian cold seep carbonate concretions in the Cretaceous strata of the Tethyan Himalayas in southern Tibet. Therefore, this project will take the Cretaceous septarian cold seep carbonates in the Tethys Himalayas of southern Tibet as the research target, and use effective techniques such as petrography and mineralogy, element and isotope geochemistry, and fluid inclusions to obtain origin, formation processes and model of them. Based on the research above, we can illustrate the architecture of the gas hydrate stable zone, and evaluate the processes that acted within this zone, which is of great significance for the paleoclimatic and paleoceanographic studies in the Cretaceous and research on gas hydrates.

天然气水合物被普遍认为是21世纪最具潜力的新型能源。冷泉碳酸盐岩，因其形成与天然气水合物释放相关日益受到科学界的关注。近年来，有一种特殊形态的冷泉碳酸盐岩—龟裂状冷泉碳酸盐岩结核被越来越多的报道，并被认为其可作为研究天然气水合物及其稳定带的媒介—还原沉积柱中天然气水合物稳定带的部分地质过程及物理架构。再者，目前对于龟裂状碳酸盐岩结核裂隙的成因机制观点不一，仍无定论。申请人在预研究中首次发现在藏南特提斯喜马拉雅带白垩纪地层中同样存在一套龟裂状冷泉碳酸盐岩结核。故本项目将以藏南地区特提斯喜马拉雅带白垩纪龟裂状冷泉碳酸盐岩为研究对象，运用岩石矿物学、元素与同位素地球化学、流体包裹体等技术手段，还原其成因机制、形成过程与模式，从而还原沉积柱中天然气水合物稳定带的部分地质过程及物理架构，这对白垩纪古气候与古海洋以及天然气水合物的研究均具有重要意义。

冷泉碳酸盐岩，因其形成与天然气水合物释放相关日益受到科学界的关注。本项目对西藏南部特提斯喜马拉雅带白垩纪地层中发现的一套龟裂状冷泉碳酸盐岩结核开展了岩石矿物学、元素与同位素地球化学等研究工作。.龟裂状结核多呈顺层分布于钙质页岩与粉砂质页岩中，大小不等，为6 cm× 8 cm～100 cm×30 cm。其由边部结核体与裂隙充填物组成，结核基质主要由微晶方解石（55~75%），石英、长石等碎屑矿物（25~45%）组成，裂隙充填物有两期：纤维状、块状方解石胶结物。. 从外缘碳酸盐岩微晶基质到纤维状方解石胶结物再到块状方解石胶结物，δ13C值呈现逐渐富集的现象（-26.81 to -1.86 ‰ VPDB）。外缘碳酸盐岩微晶基质与纤维状方解石胶结物的氧同位素值均偏负（-15.78~-5.82‰ VPDB），而块状方解石胶结物具有最负的氧同位素值（-24.7~-11.56‰ VPDB）。块状方解石最高的碳同位素值、最负的氧同位素值、高FeO低MgO、流体包裹体均一温度高（平均176.9℃）的特征，均表明其与早期的沉积物（微晶方解石沉积物、纤维状方解石胶结物）有明显差别，即形成环境明显不同。. 综合以上实验成果及分析，我们提出龟裂状结核的外缘碳酸盐岩微晶基质、纤维状方解石胶结物的形成均与甲烷的厌氧氧化相关，并且在之后结核埋藏过程中，已固结的微晶基质、纤维状方解石发生了部分重结晶。块状方解石是在结核埋藏过程中深部热液加热的海水充填结核裂隙而形成。