南海北部冷泉环境AOM驱动的自生硫化物矿物学及地球化学研究

Authigenic carbonates and sulfides are the finally produts of anaerobic oxidation of methane (AOM) and sulfate reduction,which is mediated by a consortium of methanotrophic archaea and sulphate reducing bacteria. The characteristic morphology and typical sulfur isotope of authigenic sulfide can be used as one of the fingerprint minerals of cold seeps. But further studies are needed to dicuss the formation mechanism and typomorphic peculiarities of sulfide and influence of AOM on the process of sulfide diagenesis, especially on how these processes had run and the formation of authigenic pyrite formation. In this project, the sulfides were collected from the cold seep carbonates and the core sediments in the cold seep area of northern continental slope of South China Sea. The mineral composition, chemical content, trace and rare earth elements and S, Fe isotope of sulfide were detected by the advanced analytical methods such as Field Emission Scanning Electron Microscope (FESEM), High Resolution Transmission Electronic Microprobe (HRTEM), Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS). The results show the characteristics of mineralogy, elements geochemistry and S, Fe isotopes of sulfides and can be used to explain the cause and formation mechanism of sulfide. Overall, our results will indicative of the minor changings of depositional environment during the process of AOM. All the datum will be used to set up the typomorphic mineral of sulfide to recognize the area of cold seeps.These results will provide accurate, reliable evidence for forecast and exploration of gas hydrate.

在海底冷泉渗漏环境中，甲烷厌氧氧化(AOM)和硫酸盐还原作用最终产生以碳酸盐和硫化物为主的自生矿物。其中硫化物独特的形貌及其特征的硫同位素，可以作为冷泉环境的'指纹矿物'之一。但其成因机理、标型特征以及AOM对其成岩作用的影响还有待进一步研究确定，特别是自生黄铁矿的形成过程一直存在学术争议。本项目拟以南海北部冷泉区柱状沉积物和冷泉碳酸盐岩中AOM驱动的自生硫化物为研究对象，拟采用场发射扫描电镜（FESEM）、高分辨透射电镜（HRTEM）和MC-ICPMS等先进的测试方法，研究自生硫化物的矿物组成、化学成分、微量稀土元素和S、Fe同位素特征，确定冷泉区自生硫化物矿物学、元素地球化学及同位素特征，了解其成因和形成机理，示踪AOM过程中沉积环境的微变化，建立南海北部识别冷泉环境自生硫化物指纹，为进一步勘探天然气水合物资源和科学评价提供依据。

南海北部是我国天然气水合物资源的最有潜力的区域。本项目选取了自生硫化物矿物为研究对象，其目的是为了了解自生硫化物与天然气水合物系统的关系，总结自生硫化物的特征，为水合物的勘探提供依据。X-射线衍射结果表明管状黄铁矿包括黄铁矿微晶和石墨碳;扫描电子显微镜观察表明管状黄铁矿由草莓状黄铁矿，自形晶以及无定型的黄铁矿组成；典型的草莓状黄铁矿又由八面体微晶组成，但在本项目中，在沉积物深度黄铁矿主要由带外壳结构的草莓状球和自形晶以及无定型黄铁矿组成，而且随着深度的增加，带外壳结构的草莓球占主要部分；高分辨率透射电镜（HRTEM）观察显示黄铁矿的面缺陷是白铁矿层，表明黄铁矿形成过程中环境的改变，而且在黄铁矿草莓球中发现了纳米级的石墨碳，这些白铁矿和纳米级的石墨碳共生在黄铁矿中说明沉积物深部有甲烷流体向上迁移。而且先形成的黄铁矿作为甲烷不完全氧化为元素碳的催化剂，同时，这个反应也导致环境偏酸性，更有利于白铁矿的形成；黄铁矿的S, Fe同位素研究结果表明在柱状沉积物中黄铁矿的形成机理不一样，深部黄铁矿是AOM作用下硫酸盐还原的结果。