南海神狐天然气水合物系统沉积物中自生黄铁矿的特征研究

Study on authigenic pyrite in sediments of gas hydrate geo-system in the Shenhu area,South China Sea

南海神狐海域是中国最重要的天然气水合物调查研究区之一,为了解水合物存在对沉积物地球化学环境的影响,对采自神狐海域W19B井位的沉积物样品进行了矿物学和地球化学研究。X射线衍射分析和主量元素结果显示部分层位有异常高含量的硫化物(主要为黄铁矿)。扫描电镜结果表明随着样品深度的增加,黄铁矿的晶面、晶棱更加明显,且集合体形态呈现聚莓→单莓→细粒的变化趋势,扫描电镜还观察到草莓状黄铁矿向细粒自形黄铁矿转化的中间产物。在53.0mbsf(meters below seafloor)和140.4mbsf层位均发现异常高含量的黄铁矿。其中140.4mbsf层位黄铁矿充填有孔虫壳体的现象普遍,并伴有大量柱状黄铁矿产出,可能与有机质和甲烷厌氧氧化相关,但主导作用应为甲烷厌氧氧化,该层位可能位于古硫酸根-甲烷界面(sulfate-methane interface,SMI)附近。根据所得结果,推测地质历史时期中甲烷异常渗漏事件的发生,致使向上的甲烷通量增加,推动SMI上移,导致53.0mbsf和140.4mbsf界面处因甲烷厌氧氧化而形成大量黄铁矿。多个黄铁矿富集层的存在可能表示沉积史中曾发生多期次的深部流体渗漏或者天然气水合物的分解活动。

Shenhu area of South China Sea is one of the most important research areas for natural gas hydrate in China.Geochemical and mineralogical characteristics were investigated in ten sediment samples,which included the sediments of gas-hydrate-bearing layer,collected from W19 Bsite in Shenhu area.Analyses of major elements and X-ray diffraction indicated that several layers contained pyrite with unusual high content.The SEM images revealed that the pyrite crystals became more euhedral and the morphology varied from aggregate framboids to single framboid and finally tiny grain types as sediment depth increased,and some intermediate products in the textural evolution were also identified.Both 53.0 mbsf（meters below seafloor）and 140.4 mbsf layers were observed with significant abundances of pyrite.At the 140.4 mbsf layer,pyrite filling the chamber of the foraminifer shell was common and coupled with a large amount of columnar pyrite,which might be related to the anaerobic oxidation of organic matter and methane,especially with the anaerobic oxidation of methane（AMO）.The layer of 140.4 mbsf might be close to the paleo sulfate-methane interface（SMI）.Based on the results obtained,it is proposed that anomaly high AMO derived pyrites around 53.0 mbsf and 140.4 mbsf were probably related to the high methane flux,which might be resulted from the unusual methane events.The presence of several high pyrite layers might be related to the multi-stage methane events which might be caused by tectonic activities or the decompositions of gas hydrate in the depositional history.