摘要
上扬子地区龙马溪组黑色页岩富含笔石,多以碳质薄膜形式富集于富有机质层段。前期研究多关注笔石形态和成岩后的演化过程,对笔石埋藏和早成岩阶段所经历地球化学作用的研究较少,笔石成因仍缺乏直接证据。本文利用激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)技术对宁203井龙马溪组笔石进行原位微区多元素扫描成像,对主量成矿元素分布与富集程度进行解析,发现了碳质薄膜笔石体表面富集Mg、Al、Si、Fe元素,富集倍数在1.5~10倍以上,Sr/Ba值(1.4~2.3)则明显低于围岩(>5.0),指示黏土矿物包埋是笔石碳化的主要途径,包埋形成的硫化微环境导致部分笔石发生黄铁矿化。结合面笔石率、有机质、黄铁矿、黏土矿物含量和δ^13Corg值的剖面垂向变化及相关性分析,提出早期微生物席繁盛和后期硫酸盐还原菌繁盛导致水岩界面孔隙水普遍缺氧,是笔石和有机质大量埋存的主要原因。本研究结果不仅揭示了龙马溪组笔石的埋藏矿化机制,也为有机质富集和黑色页岩形成的控制因素研究提供了新思路。
BACKGROUND: The black shale of the Longmaxi Formation in the Upper Yangtze region is rich in graptolites. Most of them were preserved as a carbonaceous film, and enriched in organic-rich layers. Previous research focuses mainly on the graptolite morphology and evolution process after diagenesis, but the direct evidence for the fossil formation is still lacking.OBJECTIVES: To explore the formation mechanism of graptolite in the Longmaxi Formation and its geological significance on organic matter enrichment.METHODS: Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry(LA-ICP-MS) was used for the in situ multi-element imaging of graptolites and surrounding rocks from the Longmaxi Formation of the N203 well. RESULTS: Analysis of the distribution and enrichment degree of major ore-forming elements indicates that Mg, Al, Si, Fe were rich in the carbonaceous film surface of graptolite, with a enrichment degree ranging from 1.5 to 10. However, Sr/Ba values(1.4-2.3) of the carbonaceous film type graptolite were lower than that of the surrounding rocks(>5.0). This indicated that embedding by clay minerals was the main process of graptolite preservation. The sulfidic micro-environment caused by embedding of clay minerals benefited the pyritization of the graptolite organism. Combing with the vertical variation and correlation analysis of the cross-section graptolite ratio, organic matter, pyrite, clay mineral content and δ^13Corg value, it can be concluded that the flourishing of the microbial mat in early stage and sulfate-reducing bacteria in later stage consumed oxygen in the pore water and caused anoxic bottom water, and should be the main reason of massive burial of graptolite and organic matter.CONCLUSIONS: The result revealed the burial and mineralization mechanism of graptolite in the Longmaxi Formation, and also provided a new method for studying the controlling factors of organic matter enrichment and black shale formation.
作者
竺成林
王华建
叶云涛
王晓梅
黄家旋
朱玉梅
杨瑞东
ZHU Cheng-lin;WANG Hua-jian;YE Yun-tao;WANG Xiao-mei;HUANG Jia-xuan;ZHU Yu-mei;YANG Rui-dong(College of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China;Key Laboratory of Petroleum Geochemistry, Research Institute of Petroleum Exploration and Development, Beijing 100083, China)
出处
《岩矿测试》
CAS
CSCD
北大核心
2019年第3期245-259,共15页
Rock and Mineral Analysis
基金
国家重点研发计划项目(2017YFC0603101)
中国科学院战略性先导科技专项(A类)资助项目(XDA14010101)
国家油气重大专项课题(2016ZX05004001)
国家自然科学基金项目(41530317)
中国石油天然气股份有限公司重点资助项目(2016A-0204,2016A-0205,2017D-5006-14)
关键词
龙马溪组
笔石
多元素成像
激光剥蚀电感耦合等离子体质谱法
Longmaxi Formation
graptolite
multi-element imaging
Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry
