鄂西宜昌地区下寒武统水井沱组草莓状黄铁矿SEM图像特征及古环境指示意义:以鄂阳页1井为例

SEM image characteristics and paleoenvironmental significance of framboidal pyrite from the Lower Cambrian Shuijingtuo Formation in Yichang area,western Hubei Province,southern China:A case study of well EYY1

草莓状黄铁矿在页岩中广泛分布,其形态特征及粒径大小等可作为指示氧化还原状态的环境指标。本文以鄂西宜昌地区鄂阳页1井水井沱组富有机质层段及岩家河组样品为研究对象,利用氩离子抛光场发射扫描电镜技术,结合图像处理软件ImageJ,快速获取SEM图像中草莓状黄铁矿相关参数,并探讨其环境指示意义。结果表明,鄂阳页1井下寒武统剖面样品中黄铁矿赋存形式多样,包括草莓状黄铁矿集合体、团块状黄铁矿、黄铁矿自形晶及有机质中星点状分散的黄铁矿等。其中,草莓状黄铁矿作为最主要的赋存形式,其粒径(D)、微晶粒径(d)、D/d值以及微晶晶型等相关参数在垂向上呈现明显变化。岩家河组草莓状黄铁矿微晶晶型以八面体为主,水井沱组富有机质层段底部以五角十二面体及近球形为主,中部以八面体、立方体为主,上部以立方体、四面体为主,自下而上微晶圆度呈逐渐减小趋势,反映了还原性减弱的过程。水井沱组富有机质页岩中草莓状黄铁矿平均粒径为4.43μm,中下部偏大,上部粒径偏小。草莓状黄铁矿微晶平均粒径为0.338μm,剖面上自下而上呈现由大到小的变化趋势。D/d值在岩家河组样品中偏小,在水井沱组富有机质层段下部最大,上部逐渐减小。综合相关参数在剖面上的变化,在岩家河组与水井沱组底部环境突变的位置,草莓状黄铁矿更多反映突变前的环境状态,水井沱组富有机质层段上部偏氧化状态下草莓状黄铁矿粒径偏小,可能当时氧化还原界面仍位于水体中,且指示物源供给中含铁矿物丰度减小。

Framboidal pyrite occurs widely in shales of all ages and its morphological and grain size characteristics can be used as a redox indicator in environmental research.In this study,micromorphology of pyrites in shale samples from the Lower Cambrian Shuijingtuo and Yanjiahe Formations in well EYY1,Yangtze area,was investigated in detail using Ar ion milling and scanning electron micrography(SEM)techniques,where relevant parameters for framboidal pyrite indicative of paleoenvironments were obtained through statistical analysis using image processing software(ImageJ).The results show that various types of pyrites including framboidal,lumpy,euhedral and anhedral pyrites are developed in EYY1.Among them,framboidal pyrite,as the most predominant type,shows obvious changes in gain size(D),microcrystalline grain shape and size(d)and D/d ratio in the vertical direction.The grain shape of microcrystalline framboidal pyrite in the Yanjiahe Formation is mainly octahedral;while in the Shuijingtuo Formation it is mainly dodecahedral or near-spherical in the lower part,octahedral or cubic in the middle part,and cubic or tetrahedral in the upper part,with decreasing microcrystalline grain roundness upward from the bottom part,indicating weaker reducing conditions in the same trend.The average grain sizes of framboidal pyrite in organic-rich shales and microcrystalline framboidal pyrite in the EYY1 core section are 4.43 and 0.338μm,respectively,and gradually decrease from bottom to top,which is opposite to the upward reduction-to-oxidation trend revealed by geochemical parameters.The D/d ratio for framboidal pyrite is relatively small in the Yanjiahe Formation and largest at the bottom of the Shuijingtuo Formation then gradually decreases upward.These results suggest that framboidal pyrite developed at the boundary between the Shuijingtuo and Yanjiahe Formations may reflect the redox condition prior to the stratigraphical change.The small grain size of framboidal pyrite in the upper Shuijingtuo Formation may suggest an aqueous redox interface and reduced abundance of iron-bearing minerals in the source supply.