湖北江陵凹陷古新统沙市组四段硫酸盐硫同位素组成及其地质意义

Sulfur Isotopic Composition of Sulfate and Its Geological Significance of Member 4 of Palaeocene Shashi Formation in Jiangling Depression of Hubei Province

江汉盆地江陵凹陷古新统沙市组四段硬石膏和钙芒硝中硫同位素组成为25.2‰～32.6‰,远远高于同时期海水硫同位素(20‰)。结合研究区古新统沙四段蒸发岩87Sr/86Sr研究,借助Mg、Ca、Sr、Ba等元素及其Mg/Ca、Sr/Ba,认为江陵凹陷沙四段时期湖盆为封闭的陆相盐湖。根据典型暖相盐类矿物(原生钙芒硝)的广泛发育和指示气候干旱程度的Fe2O3/FeO特征,可以推断研究区在沙市组四段沉积晚期的古气候属于暖旱型气候。在温暖干旱的气候环境下,硫酸盐沉积物供给率较低,因此当时的沉积环境对硫酸盐是封闭的。通过Fe、Mn、Al、Mg、Ca、Ni、V、Co、Cu等主微量元素以及Fe/Mn、(Fe+Al)/(Ca+Mg)、w(V+Ni+Mn)、V/(V+Ni)、Ni/Co、Cu/Zn的研究表明,研究区沙市组四段环境为湖盆浪基面之下的缺氧还原环境,为常年性较深水分层湖泊。上述缺氧沉积环境下存在大量的厌氧细菌,表生(≤50℃)条件下,厌氧细菌使硫酸盐SO24-还原成H2S,后者与金属离子结合形成硫化物或直接从体系中逸出,再加上当时的内陆环境对硫酸盐是相对封闭的,这种硫循环过程是造成研究区硬石膏和钙芒硝硫同位素值较高的原因,也是研究区最重要的硫同位素分馏过程。

The sulphur isotope of anhydrite and glauberite in Member 4 of Palaeocene Shashi Formation in Jiangling depression changes between 25.2‰-32.6‰,which is much higher than the sulphur isotope of synchronism sea water（20‰）.Through the study of strontium isotope in evaporate in Member 4 of Palaeocene Shashi Formation and the comparison analysis of Mg,Ca,Sr,Ba content and Sr/Ba,Mg/Ca ratio,the authors propose that lake basin of Jiangling depression is closed continental salt lake facies.Primary glauberite,typically warm facies saline minerals,is widely developed in this formation.The Fe2O3/FeO ratio represents drought climate.This feature reveals that Palaeocene Shashi Formation of south Jiangling depression in Jianghan basin is drought and warm paleoclimate.In drought and warm paleoclimate condition,because of the low supply rate of sulphate deposites,the sedimentary environment at that time is closed to sulfate.Through the study of main trace element,including Fe,Mn,Al,Mg,Ca,Ni,V,Co,Cu and Fe/Mn,（Fe＋Al）/（Ca＋Mg）,w（V＋Ni＋Mn）,V/（V＋Ni）,Ni/Co,Cu/Zn ratio,it reveals that the environment of Member 4 of Shashi Formation is anoxic and reducing environment under wave base of lake basin,which is relative deep layered lake for years.A lot of anaerobic bacteria lived in this anoxic sedimentary environment.Under hypergene condition（≤50 ℃）,anaerobic bacteria reduced SO2-4 to H2S,which reacted with metal ion to form sulfide or escaped from the system.In addition,the continental environment at that time is relative closed to sulfate.This cyclicprocess caused the high sulphur isotope value of anhydrite and glauberite,which was also the most important sulfur isotope fractionation process in the study area.