Strontium concentration,isotope composition and dolomitization fluids in the Feixianguan Formation of Triassic,Eastern Sichuan of China.
Strontium concentration,isotope composition and dolomitization fluids in the Feixianguan Formation of Triassic,Eastern Sichuan of China.
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作者：
H. Si
中国地区三叠系飞仙关组发育泥晶白云岩、粒状白云岩(具原生结构)和结晶白云岩三种主要白云岩类型，其中结晶白云岩是最重要的气藏。三种类型白云岩的锶含量相近，~(87)Sr/~(86)Sr比值均高于同时代海水。锶在方解石(或文石)和白云石中的分配系数不同是控制白云岩中锶含量的最重要因素。粒状白云岩(具有原始结构)的流体由81%的大气水和19%的海水混合而成，泥晶白云岩的流体由92%的大气水和8%的海水混合而成。但结晶白云岩流体包裹体中极低的锰含量(平均19.8ppm)、较高的锶含量(平均137ppm)和较高的均一温度不支持大气降水参与白云化作用的假说。这类白云岩的白云化流体可能是被捕获的同时代海水(年龄242~245 Ma)，可能是通过不明原因释放到白云岩层中的。微晶白云石和粒状白云石(原生结构)的~(87)Sr/~(86)Sr比值表明白云石化过程中有大气降水的存在，微晶白云石的白云石化机理可用蒸发抽运作用解释，粒状白云石(原生结构)的白云石化机理可用混合带模型解释。
Three major dolomite types including micrite dolomite,grainy dolomite(with original texture)and crystalline dolomite, occur in the Feixianguan Formation of Triassic,Eastern Sichuan of China,in which the crystalline dolomite is the most important gas reservoir.Based on the strontium concentration,isotope composition of the dolomites,the dolomitization fluids were accordingly inferred here.The three types of dolomite have similar strontium content with higher ~(87)Sr/~(86)Sr ratios to some extent than that of the coeval seawater.The difterenee of distribution coefficient of Sr in calcite(or aragonite)and dolomite is the most important control factor on the strontium content in dolomites.Calculation by strontium mixing equation of seawater with meteoric water shows that the dolomitization fluid for crystalline dolomite is mixed by 94% meteoric water and 6% seawater;the fluid for grainy dolomite(with original texture)is mixed by 81% meteoric water and 19% seawater,and that for micrite dolomite is mixed by 92% meteoric water and 8% seawater.However,very low Mn contents(average 19.8ppm),relatively high Sr contents(average 137ppm)and the homogenization temperatures measured in fluid inclusions of crystalline dolomite do not support the hypothesis that meteoric water as high as 94% involved the dolomitization.The dolomitization fluid for this kind of dolomite was probably the trapped coeval seawater (with the age of 242 to 245Ma),which may have been released into the dolomite horizon through unidentified pathways.The Mn and Sr contents,as well as ~(87)Sr/~(86)Sr ratios of the mierite dolomite and grainy dolomite(with original texture)suggest the existence of meteoric water during the dolomitization.The mechanism of dolomitization for mierite dolomite could be explained by evaporative pumping,and that for grainy dolomite(with original texture)explained by mixing-zone model.