白云岩化过程中锶含量变化及锶同位素分馏特征与意义  

Variation of Sr content and ^(87)Sr/^(86)Sr isotope fractionation during dolomitization and their implications

作　　者：王利超[1] 胡文瑄[1,2] 王小林[1,2] 曹剑[1,2] 吴海光[1] 廖志伟[1] 万野 

机构地区：[1]南京大学内生金属矿床成矿机制研究国家重点实验室,江苏南京210023 [2]南京大学能源科学研究院,江苏南京210023

出　　处：《石油与天然气地质》2016年第4期464-472,共9页Oil & Gas Geology

基　　金：国家科技重大专项(2011ZX05005-002-008HZ)

摘　　要：白云岩的87Sr/86Sr值是判识其形成流体性质及来源的重要指标,也被广泛用于古海水87Sr/86Sr值演化和地层学研究。因此,白云岩化作用对87Sr/86Sr有无影响及其影响程度直接关系到对87Sr/86Sr值地质意义的解释,但目前尚未见到对这一问题的系统研究。下扬子地区宜兴葛山剖面发育灰岩、白云质灰岩、灰质白云岩及白云岩的系列层序,为研究白云岩化过程中锶的地球化学行为提供了良好素材。通过对该剖面锶元素含量及锶同位素组成系统分析,发现随白云岩化程度的增强,锶含量显著降低,从1 358μg/g一直下降到94μg/g,而87Sr/86Sr值却不断升高,由0.708 019升高到0.708 182。分析前人发表的数据,发现这种演化特征具有一定的普遍性,因此这很可能与锶在白云岩化过程中的地球化学行为有关。进一步分析认为,在白云岩化过程中,随着Ca被Mg的大量交代,以类质同像形式存在的Sr也随之流失,导致Sr含量降低。另一方面,受质量效应的制约,在上述过程中,质量偏轻的86Sr更易迁出,从而造成87Sr/86Sr值相对增大。因此,在应用白云岩的87Sr/86Sr值判断白云岩化流体来源、建立古海水87Sr/86Sr演化曲线和分析白云岩地层年代时,必须考虑白云岩化作用对碳酸盐岩87Sr/86Sr的影响,才能得到符合客观实际的认识。87 Sr/86 Sr ratio of dolomite is an important indicator for identifying source and characteristics of fluids related to formation and evolution of dolomite.Meanwhile,the ratio has been widely used in study of seawater 87 Sr/86 Sr ratio evolu-tion of fossil seawater and strontium isotope stratigraphy.Thus,the existence and degree of influence of dolomitization on the 87 Sr/86 Sr ratio is crucial to understand the geologic significances of 87 Sr/86 Sr ratio of dolomite.However,no systematic research of this issue has been published.The Geshan section in Yixing County of Lower Yangtze region has excellent outcrops of carbonates including limestone,dolomitic limestone,calcite dolomite and dolomite,providing a chance to study the geochemical behaviors of strontium during dolomitization.In this study,we systematically analyzed the Sr contents and 87 Sr/86 Sr ratios of these carbonates.The Sr contents of these carbonates decrease significantly with enhancing dolomitiza-tion degree,from 1 358μg/g of limestone to 94μg/g of dolomite,while the 87 Sr/86 Sr ratios increase from 0.708 019 to 0.708 182.An analysis of the relevant published data shows that the evolution feature of Sr is common to some degree,in-dicating that it may be attributed to the habitual behaviors of Sr during dolomitization.As Ca is replaced by Mg during do-lomitization,the isomorphous Sr is lost simultaneously,leading to the lowering Sr content.In addition,lighter 86 Sr emi-grates from the process due to the constrint of mass effect,resulting in higher 87 Sr/86 Sr ratio.Therefore,the influences of dolomitization on 87 Sr/86 Sr ratio must be considered when identifying the source of dolomitization fluids,establishing the 87 Sr/86 Sr evolution curve of fossil seawater and analyzing the age of dolomite,so as to obtain correct understandings.

关 键 词：锶含量 87 Sr/86 Sr值 白云岩化 锶同位素地层学 下扬子地区 

分 类 号：P597[天文地球—地球化学]