摘要
困扰几代地质学家的"白云岩问题"不仅是重要的基础地质问题,而且与油气勘探开发实践密切相关。传统岩石学、地球化学研究手段在白云岩成因研究中起了重要作用,但均不能对其形成过程及Mg离子来源提供很好的约束。作为一种新兴的非传统稳定同位素,镁(Mg)同位素已经在白云岩问题研究领域受到了广泛关注。随着Mg同位素地球化学体系研究的不断深入,目前已经积累了大量不同时代、不同类型白云岩的Mg同位素数据。此外,通过理论计算、模拟实验与实际观测等方法,对白云岩(石)形成过程中的Mg同位素分馏研究已取得较大进展,为Mg同位素在白云岩问题研究中的应用奠定了坚实的理论基础。理论计算、实验模拟和实际观测的Mg同位素分馏存在较大差异,造成这种差异的原因一方面可能是模拟或实际观测的Mg同位素分馏未达到平衡,另一方面可能是实际白云岩的形成过程与理论过程不同。由于直接根据Mg同位素值判断白云岩成因存在较大难度,基于典型的白云石化地质模式,最新的研究建立了两个Mg同位素地球化学数值模型(DAR模型、AF模型),用于模拟不同白云石化过程中的Mg同位素变化,可对白云石化过程及Mg离子来源进行定量-半定量研究。其中DAR模型用于模拟海水白云石化过程的Mg同位素地球化学特征,而AF模型用于模拟流动水体白云石化过程的Mg同位素地球化学特征。通过这两个模型可以示踪白云岩的Mg离子来源并区分不同白云石化过程。尽管目前已建立的Mg同位素地球化学模型总体框架正确,但在适用范围和参数设置等方面存在一定不足,仍需要在后期研究中不断完善。
As a long standing enigma,the‘dolomite problem’not only is a basic geological problem but also has close relationships with the exploration and development of the economic hydrocarbon reservoirs. Traditional petrographic and geochemical approaches have been applied widely to the research of dolomite,but none of them could provide unambiguous solution to explain the process of the dolomitizaton and its Mg2 +origination. With the development of high precision Mg isotope analytical technique,Mg isotope become a useful tool in the study of dolomite and the Mg isotopic data of various types of dolomites from different geologic time were accumulated.Moreover,the fractionation of Mg isotope during dolomitization process has been extensively studied based on theoretical calculation,experimental calibration and actual observation. These developments provide a firm theoretical base for the application of Mg isotope on the study of dolomite origination. However,there are significant differences among theoretical calculated,experimental calibrated and actual observed fractionations. This discrepancy may ascribe either to the unequilibrium fractionation during experimental calibration and actual observation or to the different forming process between the actual forming process of dolomite and the theoretical process.Two numerical models for the Mg isotope systematics based on typical geological model of dolomitization have established to constrain the process of dolomitization and the source of Mg2 +. The Diffusion-Advection-Reaction( DAR) model simulates the contemporaneous seawater dolomitization process,in which Mg is delivered by diffusion. The Advective Flow( AF) model describes the dolomitization process in fluid flow,which can be applied to most traditional dolomitization models,such as the reflux model. Although the overall framework of the established Mg isotope geochemical model is correct,some limitations in the application scope and parameter setting still exist and are needed to be modified in the future rese
作者
甯濛
黄康俊
沈冰
NINGMeng;HUANGKangJun;SHENBing(MOE Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Sciences,Peking University,Beijing 100871,China;State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi'an 710069,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2018年第12期3690-3708,共19页
Acta Petrologica Sinica
基金
国家自然科学基金项目(41602343、41772359)资助.
