流体-岩石相互作用过程中氧同位素交换地球化学动力学评述  

Geochemical Kinetics of Oxygen Isotope Exchange During Fluid-Rock Interactions: A Review

作　　者：倪怀玮[1] 郑永飞[1] 赵子福[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《高校地质学报》2003年第2期252-267,共16页Geological Journal of China Universities

基　　金：中国科学院知识创新项目 (KZCX210 7)资助成果

摘　　要：氧同位素研究对于示踪流体 -岩石相互作用过程中流体的时间累积流量 (或流体 /岩石比 )、流动方向和组成具有重要意义。基于质量平衡原理可以建立“封闭”体系和开放体系的氧同位素交换模型。“封闭”体系又可分为封闭、批式挥发和瑞利挥发体系。瑞利挥发较批式挥发造成岩石更大的18O亏损 ,但在地质过程中两者差异并不显著。开放体系连续模型中氧同位素迁移的机制包括扩散 /弥散和平流。由流体流动速率和扩散 /弥散系数定义的Peclet数决定了上述两种机制在一定尺度上对氧同位素迁移的相对贡献。流体 -岩石交换受表面动力学控制 ,当交换速率快于流体流动时 ,可认为流体和岩石达到了氧同位素分馏平衡 ,反之则没有达到平衡。由流体流动速率和流体 -岩石反应速率常数定义的Damk hler数决定了反应接近平衡的程度。如果采用多种矿物相监控 ,则矿物内部分馏可有效地区分这两种反应模型。对流体 -岩石相互作用过程中氧同位素变化的地球化学动力学进行了系统评述 ,其原理和模型也可扩展到对其他元素的研究。Oxygen isotope studies play an important role in tracing the time integrated fluxes (or fluid/rock ratio), flow directions and sources of fluid during fluid rock interactions. Based on the principle of mass balance, oxygen isotope exchange models are built for 'closed' systems and open systems. The 'closed' systems are classified into closed, batch volatilization and Rayleigh volatilization systems. Rayleigh volatilization results in more 18 O depletion of rock than batch volatilization, but there is no significant difference between them in geological practice. Mechanisms of oxygen isotope transport in continuum mechanic models of open systems include diffusion/dispersion and advection. Peclet number, defined by the rates of fluid flow to diffusion dispersion, determines relative contribution of the above two mechanisms to oxygen isotope transport on some scales. Fluid rock exchange is controlled by surface kinetics. When exchange rate is faster than fluid flow, equilibrium in oxygen isotope fractionation between fluid and rock is achieved, otherwise the equilibrium will not be attained. Damkhler number, defined by the rates of fluid flow to fluid rock exchange, determines the extent of reaction towards equilibrium. If more than one mineral is considered as a monitoring phase, the inner mineral fractionations may distinguish these two models. This article reviews geochemical kinetics of oxygen isotope exchange during fluid rock interactions, whose principles and models can also be extended to studies on other elements.

关 键 词：流体—岩石相互作用 氧同位素 迁移 扩散/弥散 平流 

分 类 号：P597.2[天文地球—地球化学]