变质流体地球化学:从静态定性“流”向动态定量

Geochemistry of metamorphic fluids: from static-qualitative analysis to dynam ic-quantitative study

对变质岩的矿物学演化研究使岩石学家和地球化学家认识到，流体流动在变质作用过程中发挥着重要的作用。根据变质矿物共生组合、化学和同位素数据能够鉴别流体在变质体中所流经的区域，确定流体的数量、相对于温度和压力梯度以及岩石接触带的流体流动方向，并确定流体活动的时代。仅在特殊的矿物组合、流体成分、温度和压力条件下，流体可能沿矿物颗粒缝隙的网状交叉的显微通道流动。变质流体流动对于研究地壳的热和质量传输及变形机制和速率有着重要的作用。在各种地壳环境中都存在大量的含水流体，区域和接触变质岩石具有的时间积分流量远远大于紧邻岩石脱水所提供的数量。支持高水流量的证据包括不同学科种类，例如岩石学、矿物学、结构和显微构造以及稳定同位素。渗透率增大可能是促使流体流量变大的主要机制。通过同位素地球化学家与构造地质学家和岩石学家们之间的密切合作，直接对变质岩渗透率增大机制进行实验测定和理论模拟，并在野外检验渗透率增大模型，将有助于理解变质作用的热和流体循环与变形作用之间的关系。

Numerous studies of the mineralogical evolution of metamorphic rocks have resulted in recognition of the importance of fluid flow in the processes of metamorphism. Mineral paragenesis, geochemical and isotopic data can be used to identify where fluid flowed in metamorphic terranes, to measure the amount of fluid, to constrain the direction of fluid flow relative to temperature and pressure gradients and lithologic contacts, and to determine the ages of flow. Under special combinations of mineralogy, fluid composition, temperature and pressure, fluid can flow through a static, interconnected network of microscopic tubes at grain corners. Metamorphic fluid flow plays a significant role in heat and mass transfer in Earth's crust and in the mechanisms and rates of deformation. It has been recognized that there is a large amount of aqueous fluid in a variety of crustal environments. Regional and contact metamorphic rocks have time integrated fluxes far greater than amounts that can be supplied by dehydration of immediately subjacent rocks. The types of evidence supporting high water fluexes are numerous and varied, including petrologic, mineralogical, textural, microstructural, and stable isotopic data. Permeability enhancement may be the principal mechanisms to facilitate large fluid fluxes. In order to understand the relationship of heat and fluid cycles to deformation during metamorphism, future studies should be directed toward experimental and theoretical simulation of permeability enhancement with close collaboration between isotope geochemistists, petrologists and structural geologists to test models of permeability enhancement in the field.