第八讲 变质作用条件估算中热力学应用的问题

PROBLEMS IN APPLICATION OF THERMODYNAMICS TO ESTIMATION OF METAMORPHIC CONDITIONS

变质岩形成的物理化学条件,很早以来便为岩石学家所重视,原因是这些物理化学环境是由地壳的发展、演化决定的。具体地来说,一定的变质矿物组合的形成及其改变均与地壳当时的热动力场有关。为了确定变质作用与地壳的发展、演化之间的内在联系,前后有许多学者进行了详细的研究。其中重要影响的成果有:(1)Eskola(1915)通过对Orijarvi及Oslo地区的变质矿物组合的对比研究提出了变质相的概念,用以表示变质作用的热动力条件。尽管这种表述仅仅是描述性的,但也明确地提出可以用一定的矿物组合来指示变质岩形成时的温度、压力范围。(2)Miyashiro(1972)在对日本岛变质作用研究的基础上,提出了变质相系的概念,目的是把变质作用事件与大地构造发展之间建立内在的关系。(3)Winkler(1976)通过实验研究以及总结其他学者的研究成果提出了变质级和变质反应带的概念,指出各变质级之间的界限可以用特定的变质反应来标定。

It is very necessary and important to learn some fundamentals of thermodynamics in order to apply them to metamorphic geology as we have seen in the trend of study of metamorphism.To a certain degree metamorphic geology indicated that we have begun to study the conditions of formation of metamorphic rocks with semi-quantitative or quantitative methods. Our main intention is not to determine the boundaries of metamorphic facies or grades, but to examine and estimate the conditions of P.T.and dT/dP of metamorphism reflecting the physical-chemical environments and this is done by using the thermodynamic theories and equations.If k and kD are equal in quantity, we consider the system as an ideal solid solution, and vice versa. During the practical calculation, we had better follow this way such as:gettinginformation about mineral assemblage, observing their structural relationship under a microscope,calculating and discussing the results. There is an example in this paper which shows how to perform and explain the different results from using various equations.Uncertainties resulted may be due to uncertainties of thermodynamic data and of different models, to imperfection of natural minerals and to errors in analyses of mineral composition. Among the there methods of single mineral, mineral pairs, and mineral assemblage which are used to calculate the conditions of metamorphism, the method of mineral pairs is better than that of single mineral, but the mineral assemblage method is the best. There are two ways to analyze mineral composition, and one of them can be chosen according to the case demand.