Using Equilibrium Thermodynamics to Understand Metamorphism and Metamorphic Rocks

Abstract

Metamorphic rocks, formed at elevated temperature and pressure from pre-existing rocks inside mountain belts, provide a seemingly unpromising target for the application of equilibrium thermodynamics. This is because metamorphic rocks develop their mineral assemblages along a pressure-temperature (P-T) path, with pressure and temperature continuously changing along the path. However, in a successful model for the formation of such rocks, involving the essential role of fluid or melt, the mineral assemblages observed at the Earth's surface can be considered to reflect a state of frozen-in equilibrium as the rocks are exhumed towards the Earth's surface. Equilibrium thermodynamics applied to such mineral assemblages allow P-T information to be extracted. Currently the best way to do this is via calculated phase diagrams, the most powerful being P-T pseudosections. These diagrams portray the variation of mineral assemblages with P-T for a specified rock composition. Pseudosections allow the P-T conditions of the frozen-in equilibrium to be estimated, and can also give information on the P-T path followed. Such paths are an essential input in constraining the processes involved in mountain-building and the evolution of continental crust.