Minerals and Mineral Associations as Geothermometers and Geobarometers

Abstract

Mineral assemblages and textures may provide information about the conditions at which a rock equilibrated. In metamorphic rocks, we use qualitative terms such as low-grade, medium-grade and high-grade (and even ‘medium-low’ or ‘very high grade’) to describe the approximate temperature conditions of metamorphism. These designations do not provide information about pressure,and are therefore not useful for describing subduction zone rocks. Other methods for characterizing metamorphic conditions include: (a) Index minerals : Characteristic minerals that provide an indication of the temperature (and, in some cases, pressure) conditions at which a rock is formed (e.g., kyanite in metamorphosed shale; magmatic epidote in plutons and volcanic rocks).Not all rocks have a suitable bulk composition to produce index minerals.(b) Metamorphic facies : Assemblages of minerals, each characteristic for a particular bulk composition and indicating the range of pressure-temperature conditions at which the rock equilibrated. For example, high-pressure and lowtemperature conditions characterize the blueschist facies. Some igneous assemblages are also characteristic of crystallization within particular ranges of temperature/pressure conditions. This chapter deals with the determination of these methods.