Applied Mineralogy Applications in Industry and Environment
This book covers the entire spectrum of mineralogy and consolidates its applications in different fields. Part I starts with the very basic concept of mineralogy describing in detail the implications of the various aspects of mineral chemis
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CHAPTER
12 MINERALS AND MINERAL ASSOCIATIONS AS GEOTHERMOMETERS AND GEOBAROMETERS
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.
12.1 INTRODUCTION Geothermometers and geobarometers are mineral systems that may be used to estimate the absolute temperature and pressure that produced an equilibrium mineral assemblage in rocks of various litho-types, say, either igneous or metamorphic. Geothermometric and geobarometric methods are considered under two groupings: directly calibrated methods and internally consistent data set methods. Emphasis is placed on the importance of error propagation S. Mukherjee, Applied Mineralogy: Applications in Industry and Environment, DOI 10.1007/978-94-007-1162-4_12, © Capital Publishing Company 2011
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Mineral Transformations and Their Effects
calculations. They allow the calculation of specific uncertainties on temperatures and pressures due to the quantifiable uncertainties involved in the calibration and application of methods. The possibility of systematic errors, due to the unquantifiable effects of the assumptions and approximations involved, must be evaluated using the criterion of geological possibilities. Thermobarometry or the determination of temperatures and pressures during the formation of minerals or mineral associations represents a practical application of geochemical thermodynamics to petrology.
12.2 GEOTHERMOBAROMETRY Geothermobarometry is the quantitative determination of the temperature and pressure at which a metamorphic or igneous rock reached chemical equilibrium. The term “classical” thermobarometry refers to methods for calculating the PT conditions of a specific chemical reaction. Principle: Use measured distribution of elements in coexisting phases from experiments at known P-T to estimate P and T of natural sam
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