|
Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA
Information on pressure (P) and temperature (T) is a fundamental aspect of research on metamorphic terrains. Unfortunately, many workers employ thermobarometers that are not experimentally calibrated, are insensitive or too sensitive to P-T changes, depend on a priori assumptions of water pressure (such as most petrogenetic grids), or are rapidly reset on cooling. Many systems are based on inaccurate thermodynamic data, involve solids with inadequately characterized structural states, neglect effects of thermal expansion and compressibility, or require long extrapolations in P-T-X space. For instance, application of the widely used garnet-clinopyroxene KD thermometer may require extrapolation to temperatures where current thermodynamic models of pyroxenes and garnets remain uncertain. Current versions of the Mg/Fe exchange thermometer for biotite-garnet involve substantial compositional extrapolations for many applications and the biotite is easily reset while cooling from higher T. The most widely employed barometer is based on dilution of the reaction grossular + kyanite + quartz = anorthite, but failure to correct molar volumes for P-T-X may yield systematic errors of 1–2 kbar for barometry of crustal metamorphites. Application of this barometer to rocks equilibrated at T < 600–650°C is presently unwarranted in view of unknown a-X relations of garnets and plagioclases at these T. However, by careful selections, thermobarometry may be accurate to ±50°C and ±1 kbar in many metamorphic terrains if a variety of different equilibria can be applied. Well-calibrated barometers that are useful for T
600–650°C rely on continuous reactions based on equilibria such as almandine + rutile = ilmenite + sillimanite + quartz, garnet + quartz = ferrosilite + plagioclase, garnet + rutile = ilmenite + anorthite + quartz, and almandine + sillimanite = hercynite + quartz. An extensive survey of the recent literature on thermobarometry of individual metamorphic facies reveals the range of P-T encountered in each facies. Temperature estimates are in good agreement with the inferences of Turner (1968). Barometry reveals that the blueschist, amphibolite and granulite facies give way to the eclogite facies over the pressure range of 12–16 kbar.
