The current status of thermobarometry in metamorphic rocks

Abstract

Summary 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 K D 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 τ 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.

Diagenesis and Metamorphism of Clay Minerals in the Helvetic Alps of Eastern Switzerland

“…Although this barometer uses unbuffered assemblages lacking K-feldspar, it has been successfully applied in many field areas. Therefore, the statement by Essene (1989) that "it seems prudent to avoid the use of Sassi's phengite barometer for thermobarometry" seems not justified.…”

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part I: Metamorphic geology

“…In this work we concentrate on the mineral assemblages that are most useful for geological thermobarometry in granulite-facies rocks (Essene, 1989;Harley, 1989) and on the other hand, are most prominent and widespread in the LGB. This allowed us to get regional coverage on the possible change of mineral compositions and to get consistent results on the metamorphic PT regime over the mapped cross sections of the LGB.…”

Thermobarometry, Geochronology and the Interpretation of P-T-t Data in the Britt Domain, Ontario Grenville Orogen, Canada

“…The calibration of reaction (8) is consistent with GAHS and GADS barometry but is dependent on the solution models chosen. Until a consensus is reached on mixing models for garnets, pyroxenes, feldspars, biotites, and amphiboles, any exchange thermometer involving these phases must be regarded as provisional at best (Essene, 1989).…”

“…Chemical analyses of minerals assumed to be in equilibrium have been used in conjunction with barometers and thermometers to estimate the pressure (P) and temperature (T) conditions attained at or near the peak of metamorphism (Ghent et al, 1982;Chamberlain & Lyons, 1983;St.-Onge, 1984;Bohlen et al, 1985;Sandiford, 1985;Moecher et al, 1986;Essene, 1989;Anovitz & Essene, 1990;Mezger et al, 1990). In addition, chemical zonation in minerals has been used to estimate the P-T conditions at various stages in the metamorphic histories of rocks (Savage & Sills, 1980;Brown & Earle, 1983;Spear & Selverstone, 1983;Hodges & Royden, 1984;Selverstone et al, 1984;Spear et al, 1984;Selverstone & Spear, 1985;St.-Onge, 1987;St.-Onge & King, 1987;Harley, 1988;Perchuk et al, 1989).…”