thermobarometric

Thermobarometry, or geothermobarometry, is a set of methods used in geology to estimate the pressure and temperature conditions under which a rock formed or equilibrated. The approach combines mineral chemistry with phase equilibrium theory: the compositions of coexisting minerals are compared with thermodynamic models calibrated by high-pressure, high-temperature experiments to infer the P-T conditions of crystallization or metamorphism. In many rocks, multiple mineral pairs or mineral inclusions are used to solve for pressure and temperature, often by minimizing discrepancies between observed compositions and model predictions.

Common thermobarometers and thermometers rely on Fe-Mg exchange or other major element exchanges between mineral pairs, such as garnet-pyroxene, garnet-biotite, plagioclase-amphibole, and orthopyroxene-clinopyroxene, as well as barometers based on Fe-Ti oxides. Melt inclusions within crystals can also carry information about crystallization conditions in magmas, and quartz-feldspar or other phase equilibria can constrain conditions. Calibrations come from experimental petrology and thermodynamic models, sometimes supplemented by activity-composition relationships.

Data collection uses electron microprobe or LA-ICP-MS to determine major and trace element composition and, in metamorphic rocks, oxidation state. Uncertainties arise from disequilibrium, metamorphic overprinting, zoning, and model limitations; results are best interpreted as P-T histories rather than single fixed values. Thermobarometry is applied in studying igneous crystallization, metamorphism, tectonics, and planetary geology.