Geothermobarometry

Geothermobarometry is the science of estimating the temperature and pressure conditions under which rocks formed or equilibrated, using mineral chemistry and phase equilibria. It combines thermodynamic modeling with empirical calibrations to interpret mineral assemblages and compositions in metamorphic rocks. Geothermometry aims to determine temperature, while geobarometry targets pressure; in practice, many studies estimate both by applying multiple independent scales and joint constraints.

Geothermometry relies on temperature-sensitive chemical exchanges and activities in minerals. Commonly used is garnet–biotite Fe–Mg exchange thermometry, which yields temperatures from the partitioning of Fe and Mg between garnet and biotite. Other examples include garnet–orthopyroxene Fe–Mg exchange and Ti-in-quartz thermometry, which uses titanium content in quartz as a thermometer. Geobarometry uses mineral- and phase-equilibrium relationships that vary with pressure, such as phengite barometry based on silicon content, and assemblage-based barometry from garnet–pyroxene and amphibole–plagioclase reactions. In many studies, geothermometry and geobarometry are combined with thermodynamic modeling and phase diagrams or pseudosections (for example, THERMOCALC) to locate P–T conditions on a two- or three-dimensional space.

Calibration and data treatment: Estimates depend on experimental and natural calibrations, assumed activities, and the chemical evolution of rocks. Observed mineral zoning, fluid influence, and retrogression can bias results; cross-checks with independent gauges improve reliability.

Applications and limitations: Geothermobarometry is central to metamorphic petrology, tectonics, and crustal evolution, providing insights into burial, subduction, exhumation, and orogenic processes. Limitations include equilibration issues, reaction overspecification, and uncertainties in calibration.