Geothermobarometry

There are numerous extra factors to consider such as oxygen fugacity and water activity (roughly, the same as concentration) that must be accounted for using the appropriate methodological and analytical approach (e.g. Mössbauer spectroscopy, micro-raman spectroscopy, infrared spectroscopy etc...) Geobarometers are typically net-transfer reactions, which are sensitive to pressure but have little change with temperature, such as garnet-plagioclase-muscovite-biotite reaction that involves a significant volume reduction upon high pressure:[1] Since mineral assemblages at equilibrium are dependent on pressures and temperatures, by measuring the composition of the coexisting minerals, together with using suitable activity models, the P-T conditions experienced by the rock can be determined.

Upon exhumation and cooling, contrasting compressibilities and thermal expansivities induce differential strains (volume mismatches) between a host crystal and its inclusions.

Data on the geothermometers and geobarometers is derived from both laboratory studies on synthetic (artificial) mineral assemblages and from natural systems for which other constraints are available.

In natural systems, the chemical reactions occur in open systems with unknown geological and chemical histories, and application of geothermobarometers relies on several assumptions that must hold in order for the laboratory data and natural compositions to relate in a valid fashion: In natural systems elastic behaviour of minerals can be easily perturbed by high temperature re-equilibration, plastic or brittle deformation, leading to an irreversible change beyond the elastic regime that will prevent reconstructing the "elastic history" of the pair.

Some techniques include: Note that the Fe-Mg exchange thermometers are empirical (laboratory tested and calibrated) as well as calculated based on a theoretical thermodynamic understanding of the components and phases involved.