Geothermobarometry is the methodology for estimating the pressure and temperature history of rocks (metamorphic, igneous or sedimentary). Geothermobarometry is a combination of geobarometry, where the pressure attained (and retained) by a mineral assemblage is estimated, and geothermometry where the temperature attained (and retained) by a mineral assemblage is estimated.

Methodology

Geothermobarometry relies upon understanding the temperature and pressure of the formation of minerals within rocks. There are several methods of measuring the temperature or pressure of mineral formation or re-equilibration relying for example on chemical equilibrium between minerals or by measuring the chemical composition and/or the crystal-chemical state of order of individual minerals or by measuring the residual stresses on solid inclusions or densities in fluid inclusions. "Classic" (thermodynamic) thermobarometry relies upon the attainment of thermodynamic equilibrium between mineral pairs/assemblages that vary their compositions as a function of temperature and pressure. The distribution of component elements between the mineral assemblages is then analysed using a variety of analytical techniques as for example electron microprobe (EM), scanning electron microscope (SEM), Mass Spectrometry (MS). 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: 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. As different equilibrium constants of mineral assemblages would occur as lines with different slopes in the P-T diagram, therefore, by finding the intersection of at least two lines in the P-T diagram, the P-T condition of the specimen can be obtained. Despite the usefulness of geothermobarometry, special attention should be paid to whether the mineral assemblages represent an equilibrium, any occurrence of retrograde equilibrium in the rock, and appropriateness of calibration of the results. Elastic thermobarometry is a method of determining the equilibrium pressure and temperature attained by the host mineral and its inclusion on the rock history from the excess pressures exhibited by mineral inclusions trapped inside host minerals. Upon exhumation and cooling, contrasting compressibilities and thermal expansivities induce differential strains (volume mismatches) between a host crystal and its inclusions. These strains can be quantified in situ using Raman spectroscopy or X-ray diffraction. Knowing equations of state and elastic properties of minerals, elastic thermobarometry inverts measured strains to calculate the pressure-temperature conditions under which the stress state was uniform in the host and inclusion. These are commonly interpreted to represent the conditions of inclusion entrapment or the last elastic equilibration of the pair. 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. For example, one of the best known and most widely applicable geothermometers is the garnet-biotite relationship where the relative proportions of Fe and Mg in garnet and biotite change with increasing temperature, so measurement of the compositions of these minerals to give the Fe-Mg distribution between them allows the temperature of crystallization to be calculated, given some assumptions.

Assumptions in thermodynamic thermobarometry

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:

Assumptions in elastic thermobarometry

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.

Techniques

Some techniques include:

Geothermometers

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. The Ti-in-biotite thermometer is solely empirical and not well understood thermodynamically.

Geobarometers

Various mineral assemblages rely more upon pressure than temperature; for example reactions which involve a large volume change. At high pressure, specific minerals assume lower volumes (therefore density increases, as the mass does not change) - it is these minerals which are good indicators of paleo-pressure.

Software

Software for "classic" thermobarometry includes: Software for elastic thermobarometry includes:

Clinopyroxene thermobarometry

The mineral clinopyroxene is used for temperature and pressure calculations of the magma that produced igneous rock containing this mineral.