Geothermobarometry

Geothermobarometry is the science of measuring the previous pressure and temperature history of a metamorphic or intrusive igneous rocks. Geothermobarometry is a combination of "geobarometry", where a pressure of mineral formation is resolved, and "geothermometry" where a temperature of formation is resolved.

The Methodology

Geothermobarometry relies upon understanding the temperature of formation of minerals within metamorphic and igneous rocks, and is particularly useful in metamorphic rocks. There are several methods of measuring the temperature or pressure of mineral formation relying on chemical equilibrium between metamorphic minerals or by measuring the chemical composition of individual minerals.

Thermobarometry relies upon the fact that mineral pairs/assemblages vary their compositions as a function of temperature and pressure. There are numerous extra factors to consider such as oxygen fugacity and water activity (roughly, the same as concentration). The distribution of component elements between the mineral assemblages is then analysed using a Scanning Electron Microscope (SEM).

Data on the geothermometers and geobarometers is derived from both laboratory studies on artificial mineral assemblages, where minerals are grown at known temperatures and pressures and the chemical equilibrium measured directly, and from calibration using natural systems.

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 crystallisation to be calculated, given some assumptions.

Assumptions

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:

* That the full mineralogical assemblage required for the thermobarometer is present. If not all of the minerals of the reaction are present, or did not equilibrate with each other simulatenously, then any pressures and temperatures calculated for the ideal reaction will deviate from those actually experienced by the rock.
* That chemical equilibrium was achieved to a satisfactory degree. This could be impossible to demonstrate definitively, if the minerals of the thermobarometer assemblage are not all observed in contact with each other.
* That any minerals in a two-mineral barometer or thermometer grew in equilibrium, which is assumed when the minerals are seen to be in contact.
* That the mineral assemblage has not been altered by retrograde metamorphism, which can be assessed using an optical microscope in most cases.
* That certain mineralogical assemblages are present. Without these, the accuracy of a reading may be altered from an ideal, and there may be more error inherent in the measurement.

Example Geothermometers

* Ti saturation content of Biotite mica [http://www.geol.lsu.edu/henry/Research/biotite/TiInBiotiteGeothermometer.htm Ti-in biotite geothermometer, Henry et al. 2005]
* Fe-Mg exchange between garnet-biotite

Example Geobarometers

* GASP; an acronym for the assemblage Garnet-(Al₂SiO₅)-Silica(Quartz)-Plagioclase
* GPMB; an acronym for the assemblage Garnet-Plagioclase-Muscovite-Biotite

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 palaeo-pressure.

ee also

* Garnet
* Biotite
* Metamorphism

References

* Henry, D. J., Guidotti, C. V. and Thomson, J. A. (2005) The Ti-saturation surface for low-to-medium pressure metapelitic biotite: Implications for Geothermometry and Ti-substitution Mechanisms. American Mineralogist, 90, 316-328.
* Guidotti, C. V., Cheney, J. T. and Henry, D. J. (1988) Compositional variation of biotite as a function of metamorphic reactions and mineral assemblage in the pelitic schists of western Maine: American Journal of Science-Wones Memorial Volume, v. 288A, 270-292.