Geothermobarometry
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Geothermobarometry is the science of measuring the previous pressure and temperature history of a metamorphic or intrusive igneous rocks. Geothermobaromatry is a combination of geobarometry, where a pressure of mineral formation is resolved, and geothermometry where a temperature of formation is resolved.
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[edit] The method
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 inddividual minerals.
These methods rely on the fact that, for a given mineral for example garnet, within metmorphic rocks, the composition of the garnet changes both with temperature and with pressure, and may also be influenced by growth in contactwith other mierals, in this case biotite.
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 with natural systems.
For example, one of the best known and most widely applicable geothermometers is the garnet-biotite relationship where the proportion of Fe and Mg in garnet and biotite change with increasing temperature, given some assumptions.
[edit] Assumptions
Generally, geothermobaromatry relies on measuring natural systems where the chemical reactions occur in open systems with unknown geological and chemical histories, and rely on several assumptions in order to relate laboratory data with what is actually observed.
- That chemical equilibrium was achieved. This could be impossible to prove definitively.
- 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 under an optical microscope in most situations
- That certain mineralogical assemblages are present. Without these, the accuracy of a reading may be altered from an ideal, and theremay be more error inherent in the measurement.
[edit] Example geothermometers
- Ti-in biotite geothermometer, Henry et al. 2005
- Fe-Mg exchange between garnet-biotite
- Ca-in-garnet
[edit] See also
[edit] 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.
