Geothermobarometry
Encyclopedia
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.
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 an electron microprobe
or 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.
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 and Zr-in-rutile thermometers are solely empirical and not well understood thermodynamically.
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.
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
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.
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 equilibriumChemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
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
Fugacity
In chemical thermodynamics, the fugacity of a real gas is an effective pressure which replaces the true mechanical pressure in accurate chemical equilibrium calculations. It is equal to the pressure of an ideal gas which has the same chemical potential as the real gas. For example, nitrogen gas ...
and water activity (roughly, the same as concentration). The distribution of component elements between the mineral assemblages is then analysed using an electron microprobe
Electron microprobe
An electron microprobe , also known as an electron probe microanalyzer or electron micro probe analyzer , is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials...
or scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
(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.
Geothermometers
- Ti saturation content of biotiteBiotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
mica. - Fe-Mg exchange between garnetGarnetThe garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
-biotite and garnet-amphiboleAmphiboleAmphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
. - Mg-Fe systematics in pigeonites and augites
- Zr content of rutileRutileRutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
, effective for higher temperatures than the Ti-in-biotite thermometer. Requires quartz, rutile, and zirconZirconZircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
to be equilibrated.
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 and Zr-in-rutile thermometers are solely empirical and not well understood thermodynamically.
Geobarometers
- GASP; an acronym for the assemblage garnet-(Al2SiO5)-silica(quartzQuartzQuartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
)-plagioclasePlagioclasePlagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series... - GPMB; an acronym for the assemblage garnet-plagioclase-muscoviteMuscoviteMuscovite is a phyllosilicate mineral of aluminium and potassium with formula KAl22, or 236. It has a highly-perfect basal cleavage yielding remarkably-thin laminæ which are often highly elastic...
-biotite - Garnet-Plagioclase-Hornblende-Quartz.
- Hornblende
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.