Geothermobarometry

Geothermobarometry is a geochemical and petrological method for determining the formation temperature and formation pressure of a rock. It makes use of the temperature and pressure dependence of the distribution coefficients of one or more chemical elements between two or more minerals exploited.

The simplest method to determine the PT conditions of the rock afforded by the presence of certain minerals. Phase transitions, for example, the alumino-silicates ( Al2SiO5 ) and SiO2 polymorphs, can already be observed under the microscope, thus allowing a rapid assignment to specific pressure and temperature regime.

Pressure and temperature can also be determined by considering the running reactions in the rock. It must have three important categories are distinguished:

Exchange reactions are based on the exchange of certain elements between two minerals that are not depleted. At Net - transfer reactions one of the phases involved is degraded or "exhausted " and formed a new one. Solvus thermometer consider the distribution of certain elements in the segregated, coexisting phases. A classic solvus thermometer is the calcite - dolomite thermometer (distribution of Mg). Also semi-quantitative methods are used supplement.

Most geothermometer based on exchange reactions. This can be to some extent reduced to a single exchange vector. A typical example is the exchange ( FeMg ) x = ( FeMg ) y. This reaction is minimal pressure dependent and can be subsequently used for a whole range of thermobarometrischen calculations. A typical example of metabasites, as they occur in Pflerschtal for example, the exchange pair garnet - amphibole. The reaction can be summarized here in amphibole ( FeMg ) in the garnet ← → ( FeMg ).

Many Geobarometer based on Net- transfer reactions. These are characterized by small volume changes (? V ) and thus very sensitive to pressure. The Geobarometer garnet - plagioclase - hornblende - quartz is typical representative of this category and well suited for geobarometrische issues.

  • 2.1 garnet - amphibole - plagioclase - quartz barometer
  • 2.2 plagioclase - hornblende barometer
  • 2.3 GRAIL
  • 2.4 GRIPS
  • 2.5 CFSP

Geothermometer

Important geothermometer to determine the formation temperature of the rocks are uA the following methods:

Garnet - clinopyroxene exchange thermometer

The basis for this geothermometer temperature dependent exchange equilibrium between Fe2 and Mg

This determination method is mainly used for metamorphic rocks.

Garnet - phengite thermometer

This geothermometer is based on the exchange equilibrium between Fe and Mg and was developed by the authors Krogh & Raheim ( 1978) and Green & Hellman (1982 ) calibrated experimentally. It is based on the exchange equilibrium:

The formula for the calculation of the temperature is by Krogh & Raheim (1978 ) T [° K] = ( 3685 77.1 * P [ kbar ] ) / ( LNKD 3.52 )

Garnet - biotite thermometer

Also, this method is based on the Fe2 - Mg exchange between biotite and garnet.

Garnet - hornblende thermometer

The geothermometer is based on the exchange equilibrium

Hornblende - plagioclase thermometer

The basis of this method of Edenit exchange and the exchange between plagioclase - hornblende and plagioclase.

Amphibole - plagioclase thermometer

Based on a net - transfer reaction between amphibole and plagioclase ( NaSi ↔ CaAl ).

Ti content in amphiboles

The Ti content in amphiboles can be used in igneous rocks as a geothermometer ( Otten, 1984), because this is strongly temperature dependent. With decreasing temperature, the Ti content decreases. For the calculations, the formula

Used

Zr in rutile content

Rutile is an important carrier of HFSE (high field strenght elements ) such as Zr, Hf, Ta ​​and Nb. The temperature dependence of the Zr content is in the rutile by Zack et al. (2004) and Watson et al. (2006) investigated and calibrated empirically.

Geobarometer

The common Geobarometer for determining the pressure at the time of formation of the rock, the following:

Garnet - amphibole - plagioclase - quartz barometer

Plagioclase - hornblende barometer

GRAIL

GRIPS

CFSP

  • Geochemistry
  • Petrology
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