Geothermobarometry

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The geothermobarometry uses different geochemically - petrological methods ( geothermobarometers ) for determining the forming temperature [T] and the formation pressure [P] of a rock . The temperature and pressure dependence of the distribution coefficients of one or more chemical elements between two or more minerals are used. Formation temperature and formation pressure are usually determined separately using different methods ( geobarometer / geothermometer ) and the results are brought together.

The simplest method for determining the PT conditions of a rock is offered by the presence of certain minerals. Phase transitions, such as aluminosilicates (Al 2 SiO 5 ) and SiO 2 polymorphs, can already be observed on the microscope and thus enable rapid assignment to specific pressure and temperature regimes.

Pressure and temperature can also be determined by looking at the reactions taking place in the rock. There are three important categories to be distinguished:

  1. Solvus - thermometry
  2. Exchange reactions
  3. Net transfer reactions

Exchange reactions are based on the exchange of certain elements between two minerals that are not used up. In the case of net transfer reactions, one of the phases involved is dismantled or “used up” and a new one is formed. Solvus thermometers look at the distribution of certain elements in segregated, coexisting phases. A classic Solvus thermometer is the calcite - dolomite thermometer (distribution of Mg). In addition, semi-quantitative methods are also used.

Most geothermometers are based on exchange reactions. These can to a certain extent be reduced to a single exchange vector. A typical example is the exchange (FeMg) x = (FeMg) y. This reaction is minimally pressure-dependent and can subsequently be used for a whole series of thermobarometric calculations. A typical example of metabasites, such as those found in the Pflersch Valley, is the exchange pair garnet - amphibole . The reaction can be summarized here in (FeMg) in garnet ← → (FeMg) in amphibole.

Many geobarometers are based on net transfer reactions. These are characterized by large changes in volume (ΔV) and are therefore very sensitive to pressure. The garnet-plagioclase-hornblende-quartz geobarometer is a typical representative of this category and is well suited for geobarometric questions.

Geothermometer

Important geothermometers for determining the formation temperature of rocks are u. a. following methods:

Garnet clinopyroxene replacement thermometer

The basis for this geothermometer is the temperature-dependent exchange equilibrium between Fe 2+ and Mg.

Pyrope + 3 hedenbergite <=> almandine + 3 diopside

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 experimentally calibrated by the authors Krogh & Raheim (1978) and Green & Hellman (1982). The basis is the exchange equilibrium:

Pyrope + FeAl celadonite = almandine + MgAl celadonite

The formula for calculating the temperature is according to Krogh & Raheim (1978) T [° K] = (3685 + 77.1 * P [kbar]) / (lnK D + 3.52)

Garnet-Biotite Thermometer

This process is also based on the Fe 2+ -Mg exchange between biotite and garnet.

This fluid-independent geothermometer is based on the Fe 2+ -Mg exchange between garnet and biotite. The reaction that describes the cation exchange between the two minerals can be written down as follows:

Almandine + Phlogopite = Pyrope + Annite
(FeMgGrt = FeMgBt)

Fundamental problems with the use of this geothermometer arise from the incorporation of Fe 3+ in biotite. Further corrections are required by incorporating Ti in biotite and Ca and Mn in garnet.

Garnet-hornblende thermometer

This geothermometer was empirically calibrated by Graham & Powell (1984) and is based on the exchange equilibrium:

Fe pargasite + pyrope = pargasite + almandine
KD = (XFe Gt * XMg Hbl ) / (XMg Gt * XFe Hbl )

The authors recommend using this geothermometer only at T <850 ° C and at an XMn Gt <0.1.

Hornblende plagioclase thermometer

This method is based on the Edenite exchange and the plagioclase exchange between hornblende and plagioclase.

This geothermometer, which is very suitable for garnet amphibolites, was developed by Holland & Blundy (1994). The basis is formed by two exchange reactions between hornblende and plagioclase:

(1) Albite + tremolite = edenite + quartz (edenite - tremolite thermometer)
(2) edenite + albite = Richterite + anorthite (edenite - Richterite thermometer)

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 as a geothermometer in igneous rocks (OTTEN, 1984) because it is strongly temperature-dependent. The Ti content decreases as the temperature drops. The formula is used for the calculations

ln (Ti [apfu]) = (2603 / T) - 1.70

used.

Zr in rutile content

Rutile is an important carrier of HFSE (high field strength elements) such as Zr, Hf, Ta, etc. The temperature dependence of the Zr content in rutile was determined by Zack et al. (2004) and Watson et al. (2006) examined and empirically calibrated. The authors were able to show that the incorporation of the element Zr in rutile, in the presence of the "buffer" quartz + zirconium, is strongly temperature-dependent but only marginally dependent on pressure.

The calibration according to Zack et al. (2004) reads:
T [° C] = 127.8 * Ln (Zr [ppm] - 10)

The calibration according to Watson et al. (2006) reads:
Log (Zr [ppm]) = (7.36 ± 0.10) - ((4470 ± 120) / (T [K]))

Zr in the titanite content

Titanium and zircon are widely used accessories in metamorphic rocks of various origins and compositions. Their basic building blocks, Zr and Ti, are interchangeable up to a certain point. The temperature dependency of the Zr content in titanite was determined by Hayden et al. (2007) investigated and experimentally calibrated.

The calibration is: T [° C] = [7708 + 960 * P] / [10.52 - log (aSiO 2 ) - log (aTiO 2 ) - log (Zr titanite )] - 273

Geobarometer

The common geobarometers for determining the pressure [P] at the time the rock was formed are as follows:

Garnet amphibole plagioclase quartz barometer

The following net transfer reactions form the basis for this geobarometer (Kohn & Spear, 1990):

(1) anorthite + tremolite = grossular + pyrope + chermakite + quartz
(2) anorthite + Fe-actinolite = grossular + almandine + Fe-chermakite + quartz

The pressure for reaction (1) is calculated using this equation:
P [bar] = 79507 + T [K] (29.14 + 8.3144 InKeq) /10.988

The following equation applies to reaction (2):
P [bar] = 35327 + T [K] (56.09 + 8.3144 InKeq) /10.906

Plagioclase hornblende barometer

GRAIL

This geobarometer is ideal for granulite facial metapelites and is based on the composition of the minerals garnet, rutile, aluminosilicate (Al 2 SiO 5 ), ilmenite and quartz.

GRIPS

Based on the equilibrium CaFe 2 Al 2 Si 3 O 12 + TiO 2 = 2 FeTiO 3 + CaAlSi 2 O 8 + SiO 2

CFSP

This important geobarometer is based on the phases garnet, aluminosilicate, plagioclase and quartz.

literature

  • Egon Bernabè: Petrological and thermobarometric investigations on the Pflersch metabasite complex (Pflerschtal, South Tyrol - Italy). University of Innsbruck, 2009