classical thermobarometry

Figure 1 . P hotomic rograph of a blueschis t fromSivrihisar, Turkey, containing glaucophane(blue), garnet (pink), and lawsonite (white).Field of view = 4 mm.

Integrating Research and Education > Teaching Phase Equilibria > "Classical" Thermobarometry

"Classical" Thermobarometry

Donna Whitney, University of Minnesota

Thermobarometry is the quantitative determination of the temperature and pressure at whicha metamorphic or igneous rock reached chemical equilibrium. The term "classical"thermobarometry refers to methods for calculating the P-T conditions of a specific chemicalreaction.

Why do we need to know pressures and temperatures of petrologicevents?

If you know the pressure (P) and temperature (T) atwhich an igneous or metamorphic rock equilibrated, youcan determine a lot about where in the Earth the rockformed and how the rock formed. For example, sometectonic processes are associated with characteristicpressures and temperatures: e.g., low temperatures athigh pressure occur only in subduction zones.

How do we determine (paleo) pressuresand temperatures?

Mineral assemblages and textures may provideinformation about the conditions at which a rockequilibrated. In metamorphic rocks, we use qualitative

terms such as low-grade, medium-grade, and high-grade (and even 'medium-low' or 'very highgrade') to describe the approximate temperature conditions of metamorphism. Thesedesignations do not provide information about pressure, and are therefore not useful fordescribing subduction zone rocks.

Other methods for characterizing metamorphic conditions include:

(1) index minerals : characteristic minerals that provide an indication of the temperature (and, insome cases, pressure) conditions at which a rock formed (e.g., kyanite in metamorphosedshale; magmatic epidote in plutons and volcanic rocks). Not all rocks have a suitable bulkcomposition to produce index minerals.

(2) metamorphic facies : assemblages of minerals, each characteristic for a particular bulkcomposition and indicating the range of pressure-temperature conditions at which the rockequilibrated (Figure 2). For example, high-pressure - low-temperature conditions characterizethe blueschist facies.

Teaching Phase Equilibria4/23/2011 "Classical" Thermobarometry

…carleton.edu/…/classicalthermobaro… 1/4

Figure 2 . P ressure-temperature diagram showing the locations

of the metamorphic fac ies . Modified from Spear (1993). The

A l2SiO 5 phase diagram is also shown for reference. A nd =

andalus ite; Ky = kyanite; Sil = s illimanite.

Some igneous assemblages are alsocharacteristic of crystallization withinparticular ranges of temperature/pressureconditions.

Are qualitative methods fordetermining P-T conditions 'goodenough'?

Qualitative methods don't necessarilyprovide information about both pressureand temperature. In addition, qualitativetechniques in some cases give only aminimum pressure or temperature: e.g.,the presence of a high-T index mineral maynot tell you whether the rock formed at700°C or 1000°C, and the occurrence of

eclogite facies minerals may not tell you whether the rock formed at a depth of 40 km or 75 km.This degree of uncertainty makes the interpretation of metamorphic and tectonic processesdifficult.

Thermobarometry

Conceptual basis: The compositions of coexisting minerals in equilibrium are related by thethermodynamic properties of the minerals to the pressure and temperature conditions ofequilibration. Thermometers are assemblages that form by reactions that are sensitive totemperature but not (much) to pressure. Barometers are assemblages that are sensitive topressure but not (much) to temperatures. Not all rocks contain assemblages that make suitablethermometers or barometers.

To apply a thermometer or barometer (HTML File 7kB Mar31 07) to an equilibrium mineralassemblage, you need to know:

1. The compositions of all minerals that can have variable composition (e.g., solidsolutions).

2. Thermodynamic data for the phases of interest.

3. A 'calibration' that allows you to relate mineral composition to pressure ortemperature.

Thermometry

Exchange reactions. Exchange reactions involve the exchange of cations with similar sizes andcharges between two minerals. These reactions are typically very temperature-sensitive, butare not pressure-sensitive because no major change in volume is involved in the exchange.Temperature-sensitive reactions have steep slopes on P-T diagrams (Figure 3a). The most

common exchange thermometers involve exchange of Fe2+ and Mg between two minerals, e.g.,between garnet and biotite.

Solvus thermometry involves phases that form a solid solution at high-T but that 'unmix' intoseparate phases during cooling. The composition of coexisting minerals that have a solvusrelationship is an indicator of temperature (Figure 3b), although in some cases this may be atemperature representing part of the cooling path of a rock. Examples: muscovite-paragonite;calcite-dolomite; orthopyroxene-clinopyroxene; feldspars.

Figure 3 .

4/23/2011 "Classical" Thermobarometry


Schematic P -T

(a) and T -X

(solvus) (b)

diagrams

illus trating

general

charac teris tic s

of

thermometers

and

barometers .

Thermometers

have s teep

s lopes and

barometers

have shallow s lopes on a P -T diagram.

Other : The composition of coexisting plagioclase and calcic amphibole (e.g., hornblende) can beused to estimate temperature. Other thermometers involve trace element concentrations ofphases. For example, the concentrations of Ti in quartz and zircon in equilibrium with rutile(TiO2), and the concentration of Zr in rutile are very sensitive to temperature. These

thermometers can be applied to igneous and metamorphic rocks, but require an ion microprobefor analysis of trace concentrations (ppm) of Ti and Zr. Another trace element thermometer withapplications to metamorphic rocks involves the yttrium concentration of coexisting monazite andgarnet (or monazite and xenotime).

The concentration of stable isotopes of oxygen and carbon in coexisting minerals can also beused to determine the temperature at which the system became closed to exchange of these

isotopes. For example, the O18/O16 ratio in coexisting quartz and magnetite is a thermometer,

as is the C13/C12 ratio in coexisting quartz and calcite or calcite and graphite.

Barometry

Pressure-sensitive reactions (barometers) involve a significant volume change, such as thetransformation of anorthite (Ca-plagioclase) to grossular (Ca-garnet) + kyanite + quartz. Manybarometers are net transfer reactions, and have shallow slopes on P-T diagrams (Figure 3a).Other barometers involve the concentration of an element in a certain mineral in equilibriumwith a particular assemblage (e.g., Al in hornblende, Si in phengite).

Uncertainties

Uncertainties in thermobarometric calculations can be determined by propagating errors from allstages of the P-T determinations: the mineral composition analyses, the thermodynamic data,and the calibration method. Typical uncertainties are ± 50°C, ± 1 kbar.

Sources of error in thermobarometry

Common sources of error in thermobarometric calculations include:

The assumption that the minerals record equilibrium conditions may not be valid.

The choice of mineral compositions to use in thermobarometric calculations is easy ifall phases are homogeneous, but if minerals are zoned, the selection of compositionsto use can be more uncertain.

The stability of minerals may be affected by the presence of elements that are difficultto analyze with standard techniques. For example, electron microprobe analyses do

not distinguish between Fe2+ and Fe3+, but the difference is important. Athermobarometric calculation may be substantially in error if all Fe is assumed to be

Fe2+ for minerals that contain substantial Fe3+.



Thermobarometric calculations rely on thermodynamic data and a 'calibration' thatallows mineral composition to be related to temperature or pressure. The mineralcompositions and crystal structures in the rock being analyzed should not be toodifferent from the compositions and structures on which the calibrations are based.

Temperature calculations require an estimation of pressure, and pressure calculationsrequire an estimation of pressure. You therefore either need to know one variable, oruse a thermometer or barometer that is very insensitive to the other variable. Hence,the best thermometers have nearly vertical slopes on a P-T diagram and the bestbarometers have nearly horizontal slopes.

What do calculated P-T conditions mean?

For igneous rocks, calculated pressures and temperatures likely represent the conditions atcrystallization, particularly for rapidly cooled rocks. Metamorphic rocks have more complexthermal and pressure histories, but the most typical interpretation of thermobarometric resultsis that the calculated P and T represent the conditions at the thermal maximum (peak ofmetamorphism) (Figure 4). It is important to recognize that the thermal maximum may notrepresent the pressure maximum (Figure 4). Furthermore, it is important to examine thetextures of a metamorphic rock and determine if the mineral assemblages/textures areconsistent with the interpretation that peak metamorphic conditions are preserved and havenot been overprinted during later events.

Figure 4 . A common pressure-temperature path for

regional metamorphism. The firs t s tages of

metamorphism involve burial and heating. Following the

attainment of maximum pressure (depth), rocks --

which are poor conduc tors of heat and therefore are

s low to heat relative to tec tonic rates of burial -- may

reach their maximum temperature conditions during the

initial s tages of decompress ion. Therefore, the peak of

metamorphism (thermal maximum) may not coinc ide

with the depth maximum.

Resources

Thermobarometry (more info)

Dave Waters' on-line tutorial on PracticalThermobarometry

Problem Set: Calculating Pressures andTemperatures of Petrologic Events:

Geothermobarometry - Donna Whitney, University of Minnesota

Thermobarometry Problem Set - Jane Selverstone, University of New Mexico

Thermodynamic Calculation of Mineral Reactions II Lab (M ic rosoft Word 48kB Mar29 07) -This Excel-based one week exercise, provided by Dave Pattison at the University ofCalgary, includes problems sets involving equilibrium constants, activities andcalculation of thermodynamic equilibria involving impure phases, and 'conventional'thermobarometry using the GTB program.

Multi-equilibrium Thermobarometry Lab (M ic rosoft Word 53kB Mar29 07) - This Excel-basedone week exercise, provided by Dave Pattison at the University of Calgary, includesproblems sets involving multi-equilibrium thermobarometry using TWQ andThermoCalc's 'AvePT' module ('Optimal thermobarometry').



classical thermobarometry

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