Example of a geothermobarometer:

The reaction cordierite = garnet + sillimanite + quartz +

Rocks with the stable assemblage cordierite + garnet + sillimanite + quartz (± biotite) are very widespread in metamorphic series belonging to the granulite or amphibolite facies.

These constitute the kinzigitic series which sometimes crop out over several kilometres of thickness. This mineral assemblage, therefore, remains stable over a wide range of pres-sure and temperature, and as such fulfills the definition of a divariant equilibrium. The reaction is written:

3 cordierite = 2 garnet + 4 sillimanite + 5 quartz + (10)

This reaction only takes into consideration the ferromagnesian garnets (almandine and pyrope) to the exclusion of the calcic molecule, grossularite. The value of n which mea-sures the quantity of water involved in the reaction is open to discussion. This reaction theoretically extends into the kyanite stability field, but kyanite-cordierite stable assem-blages are unknown in nature. The density of cordierite (2.6 to 2.7) is low compared to that of the products of the reaction, and if water is not taken into consideration, the reaction results in a difference in molar volume on the order of -20% (cf. data in Table 2). This characteristic indicates the great barometric potential of the reaction which has been the subject of numerous theoretical and experimental investigations. The behaviour of water

Evaluation of metamorphic conditions 79

in cordierite is not quite clear, water quantities are variable, and its exact structural location in the crystal lattice is disputed. There are, therefore, some difficulties in the thermody-namic interpretation of reaction (1), which limits its use somewhat.

3.9.1 The cordierite-garnet barometer

The cordierite and garnet which coexist in reaction (1) do not have the same Mg/Fe ratio, at equilibrium cordierite always has a greater XMg than that of garnet. Reaction (1) is therefore a divariant reaction and can be treated in exactly the same way as the biotite-garnet reaction examined above. But here, because of the low entropy and high volume difference, pressure is the most significant parameter, and not temperature. The equilibrium is therefore examined in a PX diagram at constant temperature (Fig. 50). In this diagram there is a vast divariant field extending over nearly 9 kb, over which the cordierite and garnet compositions measure the advancement of the reaction as a function of pressure. A series of G-X diagrams analogous to those of Figure 47 (but at constant T and increasing P) would account for the evolution of the reaction (1) in the same way. Starting from the values of calculated from experimental and thermodynamic data, the pressure of a crd-grt assemblage in equilibrium with sillimanite and quartz may be evalu-ated between 4 and 8 kb, if the crystallization temperature is known (Fig. 50).

3.9.2 The independent geothermometer crd-grt

As in the case of the association biotite-garnet, cordierite and garnet exchange iron and magnesium as a function of the reaction:

almandine Mg-cordierite pyrope Fe-cordierite

An analogous procedure to that which was developed for garnet-biotite equilibrium leads to a linear relation between and the temperature (Fig 49b). The experimen-tal curve is much less well constrained than the preceding case, and values obtained from this relation should be viewed with caution. Several thermodynamic equations have never-theless been proposed:

where

The equations for the thermodynamic curves were established from a clearly defined expression of and naturally care must be taken not to introduce parameters into the calculation of that do not correspond to those that were used to define the equation.

The value may be used directly to complete the grid of Figure 50, which gives, with some errors (these may be important) the position of the assemblage crd + grt + sil + qtz in the P-T diagram, using the composition of garnet and cordierite in equilibrium.

3.10 “Automatic” geothermobarometry

A large number of reactions with a geothermometric and geobarometric potential have been studied theoretically and experimentally. Some, among the most utilized, are pre-sented briefly in the appendix, either in the form of thermobarometric equations, or in graphical form. It becomes difficult to manipulate all these reactions at the same time, and a computer programme becomes important in handling all the data. Several programmes have been developed, which reproduce all the reactions observed within the same meta-morphic series graphically, as a function of the composition of the phases. The crystalliza-tion condicrystalliza-tions are supposed to correspond to the best interseccrystalliza-tion of different reaccrystalliza-tion curves. Figure 51 gives an example of this type of automatic treatment.

GEOTHERMOBAROMETRY OF FLUID INCLUSIONS

The minerals of a metamorphic rock often contain fluid inclusions, microscopic cavities (sometimes in negative crystal form) filled with a mixture of liquid, gas and even solid phases (Fig. 52a). With certain restrictions, the content of the inclusion is considered as representative of the interstitial fluid phase which was present in the system at the time of metamorphic recrystallization, and which was trapped by the crystals during their growth.

This hypothesis is only acceptable if the trapping occurred in a reservoir (mineral cavity)

Evaluation of metamorphic conditions 81

both impermeable and inert, not having reacted subsequently with the imprisoned fluid.

Specialists claim that quartz crystals have these qualities and their inclusions allow an effec-tive determination of the composition of the interstitial fluid present during recrystalliza-tion.