MODELLING METAMORPHIC MINERAL ASSEMBLAGES AND PARTIAL MELTING

The mineral assemblage in a metamorphic rock evolves along a pressure--temperature (P-T) path, and may change its composition along the path, by for example addition or loss of fluid or melt. A unifying and simplifying petrological tenet, supported by substantial a posteriori evidence, is that a mineral assemblage equilibrates continuously on some scale along a P-T path while fluid or melt is present, but little or no change occurs while fluid or melt is absent. An equilibrium mineral assemblage will then tend to be preserved once fluid or melt is finally lost, this assemblage reflecting the pressure-temperature conditions experienced when that occurred.

This is a working model for understanding the mineral assemblages and textures preserved in rocks and is justified in part by the success metamorphic geologists have achieved in the last several decades by using it to establish the PT evolution of orogenic systems. Note that a rock may record different parts of its history on different scales, and rocks of different composition may record different parts of their common history. The challenge of petrology is to decode this record. The model applies equally well to, for example, the generation, evolution and crystallisation of magma, the development of hydrothermal alteration in ore systems, as to rocks metamorphosing in the crust.

With this model, the full force of equilibrium thermodynamics can be brought to bear on mineral assemblages in rocks. Forward modelling to see the consequences of envisaged processes can be undertaken, given a rock composition, a P-T path, and the way that the composition changes along the path. What can now be achieved in equilibrium modelling of metamorphic mineral assemblages, particularly using calculated phase diagrams, is outlined. What limits our ability to undertake this modelling is shortcomings in the extant thermodynamic descriptions of the minerals, silicate melt and fluids involved. Recent progress in formulating and calibrating the necessary thermodynamic descriptions is discussed.