MECHANICAL AND CHEMICAL EQUILIBRIUM IN HIGH-GRADE METAMORPHIC ROCKS: TOWARDS AN UNCONVENTIONAL BAROMETRY

Recent work on phase precipitation and corona textures in high-grade metamorphic rocks has focused on the phase equilibria modelling and description of chemical potential relationships. Further important phenomenon in metamorphic rock studies is to realize that mineral reaction cannot take place independently of the mechanical state of its immediate environment which may result in the development of pressure variations even on a hand specimen or thin section scale. This is critical for understanding the microstructural observations that reflect mineral reaction in rocks. We integrate consideration of the effect of pressure variation and diffusional relaxation during metamorphic mineral reactions in high-grade rocks. In the process, we discuss well-known models for metamorphic microstructures such as the preservation of coesite as inclusions in garnet, and the development of exsolution lamellae or reaction rims offering simple alternatives for some of them. In particular, we focus on accounting for the preservation of chemical zoning in minerals at high temperatures (>750ºC) under conditions where diffusional equilibration might be expected to be rapid. As a consequence, we can throw new light on how mineral assemblages, mineral compositions and microstructures may be preserved during higher temperature metamorphism. The suggested equilibrium approach can be used as an “unconventional” barometer. The method is based on a simple combination of Newton’s law on balanced forces, which is responsible for the development of pressure variations within a grain, and equilibrium thermodynamics, via equalisation of chemical potentials. The suggested approach avoids invoking kinetic factors for preserved mineral composition zoning.