DISENTANGLING KINETICS AND THERMODYNAMICS IN THE INTERPRETATION OF METAMORPHIC MINERAL ASSEMBLAGES AND TEXTURES

There is increasing acceptance in the metamorphic community that kinetic impediments to reaction progress during prograde metamorphism can no longer be ignored in the interpretation of metamorphic mineral assemblages and textures. Discerning departures from equilibrium relies on a solid understanding of equilibrium phase relations. There is a worryingly widely held view that current thermodynamic databases and a-x models are sufficiently advanced that we can predict with confidence natural metamorphic phase equilibria for a wide range of bulk compositions. The reality is that current thermodynamic databases and a-x models are works in progress that still have a ways to go before they can be considered to accurately reproduce a number of widely observed modal and compositional features of natural metamorphic rocks. Examples from metapelites and metabasites will be given. An initiative that may aid in the future refinement of thermodynamic data and a-x models is a publically-available database of robust, repeatable modal and compositional features of natural metamorphic rocks against which the results of future models can be tested. Complicating the above approach are the effects of kinetic impediments to reaction progress, which open the possibility of reaction overstepping, progress of metastable reactions, lack of or partial equilibration amongst minerals, and episodic vs. continuous reaction, none of which can be accounted for by equilibrium phase diagrams. Kinetic effects are anticipated to vary from setting to setting, potentially allowing their discernment from more global patterns that, by their repeatability, must be governed by equilibrium phase relations. Disentangling the effects of kinetics and the uncertainties in thermodynamically-predicted phase equilibria modelling in the interpretation of metamorphic rocks makes for a daunting challenge, but one that should be embraced as the new reality.