A Natural Example of Metastable Reactions Involving Garnet and Sillimanite

The broad objective of this work is to develop models of reactionthat can be used to understand dynamic processes. The kineticsof reactions can be significant factors affecting heat and masstransfer, and the kinetic inhibition of equilibration enablesus to trace the history of natural samples. Samples were selectedfrom a metamorphic aureole for study because they show measurabledisequilibrium, and the approximate P—T path and timeof metamorphism are known. Detailed analysis of compositional variations of crystals andreaction textures suggests that biotite was being produced bythe breakdown of garnet at a faster rate than it was consumedby reaction with sillimanite. Some of the compositional variationsof biotite and cordierite can be explained by the process ofintergranular diffusion; however, the co-existence of metastablebiotite and sillimanite and the widespread distribution of Fe-richbiotite in the sample argue for a partial equilibrium modelfor which reaction rate was controlled by surface kinetic processes. Quantitative modelling was accomplished by breaking the overallreaction into three component reactions. The rate of one ofthese component reactions through time can be estimated fromgarnet zoning profiles, and the relative abundance of productphases is used then to estimate the absolute rates of the othercomponent reactions. A plausible interpretation of the datais that rate of dissolution of sillimanite was the rate-controllingprocess, and took place at a rate of 10^–7 to 10^–8cm/year in this sample. Analysis of this sample illustrates possible significant effectsof the kinetics of reaction in dry rocks in the deep crust.Even though biotite would have been unstable at final equilibriumand the overall reaction did not involve significant gain orloss of water, the particular kinetic path under taken resultedin production of biotite and consumption of water at the stageof reaction preserved in this sample.