A Natural Example of Metastable Reactions Involving Garnet and Sillimanite

• Published in: Journal of petrology 1979-05, Vol.20 (2), p.271-292
• Main Author: LOOMIS, T. P.
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/20.2.271

The broad objective of this work is to develop models of reaction that can be used to understand dynamic processes. The kinetics of reactions can be significant factors affecting heat and mass transfer, and the kinetic inhibition of equilibration enables us to trace the history of natural samples. Samples were selected from a metamorphic aureole for study because they show measurable disequilibrium, and the approximate P—T path and time of metamorphism are known. Detailed analysis of compositional variations of crystals and reaction textures suggests that biotite was being produced by the breakdown of garnet at a faster rate than it was consumed by reaction with sillimanite. Some of the compositional variations of biotite and cordierite can be explained by the process of intergranular diffusion; however, the co-existence of metastable biotite and sillimanite and the widespread distribution of Fe-rich biotite in the sample argue for a partial equilibrium model for which reaction rate was controlled by surface kinetic processes. Quantitative modelling was accomplished by breaking the overall reaction into three component reactions. The rate of one of these component reactions through time can be estimated from garnet zoning profiles, and the relative abundance of product phases is used then to estimate the absolute rates of the other component reactions. A plausible interpretation of the data is that rate of dissolution of sillimanite was the rate-controlling process, and took place at a rate of 10−7 to 10−8 cm/year in this sample. Analysis of this sample illustrates possible significant effects of the kinetics of reaction in dry rocks in the deep crust. Even though biotite would have been unstable at final equilibrium and the overall reaction did not involve significant gain or loss of water, the particular kinetic path under taken resulted in production of biotite and consumption of water at the stage of reaction preserved in this sample.