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Abstract
Analysis of the processes, kinetics, and microstructures that characterize porphyroblast crystallization identifies the primary factors that govern the progress of metamorphic reactions and highlights the importance of feedbacks among those factors. Commonly, the kinetics of nucleation and the kinetics of intergranular diffusion are rate-limiting in porphyroblast crystallization. That finding should inspire petrologic vigilance, as it implies strong potential for significant thermal overstepping of reactions, crystallization at high levels of chemical affinity, reactions that span protracted intervals of time and temperature, and limited length scales for chemical equilibration.
Acknowledgements
The author is greatly indebted to Richard Ketcham for his vital contributions during their long-standing collaborative research efforts on several of the topics covered here, and as the list of cited references shows, the content of this review is also built upon the ideas and findings of many other colleagues and students, too numerous to list individually. To all of them the author is grateful for discussions, concepts, and interactions that have sustained his prolonged interest in the mechanisms and kinetics of metamorphic reactions. The National Science Foundation has supported this work over the years in a succession of much-valued grants: EAR-9118338, EAR-9417764, EAR-9902682, EAR-0635375. The author is particularly grateful to Allen Glazner for providing essential motivation by inviting this contribution and for exhibiting remarkable editorial patience and forbearance. Finally, the author thanks Richard Ketcham, Mark Cloos, Eric Kelly, and Stephanie Moore for careful informal reviews of the manuscript, and David Hirsch and an anonymous reader for formal reviews, all of which substantially improved many elements of both substance and presentation.