Week 15-16. Metamorphic Reactions

Metamorphic Reactions: Although metamorphism may involve a number of changes, including recrystallization of preexisting phases and diffusion, the most dramatic and useful changes involve metamorphic reactions that generate new mineral phases or modify the composition of existing ones. In this chapter, we address mineral changes that involve reactions. The classic notion of an isograd is that it is a line in the field that demarcates the first appearance of a new mineral phase as one progresses up metamorphic grade. Such an isograd is useful in the field because a worker need only be able to recognize new minerals in a hand specimen. More recently, we have recognized that an isograd can also demarcate the disappearance of a mineral as grade increases. Such isograds are commonly called mineral-out isograds (e.g., “muscovite-out” isograd, Reaction (5)), to distinguish them from the traditional (mineral-in) isograds. If the “in” or “out” is not stated, an isograd is accepted to be a traditional mineral-in type. When we realize that reactions are always responsible for introducing or consuming mineral phases during metamorphism, we gain considerably in our understanding of metamorphic processes and isograds. If we treat isograds as reactions, we can then understand what physical and/or chemical variables affect the location of a particular isograd. Some investigators have advocated that we distinguish simple field-based mineral-in and mineral-out isograds (without a specified reaction) from reaction-based isograds. Miyashiro (1994), for example, referred to simple isograds as “tentative isograds,” implying that more detailed petrographic and laboratory work on rocks below, at, and above the isograd would reveal the nature of the reaction responsible. Although every isograd must represent some sort of reaction, the exact reaction is not determined in every case, so many isograds are still characterized by, and named for, the index mineral for which they mark the appearance or disappearance. A student may wonder why, after over a century of research in metamorphic terranes, we can’t simply look up which reaction is responsible for the introduction of any particular mineral. The problem is that more than one reaction can produce any single mineral.Some of the reactions were prograde and others retrograde. Because biotite occurs in a variety of igneous rocks as well, we also know that it can be generated by reactions involving the crystallization of melts. Which reaction is responsible for the “biotite isograd” depends upon the minerals present below the isograd (the potential reactants), which, in turn, are determined by the rock composition and the metamorphic grade (P and T). Care must therefore be exercised when addressing isograds based simply on the appearance or disappearance of a mineral, and an attempt should always be made to determine the responsible reaction or reactions. If we understand the nature of the reactions that produce metamorphic minerals, the physical conditions under which any particular reaction occurs, and what physical variables affect a reaction and how, we can use this knowledge to understand metamorphic processes better. If we have good experimental and thermodynamic data on minerals and reactions we can locate a reaction in P-T-X space and constrain the conditions under which a particular metamorphic rock formed. In this chapter we will review the various types of metamorphic reactions and discuss what affects them and how.

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