Reactions in Amphibolite, Greenschist and Blueschist

Mineral assemblages in which chlorite [CHL], epidote [EPI], clinoamphibole [AMP], plagioclase [PLG] and quartz [QTZ] are major phases are characteristic of many low-grade mafic schists. The possible heterogeneous reactions in such an assemblage may be separated into two types, exchange reactions and net-transfer reactions. Only the latter alter significantly the modal proportions of the minerals. A set of linearly independent reactions defines a reaction space of as many dimensions as there are independent reactions. The space defined by the net-transfer reactions alone is a sub-space that can be portrayed in three dimensions for the above assemblage. A procedure is presented herein that gives a set of independent reactions that may be taken as basis reactions for defining such a reaction space. All other reactions that can be written for this assemblage, as well as observed whole-rock reactions, can be portrayed as vectors in these reaction spaces. These vectors connect the region (mineral facies) accessible to the above assemblage. The whole-rock reactions of Laird (1980) relating greenschist, blueschist and various low-grade amphibolites from Vermont, provide informative examples, as do the whole-rock experiments of Liou et al. (1974). Although reaction spaces apply to both equilibrium and disequilibrium assemblages the reactions selected as basis vectors correspond one-for-one to the chemical conditions for equilibrium that must obtain in any fully equilibrated assemblage. The set selected is one that provides maximum sensitivity for geothermometric, geobarometric and geohygrometric purposes.