Reactions in Amphibolite, Greenschist and Blueschist |
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Authors: | THOMPSON, JAMES B., JR. LAIRD, JO THOMPSON, ALAN B. |
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Affiliation: | 1Department of Geological Sciences, Harvard University Cambridge, Massachusetts, 02138, U.S.A. 2Institut fur Kristallographie und Petrographie, Eidgenossiche Technische Hochschule Zurich, Switzerland, CH-8092 |
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Abstract: | Mineral assemblages in which chlorite [CHL], epidote [EPI],clinoamphibole [AMP], plagioclase [PLG] and quartz [QTZ] aremajor phases are characteristic of many low-grade mafic schists.The possible heterogeneous reactions in such an assemblage maybe separated into two types, exchange reactions and net-transferreactions. Only the latter alter significantly the modal proportionsof the minerals. A set of linearly independent reactions defines a reaction spaceof as many dimensions as there are independent reactions. Thespace defined by the net-transfer reactions alone is a sub-spacethat can be portrayed in three dimensions for the above assemblage.A procedure is presented herein that gives a set of independentreactions that may be taken as basis reactions for definingsuch a reaction space. All other reactions that can be writtenfor this assemblage, as well as observed whole-rock reactions,can be portrayed as vectors in these reaction spaces. Thesevectors connect the region (mineral facies) accessible to theabove assemblage. The whole-rock reactions of Laird (1980) relatinggreenschist, blueschist and various low-grade amphibolites fromVermont, provide informative examples, as do the whole-rockexperiments of Liou et al. (1974). Although reaction spaces apply to both equilibrium and disequilibriumassemblages the reactions selected as basis vectors correspondone-for-one to the chemical conditions for equilibrium thatmust obtain in any fully equilibrated assemblage. The set selectedis one that provides maximum sensitivity for geothermometric,geobarometric and geohygrometric purposes. |
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