Metamorphic fractional crystallization and internal metasomatism by diffusional homogenization of zoned garnets |
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| Authors: | Frank S Spear |
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| Institution: | (1) Department of Geology, Rensselaer Polytechnic Institute, 12180 Troy, NY, USA |
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| Abstract: | Metamorphic fractional crystallization involves fractionation of material into the interior of a crystallizing mineral such as garnet with consequent change in the effective bulk composition. In assemblages with thermodynamic variance of 3 or greater, the mineral assemblage, compositions of coexisting phases and the zoning profile preserved in the garnet are a function of the amount of material that is fractionated. Homogenization of zoned garnet by intracrystalline diffusion at elevated temperatures operates on a time scale of millions to tens of millions of years depending on the temperature-time path and radius of the garnet. One effect of homogenization is to drive chemical reactions by a metasomatic mechanism (internal metasomatism), even in the absence of changes in pressure or temperature. This process may affect the appearance or disappearance of minerals, and may, in some rocks, be responsible for the first appearance of kyanite or the disappearance of staurolite in Barrovian terranes. |
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