Author:
Ashworth J. R.,Birdi J. J.,Emmett T. F.
Abstract
AbstractCoronas containing Ca-amphibole with aluminous minerals have been characterised optically and by scanning electron microscopy, analytical transmission electron microscopy and electron-probe microanalysis. The layers nearest to plagioclase are amphibole + epidote + kyanite, followed by amphibole + epidote + staurolite + spinel. These assemblages are consistent with waterundersaturated conditions, possibly at lower metamorphic grade than the commoner assemblage amphibole + spinel. Observed mineral proportions and compositions were used in a seven-layer model of steady-state, diffusion-controlled growth with local equilibrium. This model is not fully realistic, because the observed amphibole is strongly zoned from tschermakitic to actinolitic away from plagioclase, suggesting disequilibrium. However, the four-mineral layer has been successfully modelled assuming local equilibrium, with diffusion coefficients Lii larger for i = FeO and MgO than for SiO2, AlO3/2, CaO and FeO3/2. Retarded grain-boundary diffusion of the latter components is explicable by crystal-chemical effects. The number of minerals per layer is constrained by a modified form of the metasomatic phase rule of Korzhinskii, with the role of 'inert' components played by relatively immobile ones (having relatively small fluxes and relatively small diffusion coefficients).
Subject
Geochemistry and Petrology
Cited by
19 articles.
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