Abstract

Almandine-rich garnets in mafic amphibolites of the Llano Uplift of central Texas display remarkably well-organized coronal textures with characteristics diagnostic of diffusion-controlled reaction. The garnets have undergone static resorption reactions of three types: (1) reaction with a metamorphic fluid along fractures to produce symmetrical layers of magnetite and symplectitic labradorite + ferroan pargasite; (2) reaction with quartz to produce coronal layers of plagioclase (oligoclase-andesine) + magnetite and orthopyroxene + augite; and (3) reaction with omphacite to produce coronal layers of symplectitic labradorite + ferroan pargasite ± magnetite and magnesio-hornblende + oligoclase ± orthopyroxene. Material-balance calculations assuming volume conservation in an inertmarker reference frame demonstrate that none of these reactions can be considered iso-chemical. Instead, substantial amounts of Na, Ca, Fe, Mg, Si, and Al must have been transported across the boundaries of the reaction bands.

An open-system diffusion model for the garnet-quartz and garnet-omphacite reaction bands establishes that each of the assemblage zonations observed is stable with respect to diffusion over a substantial range of possible ratios of the phenomenological coefficients for intergranular diffusion. Differences in the relative values for the phenomenological coefficients between the garnet-quartz and, garnet-omphacite reaction bands may reflect differences in the chemistry of the intergranular fluid, particularly in the degree of silica saturation. Garnet-omphacite reaction bands exhibit petrographic evidence for the progressive replacement of orthoplroxene by hornblende, indicating that the product assemblage in this reaction band evolved over time. Calculations based on the open-system diffusion model suggest that this temporal evolution results from changes in the amounts of material transported across the boundaries of the reaction band in response to the progress of a reaction (external to the garnet-omphacite corona) in which omphacite breaks down to sodian augite + oligoclase + magnesio-hornblende. To be petrologically valid, models of diftrsion-controlled reactions must therefore address rigorously the difficult questions of the degree of chemical exchange of a reaction with its surroundings and the extent to which that exchange may vary as reaction progresses.

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