Irreversible Reactions in High-Grade Metapelitic Rocks

Abstract

At least two discontinuous reactions and additional divariant (continuous) compositional shifts were overstepped in pelitic rocks of the inner aureole around the Ronda peridotite massif, southern Spain, probably as a result of upward drag and rapid decrease of Pt during high-grade metamorphism. The resulting reactions were garnet or biotite+A1-silicate↑cor-dierite+hercynite±excess component phases present throughout reaction (quartz, K-feld-spar, ilmenite, plagioclase). Both reactant and product phases are preserved. The extent of reaction, estimated from the amount of cordierite and hercynite produced by reaction, can vary markedly within a single thin section. Coexisting phases were analysed in twelve domains representing varying degrees of reaction in one thin section. A domain, the unit of comparison, was taken as the smallest volume that contained the major reacting phases. Compositions of products in a thin section vary little from domain to domain and probably can be considered a function only of the intensive variables (P, T, μH2o); product phases appear to approach equilibrium as determined by partitioning of major elements. However, reactant compositions are a function of extent of reaction and are not generally in equilibrium with the products. Thus, the reactions are irreversible. The original assemblage before reaction (quartz-K-feldspar-sillimanite and kyanite-garnet-biotite) is preserved within unzoned cores of garnet grains. As the extent of reaction increases, Mg/Fe of biotite in the matrix migrates from the initial value toward a more Fe-rich composition. The analytical data are used to reject the local equilibrium or domain equilibrium model of reaction on the scale of a domain. Product compositions do not vary as expected in this model as a function of extent of reaction, variable reactant composition, or distance from non-equilibrium reactants. The reactant grains must be chemically isolated from the equilibrium matrix products and fluid in partial equilibrium; this model is distinguished as the 'dissolution model'. Diffusion gradients still can be present in this model and are responsible for the distribution of product phases; however, the reaction rate is not diffusion-controlled. Reaction rate can be limited by interface reactions or by diffusion within crystals. Two models of reaction are examined assuming that reaction rate is controlled by reactant dissolution. The first model of irreversible reaction examined, assuming the reacting composition to be that for which the decrease of free energy of reaction is greatest, does not satisfactorily explain the data. A second, descriptive model is proposed in which the overall irreversible reaction is divided into component reactions and the rate of these reactions is assumed to be a monotonic function of free energy change or growth rate of product phases. The model can be shown qualitatively to explain observed garnet zoning and changing biotite composition during reaction. Because reactant phases and compositions of the primary assemblage are preserved along with the equilibrium product phases in these samples, more information is available from these ‘disequilibrium’ rocks than from samples that have completely equilibrated. The variability of extent of reaction in these samples emphasizes that contact of phases in a sample is not evidence of mutual compatibility.