Evolution of mineral assemblage zoning in diffusion metasomatism

Metamorphic reaction bands consist of mineral assemblage layers, that separate incompatible mineral assemblages, and grow by reaction at their contacts and by precipitation within the layer. The stoichiometry of these reactions is determined by the relative diffusion fluxes within adjacent layers. Using the local equilibrium-steady diffusion model of Fisher (1973), the stoichiometric coefficients of layer growth reactions can be computed from the mass balance at each layer contact and independent ratios of instantaneous fluxes within each layer.Model calculations in a ternary system show that (1) variation in bulk composition across a reaction band, expressed in terms of layer sequence, mineral formulae of phases that comprise monomineralic layers and modal proportions of phases in multimineralic layers is largely determined by the relative fluxes of components that diffuse in the same direction; (2) the fraction of the width of a reaction band produced by replacement of each of the initial reactant assemblages is largely determined by the relative fluxes of components that diffuse in opposite directions; (3) the fraction of the width of a layer produced by internal precipitation is determined by temperature and the relative fluxes of components that diffuse in opposite directions; (4) in a multilayer reaction band, only one layer can grow by reaction at both contacts, and that layer contains the initial contact between initial reactants; and (5) a reaction band cannot grow by constant volume replacement of both reactant assemblages, if the system is closed to diffusion beyond layer contacts with the initial reactant rock masses.