The origin of olivine‐plagioclase coronas in metagabbros from the Adirondack Mountains, New York

Olivine‐plagioclase coronas in metagabbros from the Adirondack Mountains, New York (USA) are spatially well‐organized reaction textures consisting most commonly of sequential layers of orthopyroxene, clinopyroxene, plagioclase, and garnet; the textures are characteristic of diffusion‐controlled reaction kinetics. Although similar coronas have been interpreted by previous workers in terms of an isochemical steady‐state diffusion model, petrographical relations and material‐balance calculations establish that coronas in the Adirondack metagabbros cannot be treated as isochemical and do not form in a single‐stage steady‐state process; instead they evolve through time in a complex open‐system reaction. In this study, the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band, thereby approximating the open‐system behaviour of the coronas. The sequence and relative proportions of product minerals calculated by the open‐system steady‐state model correspond closely to those observed in coronas of the Adirondacks, over a wide range of values for the relative diffusivities of chemical components involved in the reaction, regardless of the particular method used to determine material balance in the reaction texture. Despite this correspondence, petrographical evidence for successive replacement of coronal product layers reveals that the Adirondack coronas evolved through one or more transient states, rather than forming in a single‐stage steady‐state process. There is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature, but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona‐forming reaction proceeded. This is confirmed by the fact that the evolution of the coronas over time can be replicated with the open‐system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the Ca and Si components required for reaction. These simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences: (i) orthopyroxene‐clinopyroxene‐plagioclase‐garnet; (ii) orthopyroxene‐clinopyroxene‐garnet; and (iii) orthopyroxene‐garnet. All of these stages, and the transitions between them, are observed petrographically. Coronas in Adirondack metagabbros appear, therefore, to have originated in a complex, open‐system, diffusion‐controlled reaction in which the product assemblages changed as the reaction progressed.