Origin of olivine-plagioclase coronal textures from the Adirondack Mountains, New York State

Olivine-plagioclase coronas in metagabbros from the Adirondack Mountains are spatially well-organized reaction textures consisting most commonly of sequential layers of orthopyroxene, clinopyroxene, [plus or minus] plagioclase, and garnet; the textures are characteristic of diffusion-controlled reaction kinetics. After petrographic study and electron microprobe analysis, a quantitative isochemical steady-state diffusion model was applied to these textures in an attempt to explain their origin. The modeling and prominent petrographic features reveal that the coronas of the Adirondacks cannot be treated as isochemical and do not form by a purely steady-state process. The transport of elements into and out of the coronal reaction zones in nine rocks was estimated from a material balance between reactants and products. Input for each material balance calculation consisted of the compositions of all phases, determined by electron microprobe analysis, and the relative proportions of products, determined by digital image analysis. For coronas of the Adirondacks, material balance assuming limited mobility of aluminum demands substantial transport of some elements beyond the boundaries of the reaction zones. The coronas cannot, therefore, be regarded as isochemical. Modifications to the diffusion model that account for elemental fluxes across the outer boundaries of the assemblage zone approximate, to first order, the non-isochemical behavior of the coronas. The modified version of the model successfully replicated the sequence and relative proportions of the product minerals observed in olivine-plagioclase coronas although the calculated range of relative diffusivities is not tightly constrained. Textural evidence indicates the instability of some layers within the coronal assemblage zone. Growth of euhedral crystals of a bounding layer into an 'unstable' layer, and the subsequent replacement of the unstable layer by the growing phase, are exhibited in most of the coronas. These replacement textures are interpreted as results of the following process. As each of the more rapidly diffusing components eliminates its chemical-potential gradient across the reaction zone, its chemical potential becomes buffered by equilibria outside the reaction zone. Consequently, the number of mineral phases within the reaction band must decrease by one, which requires a reaction consuming one of the coronal layers. These coronas, therefore, reflect dynamic processes characteristic of one or more transient states, rather than approximating a steady-state, time-invariant process. The possible origins of these reaction textures are, accordingly: (1) a series of alternating steady and transient states in which the former predominate, (2) a series of alternating steady and transient states in which the latter predominates; and (3) a series of overlapping transient states in which a steady state is never realized. Because some coronas exhibit no textural evidence for layer instability, formation by process (3) alone is unlikely. Discrimination between possibilities (1) and (2) is difficult. However, the preponderance of coronas across the Adirondacks that display replacement of one coronal assemblage by another may suggest that transient states prevail over steady states in the evolution of these natural coronal textures.