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

Abstract

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, fplagioclase, and garnet; the textures are characteristic of diffusioncontrolled 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) orthopyroxeneclinopyroxene-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, diffusioncontrolled reaction in which the product assemblages changed as the reaction progressed.

Arrested development – a comparative analysis of multilayer corona textures in high-grade metamorphic rocks

“…2 is a schematic reconstruction of those described by Johnson and Carlson (1990) from metagabbros in the Adirondack Mountains that they interpreted as a natural example of this corona formation mechanism. A primary igneous assemblage involving contiguous olivine and Open-system, single-stage, steady-state diffusioncontrolled growth of prograde corona layers between olivine and plagioclase (modified after Johnson and Carlson, 1990). (a) With incipient reaction, different rates of intergranular diffusion for major components manifest themselves as spatially segregated layers.…”

“…Layer thickness increases with reaction duration and no change to a corona layer sequence occurs. Chemical potential gradients evolve toward a steady-state and final configuration balancing the rate of production and consumption of each component within each layer (Korzhinskii, 1959;Joesten, 1977;Mongkoltip and Ashworth, 1983;Foster, 1986;Grant, 1988;Johnson and Carlson, 1990;Carlson and Johnson, 1991;Ashworth and Birdi, 1990;Ashworth et al, 1992Ashworth et al, , 1998Ashworth and Sheplev, 1997;Markl et al, 1998). Figure 1 illustrates incipient stages of single-stage, steadystate corona formation chemographically and in chemical potential space by considering two phases (A and D) initially at equilibrium under P 1 and T 1 , with bulk composition indicated by the circle (Fig.…”

“…Mg, Ca, Na and Al migrate into the corona and Fe and Si move out from the corona. Minor spinel occurs in reactant plagioclase as Ca and Si diffuse preferentially into the reaction band, creating an Si deficiency that stabilizes spinel in relict reactant plagioclase (Johnson and Carlson, 1990).…”

“…In contrast, large chemical potential gradients over thicker layers will cause the product (− G) to deviate further from 0, suggesting suppressed equilibration. These equations predicate the steady-state diffusioncontrolled models developed for spatially segregated reaction products in multilayer coronas arranged in order of increasing or decreasing chemical potential (Fisher, 1977;Joesten, 1977;Mongkoltip and Ashworth, 1983;Foster, 1986;Grant, 1988;Johnson and Carlson, 1990;Carlson and Johnson, 1991;Ashworth and Birdi, 1990;Ashworth et al, 1992Ashworth et al, , 1998Ashworth and Sheplev, 1997;Markl et al, 1998;Dohmen and Chakraborty, 2003;. As changing P and T induces incipient reaction between contiguous metastable reactants, components will start to migrate between the reactants.…”

“…A multilayer corona may evolve in a steady or quasi-stationary state controlled by diffusion (single-stage, steady-state growth) and then subsequently modify through retrograde reaction between two adjacent layers at constant P and T through changing composition of the effective equilibration volume as the composition of a reactant evolves with protracted reaction. Brady (1977) and Vidale (1969) introduced a modification to the steady-state model that was used to explain variability in coronal layer development between the same reactants by Johnson and Carlson (1990). Vidale (1969) modelled the development of calc-silicate bands in a system with a waning availability of certain components.…”