Quantitative Estimation of an Open-system Symplectite-forming Reaction: Restricted Diffusion of Al and Si in Coronas around Olivine

Abstract

The corona reaction between olivine and plagioclase, producing layers of orthopyroxene (adjacent to the olivine), amphibole, and amphibole-spinel symplectite, is reconstructed in detail in specimens from the synorogenic Newer Basic intrusions of NE Scotland. An amphibole–anorthite symplectite is also found. The reaction occurred during slow cooling in a retrograde, regional metamorphic environment. The proportions of the different layers, and of minerals within symplectites, were quantified by digitizing enlarged images. Combined with volume-corrected electron-microprobe analyses, the results show that the Al:Si ratio of reactant plagioclase is often preserved almost exactly in both symplectites. Al and Si are immobile (have short diffusion ranges), and control the scale of the symplectic intergrowth. Symplectites are petrological analogues, not restricted to closed systems, of the ‘duplex cell’ structure produced by eutectoid reactions and discontinuous precipitation. It is argued that, for a two-mineral symplectite to form in a general open system, at least two components must have restricted diffusion ranges. This constraint is relevant to ‘local equilibrium’ models. Since Al and Si are relatively immobile, the corona-forming reaction requires Mg, Fe and O to be mobile within the corona, moving from reactant olivine into the amphibole-spinel symplectite. Thus the central boundary between orthopyroxene and amphibole-spinel symplectite migrates towards the olivine, while also becoming ‘smeared out’ by the development of the non-symplectic amphibole layer with small-scale Al diffusion. Where the total proportion of amphibole produced is relatively small, the corona as a whole is approximately closed to Mg and Fe, and we infer a small decrease in volume. Larger proportions of amphibole are produced with minor addition of Mg, Fe, Ca and O, so that the reaction proceeds at approximately constant volume. Al and Si are probably slow-diffusing species in many symplectites.