OBSERVATIONS OF PLAGIOCLASE ZONING USING INTERFERENCE IMAGING

Interference imaging techniques (laser interference microscopy in transmitted light and Nomarski differential interference contrast in reflected light) have been used to study different types of zoning in magmatic plagioclase crystals. Samples studied come from a wide range of rock types and geological environments. Analysis of zoning profiles for individual unit zones shows that a saw-tooth pattern in which an initially high An% decreases as growth proceeds is the most common type of zoning configuration. We recognize 2 major types of zoning: Type 1, small zones usually less than 10 microns thick, with compositional amplitudes up to 10 An%; and Type 2, zones up to 50 + microns thick, with large compositional amplitudes of up to 35 An%. Detailed analysis of zoning patterns shows that oscillatory zoning is not actually oscillatory in the sense of a sine wave. It is discontinuous-unit zones are characteristically bounded by discontinuities. This common pattern is consistent with alternating periods of growth and periods of dissolution, reaction and lack of growth or possibly very slow growth. Our observations firmly establish that dissolution and magmatic reaction (involving both growth and dissolution) are common during growth of the magmatic plagioclase which we have studied. Type 1 zonation appears consistent with perturbations of local conditions in a diffusion-controlled boundary layer around the growing crystal. Type 2 zoning is probably due to large-scale conditions. Faceted planar growth surfaces appear to be the rule in magmatic crystallization although in some instances rounded or irregular (convolute) zonation can occur. The repetitive nature of many of the features we have observed is consistent with and suggestive of some form of self-organization. © 1990.