Crystallization experiments using a synthetic igneous analog have shown that grain boundary migration can take place in melt-present environments (Means and Park, 1994). In order to evaluate the importance of metamorphic processes during the textural evolution of a crystal mush, microstructural evidence is needed to link them with final igneous textures. In natural magmatic systems, plagioclase may provide a microstructural link; plagioclase solid solution enables the process of chemical zoning and preserves a record of crystal morphology (and chemistry) during growth. If metamorphic processes are common, they should affect the zoning patterns of plagioclase crystals.
A petrographic stage heater apparatus was designed and built for the purpose of observing crystallization processes with a new plagioclase analog. The analog utilized synthetic compounds within the (K, NH4)SCN system and was crystallized at low temperatures (< 172°C) for observation in situ with an optical microscope. The melting points of compounds containing XK = (0, 0.25, 0.50, 0.75, 1) were measured in both thin section and in sealed capillary tubes in order to construct a phase diagram. Compounds of intermediate composition XK = (0.25, 0.50, 0.75) created a continuous solid solution with initial and final melting points between 82°C and 171-175°C in thin section, and between 108°C and 144°C (±1) in sealed capillary tubes. The differences between the two data sets are probably due to the absorption of atmospheric water and differing abilities of the two systems to contain the water.
Textural studies of quenched compounds with composition XK = 0.8 and XK = 0.75 resulted in the formation of crystals with concentric extinction patterns (under cross polarized light) that resemble patterns produced by chemical zoning in plagioclase. Grain boundary migration was also observed between chemically homogenous grains within the solid solution. However, relatively slow rates of cooling were required to produce grain boundary migration. While grain boundary migration was only observed in unzoned solid solution crystals, further crystallization experiments utilizing faster rates of cooling may produce grain boundary migration in chemically zoned crystals.
Scott, E., 2002. The use of a synthetic binary solid solution to
igneous textural evolution.
Unpublished MSc. thesis, State University of New York at Albany. 83 pp., +x
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 2002 S26
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