THE USE OF KSCN-NH4SCN SOLID SOLUTION TO MODEL PLAGIOCLASE TEXTURAL EVOLUTION

Microstructures formed by plagioclase crystals provide valuable information about the paths of crystallization within igneous plutons. The plagioclase solid solution can generate chemically zoned crystals and a record of crystal morphology (and chemistry) during growth. Our inability to observe the crystallization of plagioclase within the plutonic environment, however, has necessitated the assumption that metamorphic processes such as grain boundary migration are uncommon. If they do occur, they may leave a signature within the zoning patterns of plagioclase crystals. While crystallization experiments using synthetic compounds have shown that grain boundary migration can take place in melt-present environments (Means and Park, 1994), the importance of the process in igneous plutons was left unclear.

In order to evaluate the importance of metamorphic processes during the textural evolution of a crystal mush, a petrographic stage heater apparatus was designed and built for the purpose of in situ observation of low temperature (< 175 ° C) crystallization of KSCN-NH₄SCN liquids. A series of reversed experiments were conducted for bulk compositions X_K=n_K/(n_K+n_NH4)=(0, 0.25, 0.50, 0.75, 1) in both thin section (open system) and in sealed capillary tubes (closed system) in order to construct a T-X phase diagram. For intermediate compositions, initial and final melting points were between 82 - 175 ° C in thin section, and between 108 -144 ° C (± 1) in sealed capillary tubes, for the NH₄ and K endmembers, respectively.

Textural studies of quenched compounds with composition X_K=0.8 and X_K=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. Slow rates of cooling, however, produced grain boundary migration between chemically homogenous grains within the solid solution.

These synthetic experiments have led to a greater understanding of crystallization processes possible within a plutonic environment. Future studies involving a full range of cooling histories may produce the co-evolution of chemical zoning and grain boundary migration necessary to form textures characteristic of metamorphic processes in melt-present environments.