| Summary: | Includes bibliographical references. Includes illustrations and maps. The snowflake troctolite zone (SFT) in the Hettasch intrusion contains textures characteristic of crystallization under supersaturated conditions. Oscillatory, patchy, and reverse zoning observed in spheroids of radially arranged plagioclase megacrysts, comb-layered plagioclase crystals, and skeletal plagioclase megacrysts is believed to have formed as a result of changes in the plag/Liq distribution coefficients. The distribution coefficient is effected by variations in growth rates and diffusion rates at the crystal- liquid interface, or by variations of the normative diopside component in the boundary layer liquid adjacent to the growing crystal, or by a combination of these and other factors. Oscillatory nucleation was the dominant process by which crystallization of the SFT zone took place. By varying the amount of plagioclase component enrichment in the liquid immediately overlying a layer of crystallizing olivine, and by varying the temperature at which crystallization took place, production of the various plagioclase textures can be explained. The mafic nature of the SFT zone requires the placement of the original SFT composition within the primary field of olivine. However, the whole rock compositions do not represent liquid compositions because an unknown amount of liquid was lost in the late stages of crystallization of the SFT zone. M.S. (Master of Science)
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