Laser-interference microscopy, Nomarski interference-contrast microscopy (on etched polished sections), petrographic microscopy, and electron-microprobe analysis have been used to study the phenocryst population of a sample of pumice from the May 18, 1980, eruption of Mount St. Helens. The pumice appears to contain more than one population of phenocrysts and may itself be a sample of different layers in a zoned magma chamber. An overlapping sequence of events can be determined for the two most common phenocryst types. A model developed from the zoning profiles involves an early stage of growth at depth followed by a period of massive resorption of all phenocryst phases. The apparent resorption of hornblende with no sign of magmatic reaction or opacite rims is remarkable. Plagioclase compositions range from An63 to An37, with the commonest composition being An46. There appears to be a general similarity between the zoning pattern in the mantles of the larger phenocrysts (type I) and the complete zoning pattern of the smaller euhedral phenocrysts (type II). We consider that this is due to contemporaneous growth. In spite of overall similarities in zoning profiles, there are inconsistencies in both the compositions and the number and type of resorption events. Exact correlation between specific events cannot be drawn pending more detailed magmatic stratigraphic correlation between phenocrysts.

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