Compositional heterogeneity of tephras from the 1980 eruptions of Mount St. Helens

摘要: Samples collected at hourly intervals on May 18–19, 1980, three sites 200 km downwind from Mount St. Helens, have made possible a detailed reconstruction of the conditions that contribute to compositional heterogeneity mineral and glass components observed in distal tephra layers. The air fall deposited during first 7 hours 18 eruption is mostly coarse grained, microlite-rich, nonjuvenile feldspar. Grain-size maxima this initial can be related cataclysmic blast 0832 subsequent pulse 1200. Juvenile, microlite-free increases relative abundance sampling beginning about 1900. Such change between juvenile 5-km increase column height associated with last major which occurred 1700 volcano. Electron microprobe study both microlite-rich pumice time series samples reveals significant differences. Interstitial few enriched SiO2 K2O depleted TiO2, FeO*, MgO glass. By comparison, element composition least evolved sampled several displays slight trend toward less composition. Least squares calculations suggest more character explained by greater fractional crystallization brought enhanced cooling cryptodome prior eruption, whereas temporal changes presence small, slightly zoned magma chamber depth. glass-coated ilmenites, magnetites, plagioclases provides following estimates physical reservoir: 865°±50°C, PH2O = 2.2 kbar -log ƒO2 11.7. Analyses bulk pumice, selected phases 25, June 12, July 22, October 16–18 pumices erupted Helens indicate (magma) compositions become andesitic time, while co-existing changed significantly post-May eruptions being highly than those eruption. An application magnetite-ilmenite geothermometer 12 22 indicates temperatures 919°±30°C 930°±50°C, respectively. such derived like into approximately 50% crystals residual liquid. partitioning liquid appears scale centimeters interpreted as consequence accelerated under reduced water pressure.