Low pressure fractionation in arc volcanoes: an example from Augustine Volcano, Alaska

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6611832

Daley, E E; Swanson, S E

Augustine Volcano, situated between the Cook and Katmai segments of the Eastern Aleutian Volcanic Arc, has erupted 5 times since its discovery in 1778. Eruptions are characterized by early vent-clearing eruptions with accompanying pyroclastic flows followed by dome-building and more pyroclastic flows. Bulk rock chemistry of historic and prehistoric lavas shows little variability. The lavas are calc-alkaline, low to medium K, porphyritic acid andesites, rare basalt, and minor dacite pumice. FeO*/MgO averages 1.6 over this silica range. Plagioclase phenocrysts show complicated zoning patterns, but olivine, orthopyroxene, and clinopyroxene phenocrysts show little compositional variation. Hornblende, where present, is ubiquitously oxidized and was clearly out of equilibrium during the last stages of fractionation. Evolved liquid compositions of vitriophyric domes are rhyolitic, and of pumices are slightly less evolved suggesting that individual eruptions become more fractionated with time. Comparison of glass compositions with experimental results is consistent with low pressure fractionation of a relatively dry silicate melt. Disequilibrium of amphiboles and the evolved nature of glasses indicate that shallow level fractionation plays a significant role in the evolution of Augustine magmas. This model is consistent with a shallow magma chamber inferred from geophysical models of the Augustine system and also with its simple, predictable eruption pattern.

• Research Organization: Univ. of Alaska, Fairbanks (USA)
• OSTI ID: 6611832
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

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ALASKA
ALKALINE EARTH METAL COMPOUNDS
CHEMICAL COMPOSITION
ERUPTION
FEDERAL REGION X
FRACTIONATION
GEOLOGIC MODELS
GEOTHERMAL SYSTEMS
IGNEOUS ROCKS
IRON COMPOUNDS
IRON SILICATES
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MAGMA SYSTEMS
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
MINERALOGY
MINERALS
NORTH AMERICA
OLIVINE
OXYGEN COMPOUNDS
PETROCHEMISTRY
PETROGENESIS
PYROXENES
ROCKS
SEPARATION PROCESSES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
USA
VOLCANIC ROCKS
VOLCANOES