Abstracts Category : Other

A Climactic Unimak Island Caldera-Forming Eruption, Aleutian Arc, Alaska: Magma storage and pre-eruption P-T-fO2-H2O conditions of the rhyodacite magma

by Olivia E Anderson

Understanding the mingling interactions between felsic and mafic end-member magmas is essential in understanding the physical processes occurring in bimodal eruptions. The Fisher caldera-forming eruption (CFE), which co-erupted rhyodacite (~69 wt.% SiO2) and basaltic andesite (~53 wt.% SiO2) ~9400 yBP, was studied to better understand the scenarios under which bimodal CFEs erupt. The shallow rhyodacite had a pre-eruptive temperature of 877 908C and the basaltic andesite had a pre-eruptive temperature of 1065 1147C. Preeruptive water contents of ~41.5 wt.% H2O for the rhyodacite and ~0 2.5 wt.% H2O for the basaltic andesite suggest that the magmas were typical for Aleutian arc magmas. This study presents new geochemical and isotopic data from glasses and minerals from Fisher and Shishaldin CFE deposits. Geochemical relationships between Shishaldin and Fisher CFEs indicates these deposits are from the same eruption, sourced from magmas underlying the Fisher Volcano. Results from differentiation models show that the rhyodacite did not form from closed-system fractional crystallization of the co-erupted basaltic andesite, similar to observations of antecrystic minerals and 18O data. Assimilation of hydrothermally-altered country rock is interpreted to cause the low 18O signature of both the basaltic andesite and rhyodacite. In this study, I test three hypotheses for magma plumbing systems that have been shown to lead to compositionally bimodal CFEs: (1) a single, compositionally-zoned magma chamber, (2) a shallow silicic reservoir and a mid- or deep-crustal basaltic andesite reservoir, with rising mafic magma injecting into the shallow chamber; and (3) an independent and complex system of magma chambers within the upper crust. Geochemical modeling results and oxygen isotope data suggest that the Fisher magmatic system likely comprised a complex system of multiple independent, shallow magma chambers with the rhyodacite and basaltic andesite evolving separately but proximal to each other. The shallow rhyodacite had a pre-eruptive temperature of 877 908C and the basaltic andesite had a pre-eruptive temperature of 1065 1147C. Preeruptive water contents of ~41.5 wt.% H2O for the rhyodacite and ~0 2.5 wt.% H2O for the basaltic andesite suggest that the magmas were typical for Aleutian arc magmas. Advisors/Committee Members: Stelling, Peter L., DeBari, Susan M., 1962-, Mulcahy, Sean.