Petrologic And Geochemical Tracers Of Magmatic Movement In Volcanic Arc Systems: Case Studies From The Aleutian Islands And Kamchatka, Russia

Abstract

Mixing, crystallization and degassing commonly affect magmas during storage, ascent and eruption from volcanoes. As these interactions cannot be observed directly, they must be characterized using chemical signatures of volcanic eruptive products. In this dissertation, geochemical tracers of magma mixing, crystallization and degassing were employed to investigate the magmatic systems of Kasatochi Island volcano, central Aleutian Islands, and Bezymianny Volcano, Kamchatka, Russia, in three studies. The purpose of the first study was to characterize eruptive products from the 2008 eruption of Kasatochi Island volcano. This eruption produced pumiceous, medium-K andesite, as well as denser, medium-K basaltic andesite, with variable bulk compositions (~52-56 wt. % SiO2 for the basaltic andesite, ~58-62 wt. % SiO2 for the andesite). The basaltic andesite and andesite must have resided in separate storage areas until just prior to eruption, with mixing between these magmas limited to just prior to eruption. The second study focused on basaltic andesite from the 2008 Kasatochi eruption, which contains two plagioclase phenocryst populations and anomalously calcic plagioclase microlites. Compositional heterogeneity in the basaltic andesite resulted from pre-eruptive mixing between mafic and silicic compositional end members. As the calcic plagioclase phenocrysts and microlites are out of equilibrium with groundmass glass, these phenocrysts and microlites likely crystallized mostly in the hotter (900-1000 �C) mafic mixing end member, while more sodic phenocrysts crystallized in the cooler (800- 950 �C) felsic end member. Mixing between the silicic and mafic end members imposed mafic end members imposed an undercooling on the mafic magma, triggering plagioclase microlite nucleation in these more calcic liquids. The purpose of the final study was to investigate magmatic evolution of Bezymianny Volcano in Kamchatka, Russia through analyses of major, trace and volatile element compositions of melt inclusions. Inclusions dominantly record pressures of inclusion entrapment <50-100 MPa, indicating a zone of magmatic storage 1-2 km below the surface. Degassing and crystallization during ascent and shallow storage produces negative correlations between concentrations of incompatible lithophile (K2O, Rb) and volatile (H2O, CO2) elements. Also, fluid ascent from depth leads to enrichment of shallowly-stored magma in lithium, on timescales of hours to days prior to eruption.