Simpson, Kristie

This thesis characterizes the transfer of magmatic volatiles through the mantle and the crust to the atmosphere through the integration of melt inclusion data for pre-eruptive volatile contents with surface measurements of volcanic degassing (recorded in micro-gravity changes and volcanic fumarole and plume gas compositions) at two contrasting volcanoes: Sierra Negra, Galápagos Islands and Kawah Ijen, Indonesia. In particular, it explores the process of fluid transfer in the mantle, the partitioning of volatile elements during mantle melting and degassing of the magma through the crust, and the effect of near-surface (e.g., interactions with groundwater and hydrothermal fluids), and surface processes (e.g., cooling and mixing with air) on the gas species. The effects of differences in initial volatile content and internal volcano structure on the types of eruptions and emissions recorded at each volcano are also discussed. The comparison of Sierra Negra and Kawah Ijen volcanoes reveals that differences in style of volcanic activity are primarily a function of magmatic plumbing system as opposed to differences in initial volatile content. In both cases, permeability of the crust and degassing style have exerted a dominant control over the recent style of activity (last century). Recent eruptions at Sierra Negra are not necessarily associated with magma recharge into shallow reservoirs but can be caused by subtle changes in the pressure regime of a magma chamber, a process which is closely associated with degassing and system permeability. Large explosive eruptions at Kawah Ijen are currently impeded by the open system (permeable) flow of magma and gas through the plumbing system. Hydrothermal systems play an important role in controlling the permeability of a system and the composition of the gases measured at the surface. The comparison of theoretically modeled gas compositions with actual measured compositions is an effective approach to studying the influence of hydrothermal systems at open vent volcanoes.