Abstract:
Geochemical studies of active and fossil fumaroles were conducted at Mount St. Augustine and the Valley of Ten Thousand Smokes (VTTS) to investigate fumarolic systems for providing information on volcanic and magmatic processes. Gases and condensates collected from high-temperature rooted fumaroles at Mount St. Augustine in 1979, 1982, and 1984 are characterized by systematic long-term trends in gas composition and stable isotopes that can be best explained by progressive magmatic outgassing coupled with increasing proportions of seawater in the fumarolic emissions. Seawater-magma interaction may initiate some of the early explosive phases of Mount St. Augustine eruptions. The distribution and morphology of rootless fumaroles formed on pyroclastic flows and a lava flow emplaced during the 1986 eruptive cycle of Mount St. Augustine were controlled by pre-eruption drainage and topography, as well as by the thickness, compaction, and settling of the flow deposits. The majority of chemical components present in encrustations collected from these active fumaroles were derived by acidic condensate leaching of the eruptive deposits. Trace-element distribution apparently followed a pattern of isomorphic substitution in the encrustation phases. A reconnaissance survey of surface Hg$\sp\circ$ contents in the VTTS supports the presence of a shallow intrusion beneath the dome-like feature known as the Turtle. Based on the Hg$\sp\circ$ data, the preferred model of the 1912 Novarupta vent is one generated by collapse of supporting vent walls into a cored-out explosive vent after the major eruptive phase. Vent morphology is funnel-like with subsidence concentrated in the narrow funnel center. The magnitude of the Novarupta Basin Hg$\sp\circ$ anomalies implies that a shallow ($\approx$1 km depth) incipient hydrothermal system has developed beneath the vent.
