Long-period seismicity at Redoubt Volcano, Alaska, 1989–1990 related to magma degassing

Abstract

The mass of exsolved magmatic H 2O is estimated and compared to the mass of superheated steam (25–50 Mtons) released through the resonating crack producing the December 13–14, 1989 swarm of long-period seismic events at Redoubt Volcano. Results indicate degassing of a H 2O-CO 2-SO 2-saturated magma upon ascending from at least 12 km to 3–4 km beneath the crater as the source of the superheated steam. The mass of exsolved H 2O (3.2–250 Mtons) is estimated from solubility diagrams of H 2O-CO 2-saturated silicate melts for the ascent history of the Redoubt magmas. Crystal size distribution, seismological, petrological, and geochemical data are used to constrain the ascent history of the two andesitic magmas prior to the eruption. Two stages of crystallization are inferred from crystal size distributions of plagioclase crystals in andesites erupted in December 1989. The first stage occurred 30–150 years before the eruption in both magmas and the second stage occurred at least 8 years and 15 years before the eruption in the dacitic andesite and rhyolitic andesite, respectively. The depths of crystallization are constrained from the spatial and temporal variations of volcano-tectonic earthquakes locations (Lahr et al., 1994) and from the P-wave and S-wave velocity structures (Benz et al., 1996). These data suggest that the rhyolitic andesite magma ascended to a depth of 7–8 km within at least 15 years of the eruption. Within at least 8 years of the eruption, the dacitic andesite magma migrated to a depth just below the other magma body where it resided until hours to days of the eruption. At this time, the dacitic andesite magma mixed with the rhyolitic andesite magma and established the reservoir for the eruption. Near the top of the reservoir, some of the mixed magma was displaced into fractures which extended 4–5 km toward the surface. This displaced magma created the eruption conduit and released the fluids related to the resonating crack. This scenario is consistent with the trends in major- and trace-element chemistry, and the stability of hornblende in the pre-eruption Redoubt magmas. It also provides a source for the SO 2 and CO 2 emissions measured during the eruption.