HR: 11:20h
AN: V52B-05 INVITED     [Abstracts]
TI: Is the 2004-05 Eruption of Mount St. Helens Tapping New Dacite From the Deep Crust?
AU: * Pallister, J S
EM: jpallist@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 Cardinal Court, Suite 100, Vancouver, WA 98683 United States
AU: Thornber, C R
EM: cthornber@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 Cardinal Court, Suite 100, Vancouver, WA 98683 United States
AB: The 2004-05 eruption of Mount St. Helens began with seismic activity and uplift of the crater floor in late September 2004, followed by phreatic explosions and extrusion of a lava dome starting on 11 October 2004 and continuing to this time (September, 2005). Since shortly after the first spine of lava appeared, samples have been collected using a steel box dredge (``JAWS '') suspended 60 or 110 feet below a helicopter. This method was developed to acquire samples from the hot and steep-sided dome, which, because of occasional explosions and frequent collapses, has been unsafe to approach on the ground. To date, 21 samples have been collected from the six spines of the new lava dome and petrologic studies of these samples are reported in this session. The lava dome is composed of dacite (65 wt% SiO2) that is geochemically uniform and slightly more evolved than the 1980-86 dacite. The typical lava is crystal-rich with ~50% phenocrysts of plagioclase, amphibole, hypersthene, and Fe-Ti oxides in a microcrystalline matrix that contains ~13% microlites, ~13% glass and ~25% vesicles. Oxide thermometer data for early spine samples cluster at 840-850° C and NNO+1 log unit. In contrast, samples erupted during the winter of 2004-05 have zoned oxides with apparent temperatures that range to >950° C. Such late-stage heating is likely due to latent heat evolved during rapid groundmass crystallization, or possibly to heating by new magma. Low volatile contents, presence of trydimite and quartz microlites and decreasing H₂O with increasing SiO₂ in the rhyolite matrix glass indicate extensive shallow (<1 km) crystallization, driven by degassing of water. Major and trace elements of the 1980-86 and 2004-2005 magma batches are similar, which led us to the initial interpretation that the dacite was magma "left over" from the 1980-1986 activity. However, new petrologic and geochemical data suggest instead that the 2004-05 eruption may be fueled by a new batch of dacite magma derived from depth. Geochemically, both Pb and Th isotopes indicate that the 2004 dacite is different from the 1985 dome (Kent et al, this session; Cooper et al., this session). The recent dacite also contains amphibole cores with Al₂O₃ contents (12-15 wt%) that are too high to be in equilibrium with the MSH dacite at pressures up to 300 MPa (Rutherford and Devine, this session) and suggest incorporation of the amphibole from a higher pressure or from a more mafic magma. Low concentrations of incompatible-elements in MSH dacites indicate derivation of the dacite by melting of lower crustal metabasaltic rocks. Consequently, the isotopic distinctiveness of the 2004-05 dacite and presence of high-Al amphiboles suggest that Mount St. Helens is now being fed by small batches of dacite coming from deeper crustal levels, perhaps in response to unloading of the magmatic system in 1980. These new magmas would have ascended through the remains of the 1980-86-conduit system where they likely mingled with 1980's vintage dacite. The 2004 dacite last equilibrated at P(H₂O) of about 140 MPa (Rutherford and Devine, this session), equivalent to depths of about 5 km. Subsequent rise to the surface caused the dacite to degas water and other volatiles, and to crystallize extensively at shallow (<1 km) depths to the point that it erupted as a rheological solid, forming fault-gouge mantled spines. These data have significant implications for the long-term eruptive behavior of Mount St. Helens, as arrival of a new batch of dacitic magma from the deep crust could herald the beginning of a new long-term cycle of eruptive activity.
DE: 3618 Magma chamber processes (1036)
DE: 3625 Petrography, microstructures, and textures
DE: 3652 Pressure-temperature-time paths
DE: 8400 VOLCANOLOGY
DE: 8425 Effusive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005