HR: 1340h
AN: V13B-0533    [Abstracts]
TI: Petrological and Experimental Constraints on the Recent Magma Plumbing System at Okmok Volcano, Alaska, USA
AU: * Izbekov, P E
EM: pavel@gi.alaska.edu
AF: Alaska Volcano Observatory/ Geophysical Institute/ University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Larsen, J F
EM: faust@gi.alaska.edu
AF: Alaska Volcano Observatory/ Geophysical Institute/ University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712-0254 United States
AB: The most recent eruption of Okmok volcano, Alaska, USA, occurred in February-May 1997, after ca. 9 yr. period of quiescence. The eruption was mainly Strombolian, producing multiple ash plumes, with heights up to 9000 meters ASL, and a 6-km-long lava flow. Here, we use petrological and experimental data to constrain the recent magma plumbing system at Okmok. The whole-rock composition of the 1997 Okmok basalt is homogeneous, and averages 52.27±0.18 wt.% SiO₂. The mineral assemblage includes plagioclase (14 vol.%), olivine (3 vol.%), clinopyroxene (<0.3 vol.%), and Ti-magnetite (<0.3 vol.%). Plagioclase phenocrysts are strongly bi-modal: their cores are calcic (An_85-95), whereas the outermost rims (and microlites) are more sodic (An_66-68). Similarly, cores of olivine phenocrysts are Mg-rich (Fo_83-85), whereas their rims and (microlites) are more Fe-rich (Fo_73-76). Glass inclusions in phenocrysts contain ca. 2.5 wt.% volatiles. Compositions of the matrix glass and the outermost rims of phenocrysts are most closely reproduced by phase-equilibria experiments at 1015-1030° C and 62-76 MPa water pressure. Compositions of cores of plagioclase and olivine phenocrysts have not been reproduced in our experiments up to 100 MPa, which suggests that they possibly originated from a deeper source. We tentatively suggest that the basaltic magma erupted in 1997 was first stored at deep depths, and then migrated and re-equilibrated at depths of about 3.3-4.1 km beneath the floor of Okmok caldera. Detailed InSAR studies indicate that the 1997 Okmok eruption was preceded by at least 5 years of a nearly continuous uplift of the caldera floor (Lu et al., 2005 and references therein). The deformation was interpreted as the result of gradual pressurization of a shallow magma reservoir by repetitive magma inputs at a depth of 3.6 km, which agrees well with our petrologic estimates. Our ongoing petrological experiments will be used to further constrain the pre-eruptive conditions for the 1997 Okmok basalt and to estimate probable magma ascent rates during the eruption.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3618 Magma chamber processes (1036)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005