HR: 0800h
AN: V31D-0656    [Abstracts]
TI: Crystal Nucleation and Growth in Mount Unzen Dacite Decompression Experiments
AU: * Almberg, L D
EM: ftlda@uaf.edu
AF: Alaska Volcano Observatory - Geophysical Institute, University of Alaska Fairbanks 903 Koyukuk Dr, 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 Dr, Fairbanks, AK 99775 United States
AU: Eichelberger, J C
EM: eich@gi.alaska.edu
AF: Alaska Volcano Observatory - Geophysical Institute, University of Alaska Fairbanks 903 Koyukuk Dr, Fairbanks, AK 99775 United States
AB: Central to understanding eruption dynamics is the interplay of decompression and degassing, which triggers crystal nucleation and growth. Microlite and microphenocryst textures are an often-used tool to decipher the rates of magma ascent for specific eruptions. It is critical to determine the depth and time scale at which these processes take place to fully understand the system behavior. Conduit material retrieved from 1500 m depth from the USDP-4 at Mount Unzen, Japan provides a snap shot of a viscous magma en route to the surface and is a perfect counterpoint to compare with laboratory experiments under controlled P, T, and XH₂O conditions. The core samples were identified as representing material from the 1991-1995 eruption based upon their elemental and isotopic composition and coincidence with a temperature maximum and alteration minimum (Nakada et al, ICDP Symposium, Potsdam, 2005). Three plagioclase crystal populations coexist in the spine emplaced at the conclusion of the eruption sequence in 1995, microlites (<20 μm), microphenocrysts (20-100 μm) and phenocrysts (>100 μm). Only phenocrysts and microlites are present in the samples extracted from 1500 m during drilling of the USDP-4 core. These textural differences are the focus of decompression experiments, with the purpose of replicating shallow level crystallization that may have occurred between 1500 m depth and the surface. It is possible that the microphenocrysts present in the dome lavas and absent in the conduit core could have formed at very shallow levels during magmatic ascent. Our experimental work delineates the role of decompression in controlling crystal size distributions in Unzen dacite, for comparison with the natural dome lavas and USDP-4 core samples. We conducted isothermal (870 ± 3°C) single and multi-step decompression experiments, equilibrated at 40 ± 3 MPa under water saturation and NNO conditions, and decompressed to 7.5 ± 0.5 MPa or 318 m depth. We ran such experiments for varying hold times (0.5-166 hr) using starting materials of Unzen spine material fused at 1000°C. After 24 hours, microphenocrysts formed and grew to a length of 52 μm during decompression from 40 MPa to the 7.5 MPa ending pressure, indicating that this is approximately the time required for their formation in the spine lava.
DE: 8400 VOLCANOLOGY
DE: 8425 Effusive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005