HR: 10:20h
AN: V12B-01    [Abstracts]
TI: Segregating Gas from Melt: an Experimental Look at Ostwald Ripening in Rhyolitic Magma
AU: * Lautze, N C
EM: nlautze@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025,
AU: Sisson, T W
EM: tsisson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025,
AU: Mangan, M T
EM: mmangan@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025,
AU: Hankins, W B
EM: bhankins@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025,
AB: Magmatic degassing is a critical process that controls explosivity of volcanic eruptions, volcano deformation, and long-period seismicity. Degassing is promoted by increasing bubble size, which is attained through 1) decompressional expansion, 2) coalescence, and/or 3) Ostwald Ripening. Although rarely considered, ripening may be an important mechanism of bubble growth and segregation in viscous magmas during sub-volcanic storage. A series of piston-cylinder experiments was conducted to examine the influence of ripening on the segregation of CO₂-rich gas from crystal-free rhyolitic magma residing at mid to upper crustal levels. Experiments were run at 400 MPa and 850°C, using a sample from Panum crater (Long Valley, CA) with added ~3 wt% CO₂, ~2 wt% H₂O, and run times of 1 day, and 1, 2, and 4 weeks. Nucleation of gas bubbles occurred in each experiment; however there was little to no migration or coalescence. Instead, each sample showed a progressive increase in bubble size and decrease in bubble number density due to ripening. Sample vesicularity ranges from ~ 15-20%, number density from ~10⁴-10⁵ mm^-3, and bubble size from ~1-150 microns. These quantitative data were obtained through image processing of 2 dimensional images of sample cross sections. 3-dimensional x-ray microtomography images for the samples have been obtained at the Advanced Light Source; Lawrence Berkeley Lab. Current work to process such images will be commented on. Preliminary data are in good agreement with theory for diffusive-controlled ripening, which predicts average radius increases at a rate of t^1/3 and number density decreases proportional to t^-1. The trend for our data suggests that bubble radius will increase one order of magnitude in 100 years, which, according to Stokes Law, translates to a two order of magnitude increase in buoyant rise velocity. Ripening therefore appears to be an effective mechanism in generating buoyancy-driven migration of bubbles to the top of reservoir, where gases could leak from the magma to generate seismicity and/or feed hydrothermal systems. These data also show that experiments designed to investigate coalescence on bubble size distributions should take into account bubble growth due to ripening.
DE: 1036 Magma chamber processes (3618)
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3630 Experimental mineralogy and petrology
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting