HR: 11:05h
AN: V42A-04 INVITED     [Abstracts]
TI: Experimental Constraints on the Development of Permeability in Bubbly Magma
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: Coalescence of bubbles, and the resulting permeability in magma, is a critical part of magma degassing, because it controls whether magma retains gas and explodes violently or loses gas and effuses passively. This study uses hydrothermal experiments to study the dynamics of bubble interaction and development of long-range connectivity in silicate melts. The melts are hydrated at high pressure and temperature, and then subjected to an initial pressure drop to nucleate bubbles. Once that set of bubbles equilibrate, the bubbly melt is decompressed further at controlled rates to lower pressure and then quenched. Bubble sizes, shapes, and three-dimensional interactions are then measured. In some cases the temperature of the experiment is lowered before the second decompression to raise viscosity of the silicate melt. Three-dimensional Computed Tomography (CT) images reveal that when viscosity is low (<10⁶ Pa s) individual bubbles separated by tens of microns can coalesce at porosities <~40 vol.%. Long-range chains of connected bubbles (more than several bubbles long) are absent, however, and thus permeability is zero. Above 40 vol.%, multiple bubbles connect to form long chains of continuous gas pathways, leading to permeability of the bubbly melt. CT images reveal that although these chains are tube-shaped, they are connected to other tubes in a 3D network. Porosity is highly variable after bubble tubes formed, and bubble sizes are more diverse, because some bubbles are isolated from the tube network and remain small, even as pressure decreases further. In contrast, at viscosities of 10⁶-10⁸ Pa s we find that bubbles can remain separate and not coalesce even as walls between them thin to less than one micron. Most bubbles remain relatively spherical and uniform in size. Preliminary results suggest that long-range permeability remains low, and is possibly negligible, even when porosity reaches 65 vol.%. At low viscosities, long-range permeability develops in less than 2 minutes, whereas at higher viscosity it takes more than 5 times as long to develop. Higher melt viscosity thus hinders and slows bubble coalescence and the development of permeability in bubbly magma.
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
DE: 8434 Magma migration and fragmentation
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005