HR: 11:35h
AN: V42A-06    [Abstracts]
TI: Pore Structure of Pumice: Comparison Between Laboratory Measurements and X-Ray Tomographic Image Analysis
AU: * Wright, H M
EM: hwright1@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272
AU: Roberts, J
EM: roberts17@llnl.gov
AF: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550
AU: Cashman, K V
EM: cashman@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272
AB: The efficiency of syn-eruptive magmatic vesiculation and gas escape influences the ability of a volcano to accumulate pressure and therefore influences the dynamics and style of eruption. We examine the pore structure preserved in pumice to learn more about the vesiculation and degassing behavior of silicic magma by combining laboratory measurements of porosity, permeability, and electrical conductivity (a measure of tortuosity) with analyses of 3-D X-ray tomographic images to better understand controls on permeable flow. Previous work has demonstrated a general power-law relationship of increasing permeability with porosity in volcanic clasts over porosities ranging from 2-85% and permeabilities of 10^-17 to 10^-11 m² (Darcian permeability) and 10^-14 to 10^-7 m (inertial permeability). However, at any given porosity there are several orders of variation in permeability. Other parameters that may affect permeability include pore aperture size, pore surface area, and pore geometry (crack-like vs. spherical shapes). We use both electrical conductivity measurements and 3-D images to constrain these variables. Pore microstructure is a term that encompasses vesicularity, vesicle size distribution, vesicle shape, vesicle connectivity, and pore aperture size. We have imaged several crystal-poor pumice samples with a range of vesicle textures using x-ray tomography. These samples include calc-alkaline rhyolitic tube pumice from pyroclastic flow and airfall deposits, dacitic pumice from pyroclastic flow deposits, vesicular obsidian from lava flows, and alkaline rhyolite from a pumice cone. Tomographic images show that vesicles are quite heterogeneous in size (um to mm diameter bubbles) and shape (e.g. 0.5 mm wide shear deformation bands in the rhyolitic airfall sample) in most samples. Only the obsidian dome sample shows relatively homogeneous pore structure. Electrical conductivity measurements of fluid-saturated porous rocks are sensitive to the interconnectivity and tortuosity of the pore space and thus provide additional information that also relates to permeability. We measured electrical conductivity, determined the formation factor, and measured permeability to investigate the relationships between permeability and tortuosity using cores oriented perpendicular and parallel to macroscopic vesicle elongation. In these samples, there is large anisotropy in tortuosity and coincident large (order of magnitude) variations in inertial permeability. We expand the range of porosities for tortuosity measurements of volcanic clasts in general and find that tortuosity values range from 1 to 65. Tortuosity generally increases with increasing porosity (with the exception of strongly anisotropic samples). It is clear that the pore structure of pumice clasts varies widely and is a complex function of many different size and shape parameters. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract W-7405-ENG-48 and supported specifically by Laboratory Directed Research and Development funding.
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
DE: 8400 VOLCANOLOGY
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005