HR: 12:05h
AN: V22A-08    [Abstracts]
TI: The Influence of Grain Size and Crystal Content on Rheology and Deformation of Pyroclastic Material
AU: * Paquereau-Lebti, P
EM: paquerep@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-5506, United States
AU: Robert, G
EM: grobert@eos.ubc.ca
AF: Volcanology and Petrology Laboratory, Department of Earth & Ocean Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
AU: Grunder, A L
EM: grundera@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-5506, United States
AU: Russell, K J
EM: krussell@eos.ubc.ca
AF: Volcanology and Petrology Laboratory, Department of Earth & Ocean Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
AB: Pyroclastic deposits undergo variable degrees of sintering, viscous deformation of particles and loss of pore space, which combine to produce the dramatic textural variations that define welded facies. We here investigate the effects of grain size and crystal content on the rheology and welding of pyroclastic material.
Uniaxial deformation experiments were conducted using sintered cores of natural rhyolite ash under conditions consistent with welding. Experiments were done in the University of British Columbia Volcanology Deformation Rig (VDR). This apparatus is designed to run experiments relevant to volcanology, by supporting low-load, high temperature, deformation experiments (Quane et al., 2004). We ran experiments at constant displacement rate (2.5.10^-6 m.s-1), under ambient water pressure ("Dry"), at temperatures of 850 and 900°C and to maximal strain of 50%. Grain-size effect was investigated using sintered cores from three different sieving fractions of Rattlesnake Tuff (RST, Eastern Oregon, USA) ash: fine ash (grain size < 0.6 mm), coarse ash (0.6 to 2mm) and row unsieved ash. The effect of crystal content was explored using cores of sintered unsieved RST ash, variably enriched in crystals of feldspars and quartz.
Unsieved and fine ash cores suffered higher total porosity reduction than coarse ash cores during deformation experiments. For cores of unsieved ash, porosity loss is facilitated by mechanical compaction, which includes orientation and organisation of different size clasts to a compact assemblage, without any deformation of individual particles. Isolated porosity decreases faster than connected porosity in coarse and fine ash cores, whereas cores of raw ash mainly loose connected porosity. This is also consistent with mechanical compaction for cores of unsieved ash, in which isolated porosity of weakly deformed individual pumice clasts or glass shards is maintained. Increasing strain causes a reduction in porosity and correlates with increase in effective viscosity for all grain sizes. Cores of unsieved raw and fine ash, for which porosity reduction is highest, show the highest values and greatest increase in effective viscosity (10^11.2 – 10^12.4 Pa.s at 850°C for raw ash and 10^11.1-10^11.8 Pa.s for coarse ash).
Crystals reduce shard-shard contacts and thus sintering of glass shards. They act as a brake to welding. We found that >15% crystal content inhibited sintering in a sample that welded under the same experimental conditions when phenocryst depleted (phenocryst content around 1% in whole Rattlesnake Tuff ash).
Reference: Quane, S.L., Russell, J.K., and Kennedy, L.A. (2004). A low-load, high-temperature deformation apparatus for volcanological studies. American mineralogist, 89, 873-877.
DE: 5114 Permeability and porosity
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting