HR: 16:30h
AN: V54B-03 INVITED    [Abstracts]
TI: The Effect of Strain Localization on the Rheology of Crystal-Rich Magmas
AU: * Caricchi, L
EM: luca.caricchi@erdw.ethz.ch
AF: ETH Zurich, Clausiusstrasse 25, Zurich, 8092, Switzerland
AU: Burlini, L
EM: burlini@erdw.ethz.ch
AF: ETH Zurich, Clausiusstrasse 25, Zurich, 8092, Switzerland
AU: Ulmer, P
EM: peter.ulmer@erdw.ethz.ch
AF: ETH Zurich, Clausiusstrasse 25, Zurich, 8092, Switzerland
AU: Faccenda, M
EM: manuele.faccenda@erdw.ethz.ch
AF: ETH Zurich, Clausiusstrasse 25, Zurich, 8092, Switzerland
AU: Gerya, T
EM: taras.gerya@erdw.ethz.ch
AF: ETH Zurich, Clausiusstrasse 25, Zurich, 8092, Switzerland
AB: The rheology of magmas containing more than 40 vol. % of crystals is of particular interest to constrain the physical conditions of magmas during the extrusion of volcanic domes and necks. In this study we have conducted series of simple shear experiments on synthetic crystal-bearing hydrous rhyolitic material and on natural rocks from the Monte Nuovo eruption (Phlegrean Fields, Italy) in a Paterson-type internally heated rock-deformation apparatus. The experiments covered a wide range of crystallinities (50-80 vol. %) and strain rates (10-6-10-3 s-1). All experimentally deformed samples lack yield strength but they exhibit a tendency to non-Newtonian and Binghamian behavior at relatively high (>10-4 s-1) strain rates. The experiments have been performed under temperature and strain rate conditions where the melt phase behaves Newtonian. Moreover, in the range of strain rates applied, viscous heating is unable to account for the observed shear thinning effects (decrease of viscosity with increasing strain rate). Consequently, the non-Newtonian effects must be due to the presence of crystals in the magma. In experiments were shear weakening was indeed observed, the samples exhibit macroscopic localization features that are oriented at around 30° with respect to the shear plane. In all the other experiments, microstructural observation of cross-sections of the samples revealed melt-enriched planes and crystals alignments with an orientation identical to the localization features observed in the samples that experienced shear thinning. Numerical simulations have been conducted that were designed to reproduce the experiments and were, therefore, computed with identical stress and strain rate conditions as the experiments. Samples from both, the experiments and the numerical simulations reveal the generation of melt-enriched bands oriented at around ±30 ° with respect to the direction of flow. The melt enrichment induces localization of the strain in these regions and leads to a geometrical redistribution of melt and particles that is responsible for the complex rheological behavior of crystal-rich magmas.
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting