[S-VC31] Mechanism of volcanic eruptions

[SVC31-P03] Permeability evolution during the ductile-brittle transition in crystal- and gas bubble-bearing magma

*Takeru Ohata¹, Satoshi Okumura¹, Harumasa Kano² (1.Tohoku university, 2.Tohoku university museum)

Keywords:crystal- and gas bubble-bearing magma, gas permeability, ductile-brittle transition, Weissenberg number

Magma viscosity and gas permeability are thought to control eruption style and intensity. High-viscosity magma deforms viscously under low deformation rates, while it shows brittle deformation when the deformation rate is high. In the viscous regime, bubbles connect each other, resulting in the formation of permeable networks, and hence outgassing results in viscous compaction. In contrast, the cracking during brittle deformation causes an increase in the permeability. However, there are no previous experiments confirming the permeability change in the transition regime between viscous and brittle deformation. In this study, we performed uniaxial compression experiments for rhyolite magma including crystals (15~25 vol%) and gas bubbles (18~38 vol%) at temperatures of 810~954°C and wide ranges of strain rates, i.e., 10^-6.37~10^-1.36 s^–1, and measured gas permeabilities of the samples before and after the deformation. We observed both viscous and brittle deformation and the cracks formed in the regime of the brittle deformation. The results of permeability measurements exhibited that the permeability increased significantly when the magma showed brittle deformation under high strain rates, while it did not show a change of the permeability or slightly decreased under viscous deformation. Based on experimental results, we infer that magma permeability increases via cracking near the conduit wall where the strain rate is high, while the magma at the central part results in viscous compaction and a decrease in the permeability through outgassing.