10:45 AM - 12:15 PM
[SVC29-P13] Permeability and pore microstructure of the dictytaxitic part of the Ogurayama lava dome, Towada volcano
Keywords:Permeability, Porosity, Evaporation-condensation, Degassing, Gas pycnometer
Lava dome formation is a common eruption style in subduction zones. As it often accompanies Vulcanian explosion and Merapi-type pyroclastic flow, it is important to understand the permeability of dome lavas that control the explosivity and excess gas pressure in bubbles. In the Ogura-yama lava dome, Towada Volano (7.6cal kyr BP; Kudo, 2010), the dictytaxitic interior is widely exposed from SSE to NNE of the dome. Sakurai et al.(2019 JpGU) examined the formation conditions of this texture in the hydrothermal experiments, and Nakamura et al.(2022 JpGU) reported the preliminary value of the normalized permeability. In this presentation, we discuss rearrangement of the melt component and degassing pathways based on the occurrence in the outcrop, permeability measurements, and pore structure characterization with pycnometer.
The outcrop surface at which we corrected the samples was assumed to consist of the cooling joint, as it was flat and smooth. The surface and other joint surfaces were purplish-red, while the fracture surface of the lava exhibited grayish color. The connected and total porosity and permeability of the grayish samples were about 30–34%, 32–36%, and 4.78×10-15~9.65×10-15m2, while those of the reddish samples were 23–27%, 27–30%, and 1.22×10-15~2.68×10-15m2. Namely, the gray samples had statistically significantly higher porosity and permeability than the reddish samples. These absolute permeabilities against total porosity are lower than those reported for the tuffasive veins, which are assumed as a degassing pathway of the lava dome (Kendrick et al., 2016), at least by two orders of magnitude. However, if dictytaxitic texture is formed pervasively in the lava dome, its effect on the dome outgassing should not be negligible. The isolated porosity is very low, supporting the idea that the low absolute permeability is due to the small pore throat radius (i.e., size effect; Nakamura et al., 2022).
The dictytaxitic structure's porosity is determined by the initial bubble volume fraction in the magma and by later evaporation-condensation processes. Because the reddish and grayish parts correspond to the joint surface and lava interior, the porosity difference should be originated from the latter processes. Namely, evaporation was suppressed in the reddish part, which was cool more rapidly, or condensation from the gas supplied through the cooling joint proceeded more extensively. We plan to address this issue with FE-SEM-EDS analyses and construct a model for porosity-permeability development.
The outcrop surface at which we corrected the samples was assumed to consist of the cooling joint, as it was flat and smooth. The surface and other joint surfaces were purplish-red, while the fracture surface of the lava exhibited grayish color. The connected and total porosity and permeability of the grayish samples were about 30–34%, 32–36%, and 4.78×10-15~9.65×10-15m2, while those of the reddish samples were 23–27%, 27–30%, and 1.22×10-15~2.68×10-15m2. Namely, the gray samples had statistically significantly higher porosity and permeability than the reddish samples. These absolute permeabilities against total porosity are lower than those reported for the tuffasive veins, which are assumed as a degassing pathway of the lava dome (Kendrick et al., 2016), at least by two orders of magnitude. However, if dictytaxitic texture is formed pervasively in the lava dome, its effect on the dome outgassing should not be negligible. The isolated porosity is very low, supporting the idea that the low absolute permeability is due to the small pore throat radius (i.e., size effect; Nakamura et al., 2022).
The dictytaxitic structure's porosity is determined by the initial bubble volume fraction in the magma and by later evaporation-condensation processes. Because the reddish and grayish parts correspond to the joint surface and lava interior, the porosity difference should be originated from the latter processes. Namely, evaporation was suppressed in the reddish part, which was cool more rapidly, or condensation from the gas supplied through the cooling joint proceeded more extensively. We plan to address this issue with FE-SEM-EDS analyses and construct a model for porosity-permeability development.