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[SVC32-P12] Permeability of dictytaxitic part of the Ogurayama lava dome, Towada volcano
Keywords:lava dome, degassing, permeability, dictytaxitic, cristobalite, evaporation-condensation
We corrected samples from an outcrop with a height of ~50 m in the south-southeast part of the cliff at the dome edge. Outcrops and samples show a light gray to dark gray, reddish-brown heterogeneity in appearance. We cut a slab ~1 cm thick out from a sample cross-section of ~15 × 10 cm and trimmed nine adjacent areas of about 9 mm cubes. EPMA analysis revealed many silica minerals and the interstitial pores of plagioclase laths 10-100 μm in length. On the surface of the silica mineral, concentric spiral growth with uniform step spacing was observed, indicating that the silica mineral grew from a low-density gas phase. The sample was highly inhomogeneous, and the part where the permeability was below the detection limit and the part where the permeability was very high (4.6 × 10-8 to 2.1 × 10-6 m2; porosity 15 to 33%) were adjacent to each other. The permeability against porosity was higher than the value of Tuffisite (Kendrick et al., 2016), which is regarded as a degassing pathway on the dome surface. It was ~2 to 4 orders of magnitude higher than the value of Kushnir (2016), indicating that the structure may be an important factor controlling the explosiveness of the lava dome. It has been experimentally confirmed that such a structure is formed in the evaporation-precipitation process of magma in the final stage of crystallization under the temperature and pressure conditions inside the lava dome (Sakurai et al. 2019JpGU). The highly porous dictytaxitic structure necessarily accompanies the impermeable groundmass with silica minerals. Therefore, silica precipitation will not reduce the permeability of the lava dome. Since volcanic ashes from active volcanoes such as Sakurajima often contain cristobalite, the evaporation-condensation process also contributes to the magma degassing of active volcanoes.