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[SVC31-10] Volatile mass balance and permeability of conduit magmas leading to Vulcanian explosions at Sakurajima Volcano, Japan
Keywords:Vulcanian explosion, Sakurajima, conduit permeability
Sakurajima Volcano, Kyusyu, Japan, has been repeating vulcanian explosions, sometimes interposed with Strombolian and ash eruptions, since the late 20th century, providing a unique opportunity to study eruption mechanisms and preceding magma ascent processes. Eruptions occurred from the two craters, the Minamidake summit crater and the Showa crater in the east frank, with varying magnitudes from small ash emission to a gigantic Vulcanian explosion erupting ~0.4 M ton juvenile pyroclasts, although eruption characteristics are typical for Vulcanian explosion. Shintani et al. (2023 VSJ fall meeting) petrologically estimated the magma emplacement depths by applying the plagioclase geohygrometry to plagioclase rims and groundmass melts, and demonstrated that the larger eruptions tapped the deeper magmas in the conduit. This indicated that the conduit cross-sectional area of the respective crater is almost constant irrespective of the eruption magnitude within a factor of ~2. By dividing the daily magma intrusion rate that characterizes the eruption styles (Iguchi et al., 2019) by the calculated conduit diameter (83 and 1.7 m2 for the summit and Showa craters, respectively), we found that the magma ascent rates leading to the Vulcanian eruptions are 4.5×103 m/day and 6.0×103 m/day for the summit and Showa craters, respectively. This similarity demonstrates that the magma ascent rate is the primary controlling factor of eruption styles and provides an example of the ascent rate for Vulcanian explosions from andesitic volcanoes with an open conduit. In this study, we further carried out a mass-balance calculation of H2O originally contained in the main magma reservoir beneath the Aira caldera to constrain the ratio of vertical (along-conduit) and horizontal (across-conduit) outgassing flux and their empirical permeabilities. The important assumptions include 1) 48% of the along-conduit component escaped, and the rest formed the “gas pocket” whose overpressure caused the Vulcanian explosions (Kazahaya et al., 2016), and 2) the vertical gas flow is derived by the lithostatic pressure gradient, while the horizontal gas flow is derived by the pressure difference between lithostatic and hydrostatic pressure gradient, and 3) the horizontal gas flux is limited by the permeable flow across the marginal zone with 1m thickness. We calculated the following two cases: Case-1) the vertical/horizontal ratio of outgassing was constant throughout the entire conduit from the deep magma reservoir beneath the Aira caldera to the near-surface gas pocket, and Case-2) all the exsolved bubbles are outgassed at the shallow magma reservoirs beneath the Sakurajima. In Case-1, the calculation results revealed that 77–84% of the exsolved gas horizontally escaped to the country rock, and the rest vertically migrated, partly forming the gas pocket. In Case-2), the proportion of the horizontal outgassing decreased to 63–72% but still dominated in the conduit from the shallow magma reservoir to the gas pocket. We estimated the integrated along-conduit (vertical) permeability in Case-1) to be 8.5×10-14 – 1.2×10-13 m2, which is lower than the previous estimation for shallow conduits by orders of 1 – 7, whereas significantly smaller vertical permeability (10-19 – 10-20 m2) was obtained owing to the far larger side surface area of the conduit than the cross-sectional area. The large permeability anisotropy requires the across-conduit outgassing to be controlled not by magma permeability but by the other paths, such as chilled margin and conduit wall.