[S-VC33] Mechanism of volcanic eruptions

[SVC33-P02] Textual analyses of the interstitial phases in the mushy gabbro: comparison between Fuji and Izu-Oshima volcanoes

*Kotone Nishigaya¹, Hidemi Ishibashi¹, megu otsuka¹, Nao Yoshida¹, Natsumi Hokanishi², ATSUSHI YASUDA² (1.Shizuoka University , 2.Earthquake Research Institute, the University of Tokyo)

Keywords:crystal mush, interstitial melt, Fuji volcano, Izu-Oshima volcano, magma reservoir

Recent volcanology suggests that the magma plumbing system is chiefly composed of crystal mush, a mixture of crystals >50vol% and interstitial phases (melt + bubble). The process of extracting interstitial melt from crystal mush is important not only for the accumulation of eruptible magma but also for the formation of plutonic rocks. However, we do not understand enough how the interstitial phase decreases to form a plutonic rock. The mushy plutonic xenoliths containing the interstitial phase sometimes found in volcanic products have information about this because they are quenched fragments of natural crystal mush. Nishigaya et al. (2024 VSJ fall meeting) analyzed the texture of the interstitial phases in the mushy gabbro xenoliths of the Fuji 1707 Hoei eruption and showed that the connectivity of the interstitial phase drastically decreased when the volume fraction of the interstitial phase, f, decreased below ~0.23, where the connectivity is defined as the ratio of the maximum cluster volume against the total volume of the interested phase. They also argued that the 3D network of the solid phase can be maintained up to f ~ 40 vol%. However, to verify whether these properties are general, comparisons with mushy plutonic xenoliths from other volcanoes and containing the interstitial melt of different compositions is required. The mushy plutonic xenoliths are also found in the ejecta of the Izu-Oshima 1986 B eruption (Yoshida et al., 2023 VSJ fall meeting). In this study, we analyzed the texture of the interstitial phase in the mushy plutonic xenoliths of the Izu-Oshima 1986 B eruption, which was studied petrologically by Yoshida et al. (2023), using FE-EPMA (JEOL JXA-8530FPlus) at the Earthquake Research Institute, University of Tokyo, to examine the relationship between f and the connectivities of the interstitial and the solid phases. Then the results were compared with those of the gabbroic xenoliths of Fuji volcano.
Otsuka et al. (2021 VSJ fall meeting) classified the mushy gabbro xenoliths from Fuji volcano into two groups; Groups A and B are characterized by interstitial glasses of relatively homogeneous, rhyolitic composition and heterogeneous compositions ranging from andesitic to dacitic. Microlite is almost absent in the interstitial phase. On the other hand, Yoshida et al. (2023) classified the plutonic xenoliths from the Izu Oshima 1986B eruption into four groups. The compositions of the interstitial glass were basaltic andesitic in Group-I, andesitic to dacitic in Group- II, and andesitic to rhyolitic in Groups-IIIA and IIIB. Plagioclase and pyroxene microlites were often found in the interstitial phases of these samples, and some crystals have visible overgrown rims.
In this study, the connectivity of the interstitial phase (CM₂) was quantified as the ratio of the sum of the volumes of the first and second largest clusters to f. The samples from Fuji volcano showed f and CM₂ of 24-39vol% and 67-86% for Group-A and 10-23vol% and 9.9-37% for Group-B, respectively. CM₂ was constant at around 75% when f >24vol%, but it decreased to < 21% when f < 21vol%. This result suggests that the percolation threshold for the interstitial phase is at around f ~ 23vol%. On the other hand, CM₂ decreased at a constant rate from 42% to 3.4% as f decreased from 31 vol% to 19 vol% for the samples from Izu-Oshima. The f for the onset of increasing the interstitial phase connectivity is ~10 vol% larger in Izu-Oshima than in Fuji volcano. This may be attributed to the differences in the composition of the interstitial melt. The connectivity of the solid phase linearly increases with decreasing f, and the relations for Fuji and Izu-Oshima volcanoes overlap. This result suggests that the relationship between f and the solid connectivity is general. In addition, since the solid connectivity remains >30% even when f>35 vol%, a 3D network of solid phase was maintained in these xenoliths, which enables them to erupt in a mushy state.