[S-VC28] International volcanology

[SVC28-04] Pheno-vesicle number density (PVND) as an indicator of overpressure in pre-eruptive magma chamber

*Indranova Suhendro^1,2、Atsushi Toramaru² (1.Department of Environmental Geography, Faculty of Geography, Universitas Gadjah Mada, Indonesia、2.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, Japan)

キーワード：pheno-vesicle, phenocryst, crystal size distribution, magma chamber, overpressure, explosive eruption

Magma chamber overpressure caused by oversaturation of preexisting volatiles (hereafter referred as pheno-vesicle) is considered as one of the most important factors for triggering explosive volcanic eruptions. However, such qualitative and quantitative information of pheno-vesicles (pheno-bubbles in volcanic rocks) which are preserved in juvenile materials (e.g., pumice and scoria) are not yet provided. Therefore, in this study, we present the first qualitative and quantitative study of pheno-vesicle by examining natural samples which were obtained from 18 volcanoes, ranging from the VEI (volcanic explosivity index) 3 to 8 eruptions and various magma compositions (basaltic andesite to rhyolite). Based on our 2D-vesicle size distributions (VSDs) data, we confirm that pheno-vesicle can be defined as vesicles with diameter larger than approximately 0.1 mm, regardless of magma compositions. We found that the number density of pheno-vesicle (PVND) in pre-eruptive magma chamber exhibit (1) strong positive correlation with VEI and SiO₂, and (2) weak negative correlation with crystallinity (φc). Particularly, mild-eruptions (VEI 3-5) have relatively small PVNDs value, less evolved chemical compositions, and higher crystallinity (avg. of 0.07 to 0.5x10¹¹ m^-3, 54.3-77.0 wt. % SiO₂, and 0.8-45.5 % φc, respectively) compared to super-eruptions (VEI 6-8; avg. of 0.7-1.6x10¹¹ m^-3, 58.8-78.5 wt. % SiO₂, and 3.3-35.8 % φc, respectively). Moreover, super-eruptions are characterized by the gentler phenocryst size distribution (CSD) slope compared to super-eruptions (0.9-4.1 and 3.5-6.4, respectively). This suggests that; a large, long-lived, and more differentiated magma chambers experience more intense second boiling thus results in higher supersaturation of volatiles and provides larger amount of overpressure.