[S-VC29] Volcanic and igneous activities, and these long-term forecasting

[SVC29-P04] Lava dome-forming eruptions in Haruna volcano-Comparison between 4 eruptions at 45-10ka and Futatsudake-Ikaho eruption

*Yuki Suzuki¹, Yurika Toeda¹, Shinya Kimura¹, Rei Tanaka¹ (1.Department of Earth Sciences, Faculty of Education and Integrated Arts of Sciences, Waseda University)

Keywords:Haruna volcano, lava dome, enclave, mush-like felsic magma, magma mixing

We have conducted comprehensive petrological study for four lava dome-forming eruptions (Haruna-Fuji, Jyagatake, Somayama and Mizusawayama) that took place in 45~10 ka in Haruna volcano (Geshi and Takeuchi, 2012). We then compared the four eruptions with Futatsudake-Ikaho eruption (late 6th ~ beginning of 7th century; studied in Suzuki and Nakada, 2007), to clarify the temporal changes and common points of magma plumbing system and eruption triggering process in activity of “younger Haruna volcano” since 45 ka (Suzuki et al., in review). Each of older four eruptions did not have explosive phase including the generation of pyroclastic flow, while Futatsudake-Ikaho eruption started with Plinian phase and ended with emplacement of lava dome. The compared data include whole-rock composition, phenocryst composition and volume. Temperature of magma was estimated by applying method of Putirka (2016) to amphibole phenocrysts. The lava samples of the older four eruptions (Haruna-Fuji, Jyagatake, Somayama and Mizusawayama) have enclave parts (SiO₂ 50.9-55.1 wt.%) and host parts (SiO₂ 59.5-64.5 wt.%). All of these are products of mixing of two endmember magmas. Bulk composition and assemblage and composition of crystal phases in felsic endmember magmas are the same among four eruptions, while bulk composition of mafic endmember magmas varies slightly with eruption. The felsic magma had SiO₂ 63 wt.% or more and a temperature of 760-860C, and contained > 60 vol.% of orthopyroxene, amphibole, plagioclase, quartz, and Fe-Ti oxides. The mafic magma had SiO₂ 48-51 wt.% and contained 0-10 vol.% of olivine. The common point between the older four eruptions (Haruna-Fuji, Jyagatake, Somayama and Mizusawayama) and Futatsudake-Ikaho eruption is the similarities of endmember magmas involved in the eruptions. The felsic magmas were mush-like with bulk compositions of silicic andesite to dacite, while the mafic magmas were aphyric or nearly aphyric with bulk compositions of basalt to basaltic andesite. These suggest that structure of magma plumbing system and eruption triggering process have been basically unchanged in past 45,000 years. The mixing ratios of mafic magma are generally small for all of five eruptions (except enclaves rarely found in Haruna-Fuji and Mizusawayama), which is probably because mafic magmas were stored in deeper place than the felsic magmas having high crystallinity. In all of five eruptions, the mush-like felsic magma is thus the main source of ejecta. The ejection of such high-viscosity felsic magma requires reduction of viscosity through interaction with mafic magma. In Futatsudake-Ikaho eruption, vent-opening by the low-viscosity magmas that resulted from mixing and heating was followed by eruption of original felsic magma (Suzuki and Nakada, 2007). On the other hand, the lavas of older four eruptions (Haruna-Fuji, Jyagatake, Somayama and Mizusawayama) are all products of magma mixing, and it seems that each of four eruptions ended at the stage corresponding to the earliest stage of the Futatsudake-Ikaho eruption. A detailed comparison of the felsic endmembers of five eruptions reveals that the mineral phases are similar, but quartz is lacking in Futatsudake-Ikaho eruption. The absence of quartz in Futatsudake-Ikaho eruption is consistent with its less-evolved bulk composition (60.0-61.5 wt.% in SiO₂) and slightly higher temperature (793-864C) than those of older four eruptions. The datasets obtained in this study will be useful in future eruptions, in assessing the magmas involving in those eruptions and confirming the presence or absence of juvenile material in volcanic ash samples.