Japan Geoscience Union Meeting 2019

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC38] Active Volcanism

Mon. May 27, 2019 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Yuta Maeda(Nagoya University), Takahiro Miwa(National research institute for earth science and disaster prevention), Takeshi Nishimura(Department of Geophysics, Graduate School of Science, Tohoku University)

[SVC38-P09] Eruption processes of the 3rd and 4th stage of Niigata-Yakeyama Volcano, Japan, inferred from microlite texture and chemical composition

*Takumi Yoshida1, Takeshi Saito1 (1.Shinshu University)

Keywords:Niigata-Yakeyama, Yakeyama lava flow, Otani pyroclastic flow, Maeyama lava flow, Hayakawa pyroclastic flow, Plagioclase microlite

Niigata-Yakeyama volcano, which belongs to the Myoko volcano group, is one of the most active and youngest volcanoes in Japan. In the 3rd stage of Yakeyama activity, Maeyama lava flows and Hayakawa pyroclastic flows were generated to the northern slope of the volcano. In the 4th stage activity, Yakeyama lava flows, which forms present lava dome at the summit, and Otani pyroclastic flows were effused. Otani pyroclastic flow deposit contains well-vesiculated lavas, while lavas from Hayakawa pyroclastic flow are poorly-vesiculated. In this study, groundmass textural analysis of the eruptive products at the 4th stage was carried out. Chemical composition of plagioclase phenocrysts and microlites of the eruptive products at the 3rd and 4th stage were analyzed in order to examine the eruption process of the 3rd and 4th stage of Yakeyama volcano.

Microlite number density (MND) and crystallinity of plagioclase in Otani pyroclastic flow deposits were obtained from groundmass textural analysis. Our results indicated that Otani pyroclastic flows showed lower MND and intermediate crystallinity compared with Hayakawa pyroclastic flow deposits. As a result of chemical composition analysis, phenocryst in the samples of the 3rd stage activity shows An#(Ca/Ca+Na) =0.65~0.84. Microlite of andesite of Maeyama lava flow has an average An# of 0.61, which is lower than that of dacite lava of Maeyama Lava flow (average An# =0.54). Phenocryst of the 4th stage activity has An# =0.45~0.89. Microlite of Yakeyama lava flow and Otani pyroclastic flow have average An# of 0.63 and 0.62, respectively. The An# of plagioclase microlite becomes lower with decreasing crystal size.

Plagioclase microlite becomes lower An# with higher degree of H2O exsolution due to decompression (e.g. Nakamura, 2011). Our results suggest that microlite in dacite lava of Maeyama lava flow has undergone high decompression compared with andesite lava of Maeyama lava flow. However, there were little difference between plagioclase microlite of lava flow and that of pyroclastic flow samples. Eruptive products of the 4th eruption have relatively homogeneous SiO2 composition. Plagioclase microlite of Otani pyroclastic flow deposit, showing lower MND and higher An# than Hayakawa pyroclastic flow, may be crystallized under a condition of lower undercooling. Another possibility is that rapid vesiculation and H2O exsolution may proceed before microlite crystallization and prevent from decreasing An#.