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Melting of hydrothermal system: Petrology of glassy-like bombs in 1895 eruption of Zao Volcano, NE Japan

Masao Ban1, Mutsumi Matsuoka1, Takumi Imura1, Kae Tsunematsu1, Motohiro Sato1, Akiko Matsumoto2, Mitsuhiro Nakagawa2

  • Affiliations: 1Faculty of Science, Yamagata University, Yamagata, Japan; 2Department of Earth and Planetary Science, Hokkaido University, Sapporo, Japan

  • Presentation type: Talk

  • Presentation time: Friday 11:00 - 11:15, Room R290

  • Programme No: 1.8.2

  • Theme 1 > Session 8

Abstract

Glassy-like bombs of 1895 hydrothermal phreato-magmatic eruption of Zao Volcano were petrologically examined. The 1895 eruption products are mainly volcanic breccia composed of mainly altered blocks of older lavas with minor amount of juvenile scoria and glassy-like bombs in a hydrothermally altered whitish gray colored matrix. The scoria is ol bg. cpx opx andesite (ca. 57--58 % in silica content), showing hyalo-ophitic groundmass texture. The glassy-like bombs have cores of dense andesite (<ca. 20 cm). The glassy-like parts cover and sometimes intrude into the core andesites. The core andesites are the same as the juvenile scoria in composition. The glassy-like parts are less porphyritic two px andesite to dacite, having glassy-like groundmass, and are sometimes vesiculated. Disaggregated andesites are observed, that indicates most phenocrysts would derive from core andesites. The glassy-like groundmass is divided into color-less, light brownish colored, and brownish colored parts. These three parts are composed of glass, glass-microlite (plagioclase, cordierite, and osumilite) mixture, and glass-nanolite (almino-silicate) mixture, respectively. The whole rock compositions of the glassy-like bombs are trachyandesitic to trachytic in composition (59--68% in silica content; K and Al-rich, Ca-poor) that cannot be explained by fractional crystallization nor melt accumulation from the andesitic composition of juvenile scoria. K and Al-rich character indicates the melting of hydrothermally altered rocks. It is probable that some parts of the altered rocks in the hydrothermal system were melted by the injection of the andesitic magma. Thereafter the melts assimilated the solidified andesites and erupted as the glassy-like bombs.