[SVC38-P09] Eruption processes of the 3rd and 4th stage of Niigata-Yakeyama Volcano, Japan, inferred from microlite texture and chemical composition
Keywords:Niigata-Yakeyama, Yakeyama lava flow, Otani pyroclastic flow, Maeyama lava flow, Hayakawa pyroclastic flow, Plagioclase microlite
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#.