V13B-3117
Pre-eruptive Magma Processes of the Catastrophic Eruption of Toya Caldera, Japan

Monday, 14 December 2015
Poster Hall (Moscone South)
Akihiko Tomiya1,2, Yoshihiko Goto3, Tohru Danhara4 and Shanaka L de Silva2, (1)Geological Survey of Japan, AIST, Tsukuba, Japan, (2)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, (3)Muroran Institute of Technology, College of Environmental Technology, Graduate School of Engineering, Muroran, Japan, (4)Kyoto Fission-Track Co. Ltd., Kyoto, Japan
Abstract:
Catastrophic caldera-forming eruptions like the VEI 7, 112-115 ka Toya caldera eruption pose significant hazard and need to be better understood. We have established a revised stratigraphy for this eruption that now identifies a sequence of Toya 1A, 1B, 2, 3A, 3B, 4, 5A, 5B, 6A, and 6B in ascending order, where 1A to 4 are mainly ash flow, 5B and 6B are pumice flow, and 5A and 6A are lag breccia. Most of the pumice is white, homogeneous, nearly aphyric high-silica rhyolite, whereas gray heterogeneous pumice is dominant in unit 6B.

We analyzed phenocrysts from juvenile clasts from unit 6B using electron microprobe and LA-ICP-MS, and divided them into three types: type-A with sodic plagioclase (An10-13), ferrous opx (Mg# 25-28), quartz, and low-Mg magnetite (Mg/Mn=0.23-0.26); type-B with calcic plagioclase (An>76), magnesian opx (Mg#>60 and MnO<1wt%), cpx, and high-Mg magnetite (Mg/Mn>5.5); and type-C with intermediate plagioclase and opx, hornblende, intermediate magnetite, and ilmenite. Type-A phenocrysts are dominant, and have homogeneous compositions. They show no reverse zoning even in magnetite, indicating no magmatic disturbance until just prior to the eruption. Type-B opx shows high Ti, Ni and low Zn compositions, indicating more primitive melt. Type-C phenocrysts show intermediate characters, and variation in composition. White pumice contains virtually only type-A phenocrysts, whereas gray pumice contains all types.

We propose the following pre-eruptive magma dynamics: A main reservoir with homogeneous rhyolitic magma containing type-A phenocrysts was underlain by a more mafic magma with type-C phenocrysts and a hybridized magma interface between them. This stratified reservoir was intruded by a more primitive mafic magma with type-B phenocrysts just prior to the eruption. Rhyolitic magma was ejected during all phases of the eruption, whereas other magmas partly mixed during phase 6B, a short time after the beginning of caldera collapse (phase 5A).