Pre-eruptive Magma Processes of the Catastrophic Eruption of Toya Caldera, 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).