Long-term evolution of subduction-zone volcano: Geology, petrology and geochemistry of Rishiri Volcano, southern Kuril Arc

Abstract:

The eruption rate is one of the useful indicators of the long-term evolution of volcanic activity. It is well known that the activity of subduction-zone volcano typically continues for 10⁴-10⁵ years and the eruption rate changes during the activity (Yamamoto et al., 2018). Why does eruption rate change during a volcanic activity? What is the dominant factor in changing eruption rates? In order to clarify this issue, we focus on the activity since ~40 ka of the Rishiri Volcano, southern Kuril Arc. The volcanic activity after ~40 ka was divided into three stages: Middle, L-1, and L-2 stages (Ishizuka, 1999). The eruption rate of the Middle stage was the highest during the volcanic activity and decreased at the L-1 stage and further decreased at the L-2 stage. After the Middle stage in which calc-alkaline andesite magmatism was active, high Na/K alkali basalt to tholeiitic andesite and low Na/K alkali basalt occurred during the Late-1 and Late-2 stages, respectively. The calc-alkaline andesite in the Middle stage show petrological evidence of magma mixing between mantle-derived mafic magma and crust-derived felsic magma, and the water content of the primitive magma was estimated to be ~5 wt.% (Taniucni et al., 2020). The high Na/K basaltic magma in the L-1 stage is suggested to have produced the tholeiitic intermediate magmas by AFC in a crustal magma chamber with a low ratio of assimilated mass to fractionated mass (Kuritani et al., 2005). The water content of the primary magma is estimated to be ~3 wt% (Kuritani, 1999). The low Na/K basalt in the L-2 stage show primitive composition and the water contents of the primary magmas are estimated to be ~2 wt% (Kuritani and Nakagawa, 2016). The more hydrous the primary magma and its derivative magma, the more extensive the melting of the crust due to the higher water flux from the solidifying magma chamber. Therefore, we propose the water content of primary magma as one of the main factors that control the eruption rate.