Abstract
The geologic and petrologic study of the Kattadake pyroclastics (around 10 ka) from the Zao volcano (NE Japan) revealed the structure of the magma plumbing system and the mixing behavior of the shallow chamber. The Kattadake pyroclastic succession is divided into lower and upper parts by a remarkable discontinuity. All rocks belong to medium-K, calc-alkaline rock series and correspond to ol-cpx-opx basaltic-andesite to andesite with 20–28 vol% phenocrystic modal percentage. All rocks were formed by mixing between andesitic magma and near aphyric basalt. The petrologic features of andesites of lower and upper parts are similar, 59–61 wt% SiO2, having low-An plagioclase and low-Mg pyroxenes, with pre-eruptive conditions corresponding to 960–980 °C, 1.9–3.5 kb, and 1.9–3.4 wt% H2O. However, the basalts were ca. 49.4 wt% SiO2 with Fo~84 olivine in the lower part and 51.8 wt% SiO2 with Fo~81 olivine and high-An plagioclase the in upper one. The percentage of basaltic magma in the mixing process was lower, but the temperature of the basalt was higher in the lower part than the upper one. This means that the shallow magma chamber was reactivated more efficiently by the hotter basalts and that the mixed magma with a 70–80% of melt fraction was formed by a smaller percentage of the basaltic magma.
Highlights
IntroductionThe overpressure caused by the intrusion of the new magma into the magma chamber is sufficient to trigger the eruption (Izbekov et al 2004 [5]; Patia et al 2017 [6]), while in others, an additional contribution (e.g., exsolution of volatiles) is required to reach the eruption conditions (Eichelberger 1980 [7]; Synder 2000 [8])
The intrusion of deeper and hotter magma into a shallow magma chamber is the cause of such a process, which may evolve from mingling to mixing depending on the time the two magmas are in contact before the eruption (Murphy et al 2000 [3]; Plail et al 2018 [4])
Whole-rock major element and trace element (Rb, Sr, Ba, Zr, Y, Nb, V, Cr, and Ni) concentrations were determined by X-ray fluorescence (XRF) analysis with a Rigaku RIX2000 analyzer (Rigaku, Tokyo, Japan) at Yamagata University
Summary
The overpressure caused by the intrusion of the new magma into the magma chamber is sufficient to trigger the eruption (Izbekov et al 2004 [5]; Patia et al 2017 [6]), while in others, an additional contribution (e.g., exsolution of volatiles) is required to reach the eruption conditions (Eichelberger 1980 [7]; Synder 2000 [8]) This may require keeping the two magmas in contact a certain time, during which they may allow chemical diffusion and facilitate magma mixing (Cooper 2018 [9]; Nishi et al 2019 [10]). The detailed petrologic examination of erupted mixed rock provides important information about the processes of the reactivation of the shallow chamber and mixing (Singer et al 2016 [11]; Morgado et al 2019 [12])
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