Magma feeding system of the 1893 CE Meiji eruption of Azumayama Volcano, NE Japan

Abstract Azumayama Volcano is one of the most active stratovolcanoes in northeastern Japan, with a ca. 1.2 million-year eruption history. Its hazard status was raised to volcanic alert level 2 (restricted access to the crater) in 2018 and 2019 due to an increase in volcanic activity, including the number of volcanic earthquakes. The latest magmatic eruption of Azumayama was the 1893 CE. This was preceded by phreatic eruptions, and was followed by Vulcanian eruptions. The products of the magmatic eruptions are the most suitable candidates for revealing the magma feeding system and the magmatic processes beneath the volcano. This information is crucial in assessing the hazard of future eruptions, and the recent unrest makes it an urgent issue. The samples were collected from juvenile bombs in the vulcanian fall deposits of the 1893 CE Meiji eruption. The bombs were divided into darker gray and lighter gray groups (D-group; 58–59 and L-group; 60–63 wt% SiO 2 ), both of which have olivine-orthopyroxene-clinopyroxene-plagioclase phenocrysts. Whole rock compositions of all samples plot along the same linear trend on SiO 2 variation diagrams. The phenocrysts showed compositional variations (e.g., 60–83 Mg# for pyroxenes and An 51–95 for plagioclases). Based on the compositional and textural features, it was deduced that low-Mg pyroxenes and low-An plagioclases, Fo-rich olivine and Cr-spinel, and intermediate-Mg pyroxenes and intermediate-An plagioclase might be from felsic end-member, mafic end-member, and mixed magmas, respectively. Using pyroxene and olivine-spinel geothermometers, and the rhyolite-MELTS program, the felsic magma was estimated to be dacitic (64–66 wt% SiO 2 ), 860–900 °C, and stored at ca. 5–10 km, and the mafic magma was estimated to be basaltic (ca. 49 wt% SiO 2 and 1138–1166 °C), precipitating Fo-rich olivine and spinel at ca. 14–20 km in depth. Based on the petrological data combined with the geophysical data, the mafic magmas might have ascended from a deeper reservoir (> 20 km in depth). The shallow dacitic crystal mush body might have been repeatedly injected by the mafic magma, which resulted in the remobilization of the crystal mush body and the formation of mixed magmas. The intermediate magmas formed by the mixing would have been vertically heterogeneous. These three magmas were incompletely mixed during the eruption. The timescale from the injection to the eruption, estimated by diffusion modelling in the pyroxene phenocrysts, was several months to years. These results are important for assessing the hazard of future eruptions; for example, they may provide important information for geophysical monitoring and the development of evacuation plans. Graphical abstract