5:15 PM - 6:45 PM
[SVC30-P10] Magma system and conduit dynamics of large-scale eruptions caused by basaltic magma in Izu-Oshima
Keywords:Mafic magma, Explosive eruption, Izu-Oshima
Explosive eruptions such as Plinian and sub-Plinian ones are generally thought to be caused by silisic magmas. However, some explosive eruptions caused by mafic magmas have also been reported (Foshag and González, 1956; Williams, 1983; Miyaji, 1984; Walker et al., 1984; Coltelli et al., 1998; Perez and Freundt, 2006; Wehrmann et al., 2006; Pioli et al., 2008). Izu-Oshima volcano, an active volcano erupting mainly basaltic and basaltic andesitic magmas, has caused explosive large-scale eruptions with erupted mass of more than 108 tons approximately every 150 years (Nakamura, 1964). The last large-scale eruption in Izu-Oshima is the An’ei eruption (Y1 eruption) that started in 1777. Y1 eruption is divided into several events, and the climax stage accompanying the most explosive event occurred approximately a year after the onset of Y1 eruption (Ikenaga et al., 2023). In this study, the ejecta of mainly Y1 eruption, but also of other large- and middle-scale eruptions, was analyzed to discuss the magma system and conduit dynamics causing explosive large-scale eruptions in Izu-Oshima.
Whole-rock compositions, core compositions of plagioclase phenocrysts, and melt compositions of the deposits from Y1 eruption were analyzed. While the core compositions of plagioclase phenocrysts and the melt compositions are constant through the eruption, whole-rock Al2O3 content increases as the eruption progresses. This increase of whole-rock Al2O3 content correlates with the increase of the volume percentage of plagioclase phenocrysts. Whole-rock Al2O3 content increases stepwise between the different eruptive events. The analyses of the whole-rock compositions of older large-scale eruptions (Y2, Y3, Y4, and Y6 eruptions) show that whole-rock Al2O3 content increased during Y3 and Y4 eruptions like Y1 eruption.
The textures of the ejecta from Y1, Y2, Y4, and Y6 large-scale eruptions and the ejecta from the middle-scale Strombolian eruption at the summit A crater in 1986 were also analyzed. Although microlite crystallizations were observed in most of the large-scale eruptions, there were almost no microlites in Unit C of Y1 eruption, which is the deposit from the most explosive event during Y1 eruption. The measurements of bubble size distributions show that Unit C of Y1 eruption is richest in small bubbles. The size distributions of plagioclase phenocrysts in Y1 and 1986 eruptions indicate that the magma tends to become richer in large plagioclase phenocrysts as the volume percentage of plagioclase phenocrysts in magma becomes larger. Plagioclase phenocrysts in large-scale eruptions show almost no compositional zoning. On the other hand, plagioclase phenocrysts in 1986 eruption accompany low-An number rim.
Principal component analysis was conducted for the whole-rock compositions of large-scale eruptions, some of which were from not single but multiple stratigraphic levels. The result of principal component analysis indicates that shallow aphyric and deep porphyritic magmas are stored beneath Izu-Oshima volcano as already discussed by Kuritani et al. (2018). It is also indicated that the deep porphyritic magma has been gradually differentiated, opposed to previous works describing that the shallow aphyric magma has been differentiated. Considering that the magma of Izu-Oshima is assumed to be rich in water (3–5 wt.%) (Hamada et al., 2011) and the size distribution of plagioclase phenocrysts, plagioclase phenocrysts may have been removed by sinking in the aphyric reservoir. There are two possibilities that the deep magma was originally porphyritic and that plagioclase phenocrysts were crystallized by decompression during magma ascent from deep to shallow reservoir.
Explosive eruptions in Izu-Oshima can be classified into two types. One type is that microlites are crystallized in the groundmass. In such eruptions, it is highly possible that microlites increase the viscosity of magma. The other type is that explosive eruptions are caused by deep magma rich in plagioclase phenocrysts. In such eruptions, large volume of porphyritic magma ascends rapidly from the deep part of magma system making the eruption more explosive.
Whole-rock compositions, core compositions of plagioclase phenocrysts, and melt compositions of the deposits from Y1 eruption were analyzed. While the core compositions of plagioclase phenocrysts and the melt compositions are constant through the eruption, whole-rock Al2O3 content increases as the eruption progresses. This increase of whole-rock Al2O3 content correlates with the increase of the volume percentage of plagioclase phenocrysts. Whole-rock Al2O3 content increases stepwise between the different eruptive events. The analyses of the whole-rock compositions of older large-scale eruptions (Y2, Y3, Y4, and Y6 eruptions) show that whole-rock Al2O3 content increased during Y3 and Y4 eruptions like Y1 eruption.
The textures of the ejecta from Y1, Y2, Y4, and Y6 large-scale eruptions and the ejecta from the middle-scale Strombolian eruption at the summit A crater in 1986 were also analyzed. Although microlite crystallizations were observed in most of the large-scale eruptions, there were almost no microlites in Unit C of Y1 eruption, which is the deposit from the most explosive event during Y1 eruption. The measurements of bubble size distributions show that Unit C of Y1 eruption is richest in small bubbles. The size distributions of plagioclase phenocrysts in Y1 and 1986 eruptions indicate that the magma tends to become richer in large plagioclase phenocrysts as the volume percentage of plagioclase phenocrysts in magma becomes larger. Plagioclase phenocrysts in large-scale eruptions show almost no compositional zoning. On the other hand, plagioclase phenocrysts in 1986 eruption accompany low-An number rim.
Principal component analysis was conducted for the whole-rock compositions of large-scale eruptions, some of which were from not single but multiple stratigraphic levels. The result of principal component analysis indicates that shallow aphyric and deep porphyritic magmas are stored beneath Izu-Oshima volcano as already discussed by Kuritani et al. (2018). It is also indicated that the deep porphyritic magma has been gradually differentiated, opposed to previous works describing that the shallow aphyric magma has been differentiated. Considering that the magma of Izu-Oshima is assumed to be rich in water (3–5 wt.%) (Hamada et al., 2011) and the size distribution of plagioclase phenocrysts, plagioclase phenocrysts may have been removed by sinking in the aphyric reservoir. There are two possibilities that the deep magma was originally porphyritic and that plagioclase phenocrysts were crystallized by decompression during magma ascent from deep to shallow reservoir.
Explosive eruptions in Izu-Oshima can be classified into two types. One type is that microlites are crystallized in the groundmass. In such eruptions, it is highly possible that microlites increase the viscosity of magma. The other type is that explosive eruptions are caused by deep magma rich in plagioclase phenocrysts. In such eruptions, large volume of porphyritic magma ascends rapidly from the deep part of magma system making the eruption more explosive.