13:45 〜 15:15
[SVC34-P04] 八丈島・西山火山におけるマグマ供給系とマグマプロセス
キーワード：マグマ供給系、マグマ過程、活火山
Hachijojima is an active volcanic island located on the volcanic front of the Izu–Bonin arc. The northwestern part of the island is mainly made up of basaltic products of younger volcanism (< ~10 ka) that constitute Nishiyama volcano. Petrological and geochemical studies have been conducted on the Nishiyama volcanic products to understand the magma plumbing system and/or magmatic processes (Isshiki 1958, 1963; Nakano et al. 1991, 1997; Tsukui and Hoshino 2002; Ishizuka et al. 2008; Aizawa et al. 2020). These studies have suggested that whole-rock compositional variation of the volcanic products is primarily controlled by plagioclase fractionation or accumulation (‘plagioclase control’). Ishizuka et al. (2008) examined whole-rock compositional variations of the products of the Nishiyama volcano, as well as those of submarine volcanic products surrounding Nishiyama, and found that some submarine samples have primitive compositions. Recently, Ishizuka and Geshi (2018) presented a detailed geological map of Hachijojima Island, which enabled us to collect samples with good temporal resolution from the Nishiyama volcano. Using this new information, we conducted petrological and geochemical analyses of subaerial eruptive products from the Nishiyama volcano to understand the magma pluming system and pre-eruption magmatic processes, particularly those of the accumulation of plagioclase phenocrysts.
The phenocryst content of the Nishiyama samples is variable, ranging from 1–40 vol.%. The most abundant phenocryst is plagioclase, and the total amount of mafic phenocrysts (olivine, clinopyroxene, and orthopyroxene) is commonly less than 2 vol.%. In some plagioclase phenocrysts, a high-An core region (>An80) consists of a glass inclusion-poor inner core and an inclusion-rich outer mantle. Whole-rock major element compositions show significant variations (49.4–54.9 wt.% SiO2), and they can be largely divided into basaltic (< 53 wt.% SiO2) and andesitic samples (> 54 wt.% SiO2). The whole-rock Sr, Nd, and Pb isotopic compositions of samples from the youngest volcanic stage (Fujitozando stage; < 0.7 ka) are homogeneous. However, some samples from the older stage (Senjojiki stage; 3–1 ka) have relatively low Pb isotopic ratios, and they characteristically have lower La/Sm and Zr/Y ratios and lower K2O contents than those of the other main Nishiyama samples.
The homogeneity of the Sr, Nd, and Pb isotopic compositions of the volcanic products from the Fujitozando stage suggests that the magmas were derived from a single parental magma. However, slightly less radiogenic Pb isotopic compositions of some basaltic samples from the Senjojiki stage suggest that another parental magma with distinct compositions (i.e., lower La/Sm, Zr/Y, and Pb isotopic ratios) might have been involved in the Nishiyama magma system before the Fujitozando stage. Considering that the submarine primitive magmas from the Hachijo NW chain have low La/Sm and Zr/Y ratios and low K2O content (Ishizuka et al., 2008), the parental magma with less radiogenic Pb isotopic compositions might have been similar to the primitive Hachijo NW chain magma.
As discussed above, the compositional variations of the subaerial Nishiyama products were essentially established by crystal–melt separation, except for the involvement of the parental magma with less radiogenic Pb isotopic compositions. Therefore, there must have been a magma chamber in which differentiation from the parental primitive magma to the magmas of the subaerial volcanic products occurred. Two-pyroxene geobarometry suggests that the main magma chamber was located at a depth of 9–12 km. The estimated depth of 9–12 km coincides with the depth range over which earthquake swarms occurred in 2002 (Kimata et al. 2004).
The core region of some plagioclase phenocrysts consists of a glass inclusion-free inner core and an inclusion-rich outer mantle, suggesting that some plagioclase crystallized in the main magma chamber, which was followed by overgrowth during magma ascent because of increasing liquidus temperatures due to decompression-induced water exsolution from the melt. The whole-rock compositions of some eruption units with different Al2O3/MgO ratios exhibit distinct plagioclase-controlled trends, which negates the possibility that plagioclase accumulation occurred in a stable magma chamber. In addition, the density of plagioclase was higher than that of the melt during the magma ascent to the surface. From these observations, it is suggested that the accumulation of plagioclase phenocrysts occurred in ascending magmas as the plagioclase settled relative to the surrounding melt.
The phenocryst content of the Nishiyama samples is variable, ranging from 1–40 vol.%. The most abundant phenocryst is plagioclase, and the total amount of mafic phenocrysts (olivine, clinopyroxene, and orthopyroxene) is commonly less than 2 vol.%. In some plagioclase phenocrysts, a high-An core region (>An80) consists of a glass inclusion-poor inner core and an inclusion-rich outer mantle. Whole-rock major element compositions show significant variations (49.4–54.9 wt.% SiO2), and they can be largely divided into basaltic (< 53 wt.% SiO2) and andesitic samples (> 54 wt.% SiO2). The whole-rock Sr, Nd, and Pb isotopic compositions of samples from the youngest volcanic stage (Fujitozando stage; < 0.7 ka) are homogeneous. However, some samples from the older stage (Senjojiki stage; 3–1 ka) have relatively low Pb isotopic ratios, and they characteristically have lower La/Sm and Zr/Y ratios and lower K2O contents than those of the other main Nishiyama samples.
The homogeneity of the Sr, Nd, and Pb isotopic compositions of the volcanic products from the Fujitozando stage suggests that the magmas were derived from a single parental magma. However, slightly less radiogenic Pb isotopic compositions of some basaltic samples from the Senjojiki stage suggest that another parental magma with distinct compositions (i.e., lower La/Sm, Zr/Y, and Pb isotopic ratios) might have been involved in the Nishiyama magma system before the Fujitozando stage. Considering that the submarine primitive magmas from the Hachijo NW chain have low La/Sm and Zr/Y ratios and low K2O content (Ishizuka et al., 2008), the parental magma with less radiogenic Pb isotopic compositions might have been similar to the primitive Hachijo NW chain magma.
As discussed above, the compositional variations of the subaerial Nishiyama products were essentially established by crystal–melt separation, except for the involvement of the parental magma with less radiogenic Pb isotopic compositions. Therefore, there must have been a magma chamber in which differentiation from the parental primitive magma to the magmas of the subaerial volcanic products occurred. Two-pyroxene geobarometry suggests that the main magma chamber was located at a depth of 9–12 km. The estimated depth of 9–12 km coincides with the depth range over which earthquake swarms occurred in 2002 (Kimata et al. 2004).
The core region of some plagioclase phenocrysts consists of a glass inclusion-free inner core and an inclusion-rich outer mantle, suggesting that some plagioclase crystallized in the main magma chamber, which was followed by overgrowth during magma ascent because of increasing liquidus temperatures due to decompression-induced water exsolution from the melt. The whole-rock compositions of some eruption units with different Al2O3/MgO ratios exhibit distinct plagioclase-controlled trends, which negates the possibility that plagioclase accumulation occurred in a stable magma chamber. In addition, the density of plagioclase was higher than that of the melt during the magma ascent to the surface. From these observations, it is suggested that the accumulation of plagioclase phenocrysts occurred in ascending magmas as the plagioclase settled relative to the surrounding melt.