[S-CG54] Volcanoes in the sea

[SCG54-P09] Differentiation process of basaltic magma of Inamura-dake, Satsuma Iwo Jima

*Sayaka Yoshimura¹, Tetsuya Sakuyama¹ (1.Osaka Metropolitan University)

Keywords:Satsuma Iwo Jima

Kikai Caldera is a Quaternary volcano located about 40 km southwest of Cape Sata, the southernmost tip of Kyushu Island, and is divided into pre-caldera, caldera-forming, and post-caldera phases. Inamura-dake, located in the central part of Satsuma Iwo Jima, is a basaltic volcano that was active during the post-caldera stage. Inamura-dake started its activity at 3.9 ka and formed three lava flows (south lava, east lava, and Isomatsuzaki lava) and pyroclastic hills, and was active until 2.2 ka. However, the details of the formation and differentiation process of basaltic magma have not been clarified. In the Kikai caldera, it is known that there was bimodal volcanism during the post-caldera stage, in which both basaltic and rhyolitic magmas existed, and that there was mixing between the two. Therefore, it is important to clarify the formation process of basaltic magma in order to understand the volcanic activity of the Kikai caldera during the post-caldera stage. In this study, we conducted rock description, whole-rock chemical composition analysis using an X-ray fluorescence spectrometer, and mineral chemical composition analysis using an electron microprobe analyzer for samples collected from Satsuma Iwo Jima with the aim of clarifying the differentiation process of basaltic magma, especially at Inamura-dake volcano.Inamura-dake erupted in the order of Minami lava, Higashi lava and pyroclastic hills, and Isomatsuzaki lava, with SiO2 and Na2O increasing and MgO, FeO*, and CaO decreasing monotonically with time. Except for the Minami Lava, which was not sampled in this study, all the samples have more than 20 vol% of crystals, most of which are plagioclase. In addition, monoclinic pyroxene, orthopyroxene, olivine, and magnetite are present in a few vol%. The An# values of the plagioclase core are higher in the order of Higashi lava, pyroclastic scoria, and Isomatsuzaki lava. The pyroclastic scoria has brown and black lithic facies, and the boundary between the two is curvilinear. The compositions of the maceralites in each area are systematically different. Monoclinopyroxene and orthopyroxene macerals in the brown area show a bimodal distribution in the Mg# frequency distribution map, while those in the black area show a unimodal distribution. The plagioclase core has a wider An# distribution in the brown part than in the black part. These results suggest that magma mixing may have occurred immediately before the eruption of the pyroclastic scoria. On the other hand, the Higashi lava, which is thought to have erupted at about the same time, and the Isomatsuzaki lava of the later stage of the eruption do not show lithic heterogeneity as in the scoria samples. We used the least-squares method to investigate whether the total chemical composition range of the Inamura-dake basaltic samples can be explained by crystal fractionation, and found that the major elemental composition of each sample can be generated by fractionation of minerals contained in each other as crystals. The magma temperatures estimated using both pyroxene thermometers were 986±9°C for the eastern lava, 959±20°C and 1051±4°C for the scoria, and 985±20°C for the Isomatsuzaki lava.
Based on these results, the following magma differentiation process is considered. As the temperature of the southern lava decreased, a two-layered magma reservoir was formed, consisting of the eastern lava magma (986°C) at the center and low-temperature magma (959°C) in the scoria at the periphery. A small amount of high-temperature magma (1051°C) was supplied from the deeper part of the magma reservoir, and the magma erupted as if pushed out. At the same time, magma mixed with low-temperature magma from the periphery of the magma reservoir and the supplied high-temperature magma erupted and formed pyroclastic hills. Later, further differentiated magma erupted as Isomatsuzaki lava.