著者

津久井 雅志 荒牧 重雄

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.35, no.2, pp.231-248, 1990-07-02

被引用文献数

5

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22,000 years ago, about 100 km^3 of magma erupted from the northrn end of Kagoshima Bay, southern Kyushu. The eruption produced 5 units of pyroclastic deposits; (1) 98 km^3 of airfall pumice (Osumi pumicde fall, OS), (2) 13 km^3 of oxidized, fine-grained Tsumaya pyroclastic flow (TSU), (3) Kamewarizaka breccia (KM) of the new vent-opening and enlargement stage, (4) 250 km^3 of Ito pyroclastic flow (ITO) at the climactic stage, (5) >50 km^3 of co-ignimbrite ash fall (AT ash). Phenocryst mineral assemblage throughout the whole sequence is ubiquitously plag+qtz+opx(Mg#45-60) +mt+il. One exceptional sample (ITO 11c) carries Fe-rich oliv (Fo 26-28) and cpx beside other phases. Fifty-five new XRF analyses of 10 major and 15 trace elements show that the majority of the erupted magma consisted of a remarkably homogeneous, high-silica rhyolite with SiO_2 74-76.5%(H_2O free and 100% normalized). The maximum fluctuation found both in major and trace elements is ±40%. These variations can be explained by the crystal-liquid separation near the roof of the magma reservoir. Mt-il temperatures and opx-mt-qtz pressures show narrow ranges, i e., 770±20℃ and 3-5 kb, respectively. Although the sample ITO 11c shows similar temperature, its calculated pressure is close to 0 kb. The bulk and mineral chemistry and the temperature-pressure estimation suggest that the magma reservoir was not distinctly zoned but was very homogeneous throughout prior to eruption.