1940年・1962年噴出物の岩石学的研究から見た近年の伊豆諸島三宅島火山のマグマ供給系

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Miyake-jima volcano, one of the tholeiitic volcanoes in Izu-Mariana arc, has erupted at least 14 times since 1085. After 66-year repose period since the 1874 AD eruption, the largest eruption during the historic period occurred in 1940 AD. Since then, eruptions have repeated about every 20 years (1962 and 1983 AD). These recent three eruptions are characterized by effusion of lavas and scoria from fissures at flank of the volcano, and only 1940 one accompanied the summit eruption. Although most of the products are nearly aphyric augite basaltic andesites and basalts, ejecta erupted from northern fissure during the 1940 eruption are characterized by the presence of plagioclase and olivine megacrysts. These megacrysts should not be comagmatic but accidental, because the olivine megacrysts often show deformed texture (kink-bands) and also because plutonic xenoliths consisting of olivine and plagioclase with the same compositions as the megacrysts are found. Except for the megacrysts, phenocrystic minerals in all of the 1940 ejecta are divided into two types, B (basalt type) and A (andesite type). The B type consists mainly of calcic plagioclase, magnesian olivine and augite, each showing normal zonation. By contrast, the A type is characterized by the presence of Ti-magnetite and by more evolved composition of phenocrysts than those in the B type. Phenocrysts in the A type are usually reversely zoned. These suggest that the 1940 ejecta are produced by magma mixing between basaltic and andesitic magmas which were derived from different magma storage systems. On the other hand, the 1962 and 1983 products contain only A type phenocrysts with minor evidence of magma mixing. In contrast to the 1940 eruption, both the 1962 and 1983 eruption events effused nearly homogeneous magmas. The whole-rock chemistry and mineral chemistry shows that magma has become systematically more basic from 1940 to 1983, indicating that the 1962 and 1983 ejecta are not products of simple fractional crystallization of the 1940 andesitic magmas. In each eruption episode, the products from different fissures can be distinguished mineralogically and chemically from each other. This fact requires independent magma transport systems, which connected the magma storage systems with respective fissures.
特定非営利活動法人日本火山学会の論文
1998-12-28

1940年・1962年噴出物の岩石学的研究から見た近年の伊豆諸島三宅島火山のマグマ供給系

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