Evolution of the Magma Chamber beneath Usu Volcano since 1663: a Natural Laboratory for Observing Changing Phenocryst Compositions and Textures |
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| Authors: | TOMIYA AKIHIKO; TAKAHASHI EIICHI |
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| Institution: | 1 INSTITUTE OF GEOLOGY AND GEOINFORMATION, GEOLOGICAL SURVEY OF JAPAN, AIST, TSUKUBA CENTRAL 7, 1-1-1 HIGASHI, TSUKUBA, IBARAKI, 305-8567, JAPAN
2 DEPARTMENT OF EARTH AND PLANETARY SCIENCES, TOKYO INSTITUTE OF TECHNOLOGY, 2-12-1 OOKAYAMA, MEGURO-KU, TOKYO, 152-0033, JAPAN |
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| Abstract: | We have investigated the evolution of an active silicic magma-feedingsystem beneath Usu volcano, Japan, where eight eruptions havebeen recorded since AD 1663. All magmatic products contain similartypes of plagioclase and orthopyroxene phenocrysts that consistof homogeneous cores with uniform compositions, and a zonedmantle that increases in size with time. The compositions ofplagioclase and orthopyroxene phenocrysts vary gradually andregularly with time, as do the bulk-rock compositions. The textureof these phenocrysts also changes systematically, caused byprogressive crystal growth, dissolution and diffusion. On thebasis of these observations, we conclude that the same magma-feedingsystem has persisted at Usu volcano since AD 1663. Compositionalvariation of magnetite phenocrysts differs from that of plagioclaseand orthopyroxene, because magnetite has large diffusion coefficientsand should represent magmatic conditions immediately beforethe eruption. Most pumices from Usu volcano contain two typesof magnetite phenocryst, each with a different composition andcrystallization temperature, indicating that two magmas mixedbefore each eruption (approximately several days before). Theend-members changed with time: rhyolite + basaltic andesite(1663); dacite ± rhyolite (1769, 1822, 1853); dacite± dacite (1977, 2000). The temperature of the magma apparentlyincreases with time, and the increase can be explained by sequentialtapping from a magma chamber with a thermal and chemical gradientin addition to injection of high-temperature magma. KEY WORDS: continuous existence of magma chamber; dacite; dissolution and diffusion of phenocrysts; magma mixing; magnetite |
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| Keywords: | : continuous existence of magma chamber dacite dissolution and diffusion of phenocrysts magma mixing magnetite |
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