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Experimental Petrology of the 1991-1995 Unzen Dacite, Japan. Part I: Phase Relations, Phase Composition and Pre-eruptive Conditions |
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| Authors: | HOLTZ FRANCOIS; SATO HIROAKI; LEWIS JARED; BEHRENS HARALD; NAKADA SETSUYA |
| Institution: | 1 INSTITUTE FOR MINERALOGY, UNIVERSITY OF HANNOVER, WELFENGARTEN 1, D-30167 HANNOVER, GERMANY 2 DEPARTMENT OF EARTH AND PLANETARY SCIENCES, FACULTY OF SCIENCE AND GRADUATE SCHOOL OF SCIENCE AND TECHNOLOGY, KOBE UNIVERSITY, KOBE, 657-8501 JAPAN 3 EARTHQUAKE RESEARCH INSTITUTE, UNIVERSITY OF TOKYO, YAYOI 1-1-1, BUNKYO, TOKYO, 113-0032 JAPAN |
| Abstract: | Crystallization experiments were conducted on dry glasses fromthe Unzen 1992 dacite at 100–300 MPa, 775–875°C,various water activities, and fO2 buffered by the Ni–NiObuffer. The compositions of the experimental products and naturalphases are used to constrain the temperature and water contentsof the low-temperature and high-temperature magmas prior tothe magma mixing event leading to the 1991–1995 eruption.A temperature of 1050 ± 75°C is determined for thehigh-temperature magma based on two-pyroxene thermometry. Theinvestigation of glass inclusions suggests that the water contentof the rhyolitic low-temperature magma could be as high as 8wt % H2O. The phase relations at 300 MPa and in the temperaturerange 870–900°C, which are conditions assumed to berepresentative of the main magma chamber after mixing, showthat the main phenocrysts (orthopyroxene, plagioclase, hornblende)coexist only at reduced water activity; the water content ofthe post-mixing dacitic melt is estimated to be 6 ± 1wt % H2O. Quartz and biotite, also present as phenocrysts inthe dacite, are observed only at low temperature (below 800–775°C).It is concluded that the erupted dacitic magma resulted fromthe mixing of c. 35 wt % of an almost aphyric pyroxene-bearingandesitic magma (1050 ± 75°C; 4 ± 1 wt % H2Oin the melt) with 65 wt % of a phenocryst-rich low-temperaturemagma (760–780°C) in which the melt phase was rhyolitic,containing up to 8 ± 1 wt % H2O. The proportions of rhyoliticmelt and phenocrysts in the low-temperature magma are estimatedto be 65% and 35%, respectively. It is emphasized that the strongvariations of phenocryst compositions, especially plagioclase,can be explained only if there were variations of temperatureand/or water activity (in time and/or space) in the low-temperaturemagma. KEY WORDS: Unzen volcano; magma mixing; experimental study |
| Keywords: | : Unzen volcano magma mixing experimental study |
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