Journal of Petrology Advance Access originally published online on October 28, 2004
Journal of Petrology 2005 46(2):319-337; doi:10.1093/petrology/egh077
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Journal of Petrology vol. 46 issue 2 © Oxford University Press 2004; all rights reserved
Experimental Petrology of the 1991–1995 Unzen Dacite, Japan. Part I: Phase Relations, Phase Composition and Pre-eruptive Conditions
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
Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100–300 MPa, 775–875°C, various water activities, and fO2 buffered by the Ni–NiO buffer. The compositions of the experimental products and natural phases are used to constrain the temperature and water contents of the low-temperature and high-temperature magmas prior to the magma mixing event leading to the 1991–1995 eruption. A temperature of 1050 ± 75°C is determined for the high-temperature magma based on two-pyroxene thermometry. The investigation of glass inclusions suggests that the water content of the rhyolitic low-temperature magma could be as high as 8 wt % H2O. The phase relations at 300 MPa and in the temperature range 870–900°C, which are conditions assumed to be representative of the main magma chamber after mixing, show that the main phenocrysts (orthopyroxene, plagioclase, hornblende) coexist only at reduced water activity; the water content of the post-mixing dacitic melt is estimated to be 6 ± 1 wt % H2O. Quartz and biotite, also present as phenocrysts in the dacite, are observed only at low temperature (below 800–775°C). It is concluded that the erupted dacitic magma resulted from the mixing of c. 35 wt % of an almost aphyric pyroxene-bearing andesitic magma (1050 ± 75°C; 4 ± 1 wt % H2O in the melt) with 65 wt % of a phenocryst-rich low-temperature magma (760–780°C) in which the melt phase was rhyolitic, containing up to 8 ± 1 wt % H2O. The proportions of rhyolitic melt and phenocrysts in the low-temperature magma are estimated to be 65% and 35%, respectively. It is emphasized that the strong variations of phenocryst compositions, especially plagioclase, can be explained only if there were variations of temperature and/or water activity (in time and/or space) in the low-temperature magma.
KEY WORDS: Unzen volcano; magma mixing; experimental study
* Corresponding author. E-mail: f.holtz{at}mineralogie.uni-hannover.de
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