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Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100–300 MPa, 775–875°C, various water activities, and  f O 2  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 % H 2 O. 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 % H 2 O. 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 % H 2 O 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 % H 2 O. 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.

Experimental Petrology of the 1991–1995 Unzen Dacite, Japan. Part I: Phase Relations, Phase Composition and Pre-eruptive Conditions