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

Abstract

Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100–300MPa, 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 lowtemperature 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 lowtemperature magma could be as high as 8 wt % H2O. The phase relations at 300MPa 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 pyroxenebearing 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.