Experimental petrology of the 1991-1995 Unzen dacite, Japan. Part I: Phase relations, phase composition and pre-eruptive conditions

Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100-300 MPa, 775-875degreesC, various water activities, and fO(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 +/- 75degreesC 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 870-900degreesC, 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 800-775degreesC). 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 +/- 75degreesC; 4 +/- 1 wt % H2O in the melt) with 65 wt % of a phenocryst-rich low-temperature magma (760-780degreesC) 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.