Storage conditions and evolution of andesitic magma prior to the 1991–95 eruption of Unzen volcano: Constraints from natural samples and phase equilibria experiments

The compositions of homogenized melt inclusions trapped in plagioclase (Pl) microphenocrysts (40–200 μm length) from mafic enclaves within dacitic rocks erupted at Unzen volcano in 1991–95 were investigated. The SiO2 contents of the melt inclusions vary from 58 to 70 wt.% and Pl anorthite content is An50–70. The stability fields (in terms of temperature and water activity) of natural Pl and coexisting melts from the melt inclusions were estimated from data of phase equilibria experiments performed with a synthetic andesite composition at T = 900–1140 °C, P = 200 MPa, log fO2 = NNO−2–NNO, and water activity of about 0–1. This composition is representative of the average composition of mafic enclaves from the 1991–95 eruption and nearly identical to the composition of andesitic lavas from 1663 Unzen eruption. The temperatures and H2O melt concentrations, calculated using the compositions of coexisting Pl and melt inclusions, provide an estimation of the conditions of andesitic magma evolution within the mafic enclaves prior to eruption. The formation of melt inclusions in plagioclase microphenocrysts occurred at a maximum temperature of ~ 1010 ± 35 °C for a melt containing 2 wt.% H2O and a minimum temperature of ∼ 945 °C ± 30 °C for a melt with ∼ 4 wt.% dissolved H2O. The compositional range of the melt inclusions indicates that the composition of the mafic enclave was not significantly contaminated by the host magma when inclusions were formed. The difference between concentrations of dissolved S (up to 0.06 wt.%) and Cl (up to 0.05 wt.%) in melt inclusions in Pl of mafic enclaves and concentrations of S (< 0.005 wt.%) and Cl (0.05–0.11 wt.%) in melt inclusions in phenocrysts of the dacitic magma clearly implies that two distinct sources for S and Cl in the 1991–95 eruption of Unzen volcano need to be considered. Sulfur degassing was generated by a release of fluids from the high-temperature andesitic magma whereas Cl was degassed from the low-temperature silicic magma. mbination of data from melt inclusions and phase equilibria experiments indicates that the mafic end-member magma at Unzen was already partially crystallized and contained significant proportions (20 to 40 wt.%) of Pl and orthopyroxene (Opx) when melt inclusions started to form. Since clinopyroxene (Cpx) and magnetite (Mt) crystallize after Pl and Opx, the temperatures derived for the mafic end-member magma from coexisting Opx–Cpx and Ilm–Mt pairs do not represent temperatures near to the liquidus. Assuming that the injected mafic magma was nearly aphyric, its initial temperature might have been higher than estimated in previous studies.