Origin of volcanic gases discharging from a cooling lava dome of Unzen volcano, Japan

Abstract

Chemical and isotopic compositions were measured for volcanic gases discharged from the recently erupted Heisei-shinzan dome of Unzen volcano. The volcanic gases are largely mixed with the air, whose content ranges from 60 to 90%. Chemical and isotopic compositions of the gases are corrected for the air mixing accounting not only for the major air components but also for contribution of the atmospheric moisture and CO 2. The variation of the corrected composition revealed that these gases are derived from various sources including; 1) the air circulating through the lava domes, 2) the local meteoric water at various degrees of evaporation and vapor loss, 3) the magmatic CO 2 likely originated from a deep source, and 4) the gases discharged from the cooling lava dome. The gases discharged from the cooling lava dome are characterized by the oxygen and hydrogen isotopic composition typical of the subduction-type volcanoes but with high Cl content and high Cl/S and F/S ratios. In contrast, discharges of CO 2 with the magmatic δ 13C values are not correlated with discharge of heat or acid gas species, suggesting the ascent of CO 2-rich fluids from a deep source, possibly through the volcanic conduit.