Chemical and isotopic composition of fumarolic gases at Iwate volcano, Japan, during and after seismic activity in 1998: implications for the modification of ascending volcanic gases

摘要

In 1998, there were many volcanic earthquakes recorded at Iwate volcano, Japan. Although an eruption was anticipated, it never occurred. Fumarolic gases were sampled at the volcano on six occasions during 1998 and 1999, and were analyzed for their chemical compositions and isotope ratios. The fumarolic gases were again sampled in 2004 and 2006, after the period of seismic activity. The HCl concentrations and isotope ratios of the H2O in the gas samples collected from an active geothermal area at the volcano were high in 1998 and 1999, but decreased significantly after 2004, irrespective of the relatively stable concentrations of CO2 and sulfur-bearing gases. A notable feature of the fumarolic gases is the high isotope ratio of H2O, similar to that of pure magmatic gas, which is typical of andesitic volcanoes, although the temperature at the outlet was less than 146°C in the present case. These features can be explained if the enthalpy of the magmatic gas was equivalent to that of water vapor at 252°C to 370°C under a pressure of 1 bar. The correlations observed among CO2/H2O and HCl/H2O, and the δ18O values of the fumarolic gases suggest partial condensation of H2O vapor during the ascent of the volcanic gas to the surface. The estimated CO2/H2O molar ratio of the magmatic gas was 0.008, which is less than the 0.03 reported previously for magmatic gas sampled during an effusive eruption at Unzen volcano, Japan. These data suggest that the magma at Iwate volcano is depleted in volatiles, because CO2 is preferentially degassed from the silicate melt relative to H2O. Such depletion in CO2 might explain the failed eruption at Iwate volcano in 1998.