[S-VC55_1PO1] Active Volcanism

[SVC55-P20] Remote temperature sensing on fumaroles in active volcanoes using stable isotopes of trace gases in volcanic plume

*Daisuke KOMATSU¹, Urumu TSUNOGAI¹, Fumiko NAKAGAWA² (1.Graduate School of Environmental Studies, Nagoya University, 2.Graduate School of Science, Hokkaido University)

Keywords:fumarolic gases, carbon dioxide, molecular hydrogen, stable isotopes, isotopic exchange equilibrium, remote temperature sensing

Molecular hydrogen (H₂) in a high-temperature volcanic fumarole (> 400 degreeC) reach to the hydrogen isotope exchange equilibrium with coexisting fumarolic H₂O under the outlet temperature of the fumarole. In this study, we applied this hydrogen isotope exchange equilibrium of fumarolic H₂ as a tracer for the remote temperature sensing on the fumarolic area in the 1^st crater of Mt. Naka-dake (Aso volcano) where direct measurement on fumaroles was not practical, by deducing the hydrogen isotopic composition (dD value) of fumarolic H₂ remotely from those in volcanic plume. The reciprocal of H₂ concentration in the plume samples showed a good linear relationship with the dD values. The linear relationships suggested that both the concentrations and the dD values of H₂ in the plume samples can be explained by simple mixing between two end-members, both of which can be classified to a single category at least for the dD values of H₂. By extrapolating the linear relationship between 1/H₂ and dD to 1/H₂=0 to exclude the contribution of the tropospheric H₂ from the dD value of each sample, we estimated that the dD value of fumarolic H₂ to be -172+-16 per mil vs. VSMOW and the apparent equilibrium temperature (AET_D) to be 868+-97 degreeC. Although the estimated temperatures using the IR thermometers were much lower than the AET_D, we concluded that the AET_D represented the highest outlet temperature of the fumaroles in Aso volcano and that the dimensions of the fumaroles at surface smaller than the pixel of the IR thermometers was responsible for the temperatures lower than the AET_D. That is to say, temporal variation in the dimensions of fumaroles at surface, probably due to variation in the emission flux of fumarolic gases, was responsible for the temporal variation in the temperature determined by the IR thermometers, while the actual outlet temperature of the Aso fumaroles keeps the high temperature almost equal to the equilibrium temperature of fumarolic gases.