The Variations Of Fumarolic Gas Compositions In Tatun Volcano Group, Northern Taiwan

Hydrothermal activity is commonly found in northern Taiwan. Helium isotopic compositions of fumarolic samples show that more than 60% mantle source was involved in its gas sources. Carbon and nitrogen isotopic results also show significant mantle signature. It infers that a magma reservoir may exist underneath northern Taiwan. For monitoring purpose, therefor, we systematically analyze the compositions of representative fumarolic gases from Tatun Volcano Group (TVG) by utilizing the Giggenbach-bottle technique. The results show that the dry gas compositions of the gas samples are typical compositions of low temperature fumaroles, which dominated with CO2 (65~85%) and H2S (>10%) and low SO2 (<4%) contents. The time series results show that the major gas composition and helium isotopic ratios did not show significant variations since 1999. It implies that the degassing system in studied area was quite steady in last few years. However, since August 2004, there is not only a progressive increase of HCl concentrations but an increase of SO2/H2S ratio in fumaroles from Da-You-Keng (DYK), where has the highest helium isotopic ratio been analyzed in this area. The temperature of fumaroles in the TVG is also higher than that in the past, especially in DYK. Both HCl concentration and SO2/H2S ratio fluctuate with temperature. We propose two possible processes to explain observed phenomenon. One is more magmatic gases were released from recent opened fractures; another is magma ascending. These two processes can cause the similar variations of gas compositions. The present surface temperature of fumaroles in DYK is around 116°C and does not show much rising in last few months so that we consider these variations may be related to the local volcanic activity. Although the GPS monitoring results show that there is a continuous uplifting of ground in this area (Lin, C.H. unpublish data), we need to closely monitor the variations of fumaroles and hot springs to check/clarify the possible relationship between the possible local magma activity and other parameters, e.g., seismicity, and flux of fumarolic steams etc.