GAS EXHALATION FROM MUD VOLCANOES IN SW TAIWAN

Many mud volcanoes are distributed along the tectonic sutures in SW Taiwan. Most mud volcanoes are exhaling CH4-dominated gases with low helium isotopic ratios (3He/4He <0.2Ra, where Ra is the air ratio), which indicates a crustal source in origin. Nevertheless, bubbling gases from a mud-pool (ca. 100 m2) in Chung-lun (CL), which is situated in the Chu-ko (CK) active fault zone with a potential for re-activation in the future, show much higher helium isotopic ratios (3He/4He=5.2~6.6Ra), with CO2-dominated compositions. This implies that there are multiple sources for gas exhalation in SW Taiwan.

A monitoring station was set up at the CL mud-pool and gases collected through the water replacement method using an up-side-down funnel. Gases collected were continuously transferred into the analyzing system. This setup enabled measurement of gas compositions and gas flow of bubbling gases from the mud-pool at intervals of less than two minutes. The major compositions of gases exhaling from this site were 80~85% of CO2 and 8~12% of CH4, respectively. Meanwhile, the gas flow ranges from 0.1 to 15 litre/min. The funnel covering about 1/4 bubbling gases of the mud-pool, hence, the amount of gases exhaling from the mud-pool can be estimated to be about 1.5 ton/year for CH4 and 34 ton/year for CO2, respectively. The exhalation flux of CH4 will be 15000 t km-2 y-1, which is much higher than those from other mud volcanoes in the world.