Gas compositions and helium isotopic ratios of fluid samples around Kueishantao, NE offshore Taiwan and its tectonic implications

Abstract

Kueishantao is a Holocene volcanic islet (<7,000 yrs) located at NE offshore Taiwan, and tectonically is part of western extension of the Okinawa Trough. Magmatic activities are considered very active around this area on the basis of the fact that on-land fumaroles and submarine hydrothermal systems are prevailing currently. Representative bubble gas samples from submarine hydrothermal vents were collected for gas composition and helium isotope analysis. The gases show similar compositions of low temperatre fumaroles in the world, i.e., with high CO 2 and H 2 S but low SO 2 and HCl contents. They exhibit consistent high 3 He/ 4 He ratios (7.3-8.4 R A , where R A is the 3 He/ 4 He ratio of air), probably the highest 3 He/ 4 He values of gases ever reported in active hydrothermal areas of the western Pacific region. Meanwhile, seawater samples around Kueishantao and other fluid samples from I-Lan Plain, the land area closest to the Kueishantao and also the southernmost part of the Okinawa Trough, show a significant excess of 3 He compositions as well. This indicates that the mantle component plays an important role for their gas sources, and implies that the mantle fluids may have invaded into I-Lan Plain. The westward opening of the Okinawa Trough may have caused thinning of the continental crust and produced deep normal faults and hence, the primordial 3 He is able to degas from mantle source region without significant crust contamination.

Insight into the Geothermal Structure in Chingshui, Ilan, Taiwan

“…3 and 4, we propose that the existence of the magma chamber in the shallow crust and the intrusive igneous rock results in a high heat flow and geothermal gradient, and the NE deep fault system within the meta-sandstones provides meteoric recharge from a higher elevation to artesianally drive the geothermal system. The He ratio of the gas samples collected in the Chingshui geothermal field shows significant mantle input, 14% to 19%, and implies that mantle fluids may have invaded the Chingshui geothermal field (Yang et al 2005). This confirms that the geothermal fluid had deeply circulated within the fracture system and was heated by a deep hot body of rock.…”

Methane Venting in Gas Hydrate Potential Area Offshore of SW Taiwan: Evidence of Gas Analysis of Water Column Samples

“…The highest helium and methane concentrations, 848 and 115356 nL L -1 , respectively, were obtained near the bottom of site A. The degassing trace gases, like helium and radon, are usually carried by their carrier gases and emitted through fractures/faults and/or by hydrothermal activity from deep sources onto the surface of the Earth (e.g., Yang et al 2003bYang et al , 2005a. In seawater samples taken at greater depths in general contain higher concentrations of dissolved gas, which may simply be due to the higher solubility of gases at lower temperature (e.g., Takahata et al 2004;Sano and Takahata 2005).…”

Silica Geothermometry Applications in the Taiwan Orogenic Belt

“…7a). This existing heat source is supported by other methods and results, the helium isotopic ratio of soil gases, seismic data, geomagnetic and magneto telluric data and borehole information (Yu and Tsai 1979;Yang et al 2005;Lan et al 2006;Tong et al 2008).…”