ABSTRACT

The chemical and isotopic compositions of gases discharging from shallow-water hydrothermal vents at Kueishantao (KST, northeast Taiwan) have been studied since 2000. In this paper, we present new data gathered from 2010 to 2014. The main component is CO₂ (162–882 mmol/mol), followed by N₂ (33–634 mmol/mol), CH₄ (18–190 mmol/mol), and H₂S (b.d.l.–172 mmol/mol). Helium isotope values indicate that more than 70% of the helium is mantle-derived. O₂ was mostly a contaminant from ambient seawater during sampling, or from air after sampling. By subtracting the contaminant fractions of N₂ and Ar, using the O₂ concentrations plus the N₂/O₂ or Ar/O₂ ratios in air, the residual N₂ and Ar were positively correlated to a significant extent, and exhibited N₂/Ar ratios close to air values. These corrected fractions of N₂ and Ar were derived from seawater percolated during the discharge stage. The δ¹³C values of CO₂ ranged from –8.2 to –5.5‰ (VPDB), whereas the CO₂ contents were largely controlled by its dissolution in the fluid phase. The estimated endmember contributions for CO₂ were: mantle (8–32%), sediment (14–27%), and marine limestone (54–72%). CH₄ was the main hydrocarbon in the KST gases, and exhibited a linear relationships with helium. The CH₄/³He ratios increased dramatically from 0.1–20 × 10⁶ during 2000 and 2003 to 3–5 × 10⁸ after 2010, either due to the increasing CH₄ contents or the decreasing input of magmatic helium. In the C₁/C₂₊ ratios versus δ¹³C(CH₄) (–26.8 to –24.5‰) diagram, the KST gas samples lie at the boundary between thermogenic and abiotic, suggesting that CH₄ may be derived from these two sources. Alternatively, according to the log(C₂/C₃) versus log(C₁/C₂) diagram and the C₁–C₅ distribution patterns, the C₂₊ hydrocarbons and less than 2% of the CH₄ may have originated from either a kinetically-controlled Fischer Tropsch-Type (FTT) reaction or a thermogenic process, whereas more than 98% of CH₄ was in equilibrium with CO₂. In addition, the linear-logC₂–C₅ patterns suggest that no significant secondary processes have occurred. This is the first study of KST gases to include carbon stable isotope ratio measurements, which provides important information about gases released from a shallow-water submarine hydrothermal field at a subduction zone.