The chemical and isotopic compositions of gas discharge from shallow-water hydrothermal vents at Kueishantao, offshore northeast Taiwan

摘要

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 N-2 (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. O-2 was mostly a contaminant from ambient seawater during sampling, or from air after sampling. By subtracting the contaminant fractions of N-2 and Ar, using the O-2 concentrations plus the N-2/O-2 or Ar/O-2 ratios in air, the residual N2 and Ar were positively correlated to a significant extent, and exhibited N-2/Ar ratios close to air values. These corrected fractions of N-2 and Ar were derived from seawater percolated during the discharge stage. The delta C-13 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/He-3 ratios increased dramatically from 0.1-20 x 10(6) during 2000 and 2003 to 3-5 x 10(8) after 2010, either due to the increasing CH4 contents or the decreasing input of magmatic helium. In the C-1/C2+ ratios versus delta C-13(CH4) (-26.8 to -24.5 parts per thousand) 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(C-2/C-3) versus log(C-1/C-2) diagram and the C-1-C-5 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-logC(2)-C-5 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.