Geochemistry of some gases in hydrothermal fluids from the southern Juan de Fuca Ridge

Abstract

Five samples of hydrothermal fluids from two vent areas on the southern Juan de Fuca Ridge were analyzed for dissolved gases. Concentrations in the end‐member hydrothermal fluid of H 2 (270–527 μmol/kg), CH 4 (82–118 μmol/kg), and CO 2 (3920–4460 μmol/kg) are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems. The carbon isotopic ratios of the CH 4 (δ 13 C = −17.8 to −20.8) and CO 2 (δ 13 C = −3.6 to −4.7) suggest that at least some of the CH 4 and CO 2 in the fluids is basalt‐derived. The range of δ 13 C values for the basalt‐derived CO 2 is −6.8 to −9.7, calculated by assuming conservation of recharge ΣCO 2 during hydrothermal circulation. Apparent temperatures of equilibration between the CH 4 and the basalt‐derived CO 2 range from 640°C to 750°C. Small amounts of ethane (C 2 H 6 /CH 4 ≅ 0.9 × 10 −3 −2.2 × 10 −3 ), propane, and butane detected in the samples may also have formed in the basalt. One sample of almost pure (95.5%) hydrothermal fluid contained a significant fraction, up to 63% and 74%, respectively, of the recharge Ar and N 2 . This suggests that the fluid has not undergone extensive vapor‐liquid phase separation.