Gas geochemical characteristics of hydrothermal plumes at the HAKUREIand JADE vent sites, the Izena Cauldron, Okinawa Trough

摘要

Hydrothermal plumes above the HAKUREI and JADE sites, two high-temperature hydrothermal vent sites in the Izena Cauldron at the mid-Okinawa Trough, were investigated in order to gain a preliminary understanding of gas geochemical characteristics at underlying hydrothermal vent sites. Three geochemical tracers, H_2, CH_4 and δ~3He, covary with each other above the HAKUREI site but only CH_4 and δ~3He are correlated above the JADE site. The carbon isotope ratio of methane within the Izena Cauldron can be accounted by a combination of the fluid dilution by ambient seawater and microbial consumption with the kinetic carbon isotope effect (KIE) of 1.007. An estimated endmember δ~(13)C value of –32% in the HAKUREI fluid was obtained. Both the plumes above the HAKUREI and JADE sites showed C_1/C_2 ratios between 10~310~4. Only the bottom water around the HAKUREI site showed significant N_2O excess with isotopically light δ~(15)N and δ~(18)O, suggesting N_2O input from microbial activity in the sediment. A linear correlation between H_2 and CH_4 in the HAKUREI plume gives a H_2/CH_4 ratio of the HAKUREI fluid of more than 0.022. The estimated H_2/CH_4 ratio in the HAKUREI fluid is significantly higher than that of the JADE fluid, comparable with those of fluids venting at other sediment-related hydrothermal systems, and also comparable with those of thermogenic gases produced by hydrothermal sediment experiments. These facts suggest that fluid-sediment interaction during fluid upwelling appears to modify gas geochemical characteristics at the HAKUREI site but have little influence at the JADE site. This study demonstrates the availability of the Izena Cauldron hydrothermal field and the HAKUREI and JADE sites as a natural laboratory for investigating the fluid-sediment interaction during fluid upwelling.