Venting of carbon dioxide-rich fluid and hydrate formation in mid-Okinawa trough backarc basin

Carbon dioxide-rich fluid bubbles, containing approximately 86 percent CO₂, 3 percent H₂S, and 11 percent residual gas (CH ₄ + H₂), were observed to emerge from the sea floor at 1335- to 1550-m depth in the JADE hydrothermal field, mid-Okinawa Trough. Upon contact with seawater at 3.8°C, gas hydrate immediately formed on the surface of the bubbles and these hydrates coalesced to form pipes standing on the sediments. Chemical composition and carbon, sulfur, and helium isotopic ratios indicate that the CO₂-rich fluid was derived from the same magmatic source as dissolved gases in 320°C hydrothermal solution emitted from a nearby black smoker chimney. The CO₂-rich fluid phase may be separated by subsurface boiling of hydrothermal solutions or by leaching of CO₂-rich fluid inclusion during post-eruption interaction between pore water and volcanogenic sediments.