Hydrothermal venting at pressure-temperature conditions above the critical point of seawater, 5°S on the Mid-Atlantic Ridge
Hydrothermal circulation within oceanic crust depends on pressure(P) and temperature (T); the critical point (CP) of seawaterat 298 bar and 407 °C represents the threshold between subcriticaland supercritical conditions. Here we present data from thefirst hydrothermal system in which the sampled fluids fall onand above the CP. The vent system discovered at 5°S on theMid-Atlantic Ridge is characterized by multiple fluid emanationsat variable temperatures in water depths of ~3000 m. Vigorousvapor phase bubbling, stable emanation of superhot fluid at407 °C, and decreased salinity indicate phase separationat conditions above the CP at one site. At another site themeasured maximum T of 464 °C during a 20 s interval is byfar the hottest fluid ever measured at the seafloor and fallsinto the vapor-phase supercritical region of seawater. Besidesthese two separate fields with ongoing phase separation andextremely hot fluids, a third vent field emanates non-phase-separatedfluids at 349 °C and is used as a reference site. Fluidchemistry shows that supercritical fluids evolve differentlythan subcritical fluids, making this vent system a unique naturallaboratory to investigate processes at high P-T conditions.The stability of the high temperature and fluid geochemistrymeasured in 2005 and 2006 after the assumed seismic triggerevent in 2002 supports this as an exceptional site along theMid-Atlantic Ridge.