Hydrothermal circulation within oceanic crust depends on pressure (P) and temperature (T); the critical point (CP) of seawater at 298 bar and 407 °C represents the threshold between subcritical and supercritical conditions. Here we present data from the first hydrothermal system in which the sampled fluids fall on and above the CP. The vent system discovered at 5°S on the Mid-Atlantic Ridge is characterized by multiple fluid emanations at variable temperatures in water depths of ~3000 m. Vigorous vapor phase bubbling, stable emanation of superhot fluid at 407 °C, and decreased salinity indicate phase separation at conditions above the CP at one site. At another site the measured maximum T of 464 °C during a 20 s interval is by far the hottest fluid ever measured at the seafloor and falls into the vapor-phase supercritical region of seawater. Besides these two separate fields with ongoing phase separation and extremely hot fluids, a third vent field emanates non-phase-separated fluids at 349 °C and is used as a reference site. Fluid chemistry shows that supercritical fluids evolve differently than subcritical fluids, making this vent system a unique natural laboratory to investigate processes at high P-T conditions. The stability of the high temperature and fluid geochemistry measured in 2005 and 2006 after the assumed seismic trigger event in 2002 supports this as an exceptional site along the Mid-Atlantic Ridge.

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