Ashes vent "flaming" due to phase separation. View the movie from ROV dive.

Phase Separation

The process of phase separation, which takes place either as normal boiling or as brine condensation at supercritical pressures (see figure below), plays a crucial role in determining vent fluid composition. Elements are fractionated between the liquid and vapor phases, with ionic species partitioned into the liquid phase and volatile species into the vapor phase. Phase separation may take place under a variety of circumstances. In an aqueous salt solution, normal boiling (i.e. production of low salinity vapor) occurs when the fluid crosses the two-phase curve below critical pressure and temperature. However, if the fluid crosses the two-phase curve at a pressure and temperature above the critical point, a high-salinity brine condenses out of the fluid. These two types of phase separation produce very different chemistries.

A perfect example of subcritical phase separation in a hydrothermal system occurs at Axial Volcano. Based on chemical systematics (Butterfield et al. 1990), a single hydrothermal endmember ascending in an upflow zone along the caldera-bounding fault decompresses and crosses the two-phase curve, producing a vapor phase that migrates away from the higher-salinity, residual liquid phase. Different mixtures (or unmixtures, if you will) of the vapor and liquid phases exit the seafloor in distinctly different types of vents . There are different mechanisms that can result in the vapor phase becoming segregated from the liquid phase; the point is that it does happen in many hydrothermal systems.