Disequilibrium Two-Phase Flow? The Case at A Vent EPR 9° N, April 1991 and at Endeavor, JDF, June, 1999

During the two week period immediately following the magmatic eruption on the East Pacific Rise near 9° N, fluid discharging at A vent exhibited temperatures near 400° C, placing the fluid in the two-phase regime at a seafloor pressure of 25 MPa. The vent salinity was less than 0.1 that of seawater, indicating a vapor phase. Similarly, in June 1999, following a magma/tectonic event, salinity at Puffer and Sully vents on Endeavor, Juan de Fuca Ridge, suddenly dropped to less than 0.1 that of seawater, while temperature remained in the two-phase regime corresponding to a seafloor pressure of 22 MPa. Despite these low salinities, however, the observed vent salinity is greater than that corresponding to the temperature and pressure at the respective vents, and thus the apparent vapor phase does not correspond to thermodynamic equilibrium. Because the equilibrium brine phase is generally denser than seawater and tends to sink, and because the salinity of the equilibrium vapor phase tend to increase with pressure, we argue that the vapor phases at A vent in April 1991 and at Puffer and Sully vents in June 1999 represent vapor phases that were in equilibrium at greater depths in the system. Because of their rapid ascent, these low density vapor phases remain in disequilibrium with their seafloor environment. Assuming disequilibrium, the time series at A vent allows us to estimate the rate of ascent and indicates that the subsurface permeability there is ∼ 10^-13 m².