Heat-pipe Solutions and Brine Formation in NaCl-H2O Hydrothermal Systems

Geothermal heat pipes represent a classical approach to understanding heat transfer in two-phase hydrothermal systems. Because of the presence of salt, however, heat pipe behavior in NaCl-H₂O two-phase systems is significantly different from its pure water counterpart. We have used the numerical simulator GTHSW to explore heat-pipe solutions in a seawater hydrothermal system as a function of parameters such as subsurface heat flux, crustal permeability and porosity, and seafloor pressure. Our results show that the principal feature of these systems is the rapid development of a saline brine layer at the base of the system, overlain by a low-salinity counter flow region. The size and the salinity of the basal brine layer increase as the counterflow region grows. At low basal heating rates, a region of weak counterflow occupies most of the system; and heat is mainly transferred by thermal conduction. The counterflow region expands more rapidly with increased heating rate and permeability. If the permeability is relatively high, this region is initially liquid-dominated, whereas it is vapor-dominated for a low permeability system. Eventually even the liquid-dominated counterflow region is replaced by vapor-dominated counterflow that begins to develop near the top and expand downward. A "dry" zone consisting of vapor and halite may develop near the base of the system. These heat-pipe solutions may represent the initial phase of development in a two-phase seafloor hydrothermal system. The evolution of salinity described by the model followed by recharge of surrounding deep water at the base as heat flux decreases could explain, in part, the chlorinity at "F" vent at EPR 9° N, changed from vapor (46.5 mmol) to brine (846 mmol) between 1991 and 1994.