Physical Controls On The Salinity Of Mid-Ocean Ridge Hydrothermal Vent Fluids

The salinities of mid-ocean ridge hydrothermal systems range from ~0.1 to 7-8 wt% NaCl. The observed enrichments and depletions relative to seawater (3.2 wt% NaCl) are attributed to phase separation followed by segregation of the resulting brine and vapor phases but such models do not fully account for the maximum observed venting salinities. Simple models of phase separation predict the formation of fluids with substantially higher salinities and such high salinity fluids are commonly observed in fluid inclusions in rocks recovered from fossil reaction and upflow zones. In order to investigate the physical controls on maximum venting salinities, we use single-phase, two-dimensional numerical models of hydrothermal circulation with realistic fluid properties and differing bottom temperatures, Nusselt numbers and permeability structures. For each model, we determine the densities of a fluid that would be neutrally buoyant at all depths within the upflow zone and we then equate these densities to the maximum salinity of a fluid that could rise through the whole system. Models with uniform permeability and venting temperatures that match black smokers (~350 °C) predict that fluids with salinities of up to ~15-17 wt% NaCl are buoyant enough to vent at the seafloor. When a high permeability layer is added to the top of the model to simulate the extrusive layer, both the temperature and maximum salinity of venting drop. High salinity fluids are trapped at the permeability interface and the upwelling fluids cool substantially in the high-permeability layer. Models that matched the observed maximum venting salinity have venting temperatures well below those of black smokers or have infeasible bottom temperatures. To match both the venting temperature and maximum venting salinity, we find that it is necessary to `thermally insulate' the upflow zone by surrounding it with a low permeability region in layer 2A. Such models are consistent with the observations of stockworks in ophilolites where it is found that the upflow zone is surrounded by a zone of mineral precipitation and alteration that is inferred to have low permeabilities.