Circulation Zones

The permeability structure of the ocean crust and the location of the heat source determine how hydrothermal fluids circulate. Conceptually, we break the process into three phases. In the recharge zone, cold seawater penetrates into the ocean crust and is gradually heated along its flow path. There is a strong thermodynamic drive to remove Mg from the fluid into the rock, and acid is produced in the process, which then drives leaching of other major elements and transition metals from the rock (see Seyfried 1987) into the hydrothermal fluid. Seawater sulfate is removed from solution by precipitation as anhydrite and by reduction to hydrogen sulfide.

Fluids reach their maximum temperature in the high-temperature reaction zone near the heat source. Important parameters in determining the high-temperature fluid composition are pressure, temperature, water/rock ratio, rock composition, recharge fluid composition, and reaction time (or residence time). Reactions are fast at high temperature, and fluids may approach equilibrium with a set of alteration minerals.

Hot hydrothermal fluids are buoyant and rise toward the ocean floor in the upflow zone. The residence time in this zone is probably very short, but fluids may continue to react with the rock through which they flow. Fluids cool slightly through decompression (adiabatic cooling) and may cool significantly by losing heat to the surrounding rock (conductive cooling) or by mixing with cold seawater-like fluids below the sea floor. Cooling may cause the hydrothermal fluid to become supersaturated and precipitate secondary minerals (metal sulfides, quartz, etc.), so that the fluid chemistry may change slightly between leaving the reaction zone and arriving at the sea floor.

Hydrothermal vents are the interface between the hot, anoxic upflow zone and cold, oxidized seawater. When hot fluids mix with cold seawater, many hydrothermal minerals precipitate within seconds to form the dense particle plumes characteristic of black smokers. The particles are predominantly a mixture of sulfides (e.g. pyrrhotite FeS, sphalerite ZnS, chalcopyrite CuFeS₂, etc.) and sulfates (anhydrite CaSO₄, barite BaSO₄). Some of these minerals are formed as part of the chimney structures that build up on the sea floor, while others are formed within the plume and dispersed through the water.

Another form of hydrothermal venting is the diffuse vent, where lower temperature fluids exit the seafloor as shimmering water through cracks surrounding high-temperature smokers, or in isolated areas venting directly from basalt. The lower temperature fluids generally do not contain enough dissolved metal and sulfide to form "smoke". There is evidence that the cooler diffuse vent fluids are in most cases formed by sub-sea floor mixing of hot, smoker-like fluids with cold seawater-like fluid, and in many cases they must precipitate minerals below the sea floor.