The effect of temperature on metal mobility in subseafloor hydrothermal systems: constraints from basalt alteration experiments

Crystalline basalt was reacted with a NaCaKCl fluid of seawater chlorinity at 400, 350 and 300°C, 400 bar and a fluid/rock mass ratio of 1, to assess the effect of temperature on metal mobility in ridge-crest hydrothermal systems. Basalt alteration at 400°C yielded a fluid chemistry characterized by high Zn, Cu, Fe, Mn, SiO₂, H₂S and H₂ concentrations. Lowering temperature to 350 and then to 300°C, caused dissolved metal concentrations to decrease drastically. "Black smoker" vent fluids generally have measured temperatures ≤ 355°C. The dissolved concentrations of Zn, Cu, Fe, and Mn observed in these fluids, however, could only be achieved or exceeded during the 400°C phase of the experiment indicating that hot-spring fluids have lost heat and possibly dissolved Fe and base metals on ascent to the seafloor from a high-temperature reaction zone. The relative rates at which Zn, Cu, Fe, and Mn achieve new steady-state concentrations in response to temperature reduction during basalt alteration are Cu > Fe > Zn > Mn. Thus, dissolved Zn and especially Mn concentrations are more likely to record accurately temperatures characterizing subseafloor reaction zones, while Fe and Cu, which respond rapidly to changes in temperature, are likely to precipitate along fluid flow paths, especially in near-seafloor locations. This trend is confirmed by the relative abundance of dissolved metals in hot-spring fluids variably influenced by heat loss processes. Calculations suggest that hot-spring fluids may lose heat by conduction through sulfide mounds and chimney walls at flow rates observed for many vent systems. Since heat loss processes influence the chemistry, especially dissolved metals, as well as measured temperatures of axial hot-springs fluids, these effects must be taken into account to model accurately phase relations and fluid-mineral equilibria in deeply seated subseafloor reaction zones.