Chemical exchange during hydrothermal alteration of basalt by seawater—II. Experimental results for Fe,Mn, and sulfur species

Fresh mid-ocean ridge basalts of varying crystallinity and an andesite were reacted with seawater and with a Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. $\text{Water}\text{rock}$ mass ratios of one to three were used and durations ranged from two to twenty months. The concentrations of Fe, Mn, and reduced and oxidized sulfur species in solution reached steady state in most of the experiments at 400–500°C, but not in those at 200–300°. The concentrations of Fe and Mn were a few ppm at 200–300° and increased greatly with temperature between 300 and 500°. The low values at 200–300° are probably related to the uptake of Fe and Mn by smectite at the in situ pH, which was slightly acid at 200° and slightly alkaline at 300°. The quench pH values decreased with increasing temperature above 300°. The only reliable data for the concentration of Zn in solution were obtained at 400°, where values 1–2 ppm were found. Copper was extensively leached from basalt and andesite and was deposited as part of a Cu-Au alloy in the capsule walls or, in some experiments, as chalcopyrite.Reduced sulfur was readily leached from basalt into solution, and was also produced by the reduction of seawater sulfate by ferrous iron derived from the basalts. The proportion of seawater sulfate which was reduced in the experiments with a $\text{water}\text{rock}$ ratio of one varied from 5–10% at 300°C to > 95% at 500°. The rate of sulfate reduction depended on the run temperature, on the crystallinity and initial sulfur content of the rocks used as starting materials, and on the $\text{water}\text{rock}$ ratio. The final concentration of reduced sulfur in solution increased greatly with temperature, and generally exceeded that of Fe on a molal basis.The oxide-sulfide assemblages produced in the experiments resemble those in the basalt-seawater geothermal system at Reykjanes, Iceland, and in hydrothermally altered basalts and gabbros from the oceanic crust; they include pyrite, pyrrhotite. chalcopyrite, hematite, and probably magnetite. The particular assemblage varied systematically with the temperature, rock type, and crystallinity of each run. Anhydrite precipitated in all experiments with seawater, at all temperatures from 200–500°C. However, its persistence to the end of the runs was apparently metastable, as it should have reacted with the final solutions to produce pyrite or pyrrhotite.