Chemical processes in buoyant hydrothermal plumes on the East Pacific Rise near 21°N

Abstract

Hydrothermal plume water containing suspended particles of precipitated black “smoke” has been sampled from three hydrothermal vent fields on the East Pacific Rise near 21°N. The samples were taken within the lower 22 m of the buoyant plumes, directly above 273–350°C black smoker vents, using Go-flo bottles mounted on the submersible Alvin. Based on Li as a conservative tracer of the high-temperature endmember solution, mixing ratios of 10 2 to 10 4 g seawater/g vent water are achieved within the lower 22 m of the plumes. The particle concentrations sampled in the lower 13 m of the plumes are much larger than predicted for these mixing ratios, suggesting that the particles are settling within the lower part of the plume. The major minerals in the smoke are pyrrhotite, Fe-rich sphalerite, pyrite, unidentified Fe − S ± Si and Fe − Si ± S phases, chalcopyrite, amorphous silica, sulfur, Fe-oxyhydroxides (including goethite) and anhydrite. Present in trace quantities are barite, isocubanite, wurtzite, covellite, marcasite(?), and unidentified silicates and Alsilicates. Organic matter is common. The composition of the plume solutions indicates that 35 ± 25% of the hydrothermally injected Fe remains in dissolved form (i.e., < 0.45 μm) within the lower 22 m of the plume, in spite of a 3- to 9-fold excess of H 2S in the vent solutions. Nearly all of the Mn and Si and most of the Ba also remain in solution. H 2S, by contrast, has been largely (70–100%) removed—by precipitation as Sulfides and sulfur, but mainly by oxidation to dissolved species with intermediate oxidation states. Its early removal implies that 50 ± 30% of the injected Fe will eventually precipitate as primary oxides rather than as Sulfides and that Fe can be fractionated from the other chalcophile metals during sulfide precipitation. Compared with the vent water, the particles are enriched in Cu, Co, Ag, Cd, Al, Zn, Pb, and Ni by factors of 4 to 20 relative to Fe and depleted in Ba, S, Si, and Mn. Scavenging from seawater may play a role in Co and Cu enrichment at the NGS field, and for Ag, Cd, and Ni at all three fields, but is negligible for Pb and Zn. These results indicate that two distinct metalliferous components are delivered to the distal regions of a plume as a result of reactions in the lower part of the buoyant plume: 1. 1) a dissolved component that includes about half the Fe and all of the Mn and will eventually precipitate as oxides, and 2. 2) a particulate component consisting of fine-grained sulfide minerals that are enriched in ore metals by factors of 4 to 20.