Alvin's manipulator reaches toward a black smoker chimney, seen through the sub's viewport, at 17°S on the East Pacific Rise. Hot hydrothermal fluids surge through the chimney at velocities of 1 to 5 meters per second. The "black smoke" consists of an abundance of dark, fine-grained, suspended particles that precipitate when the hot fluid mixes with cold seawater. (Photo by Meg Tivey, Woods Hole Oceanographic Institution)
During Stage 1 of black smoker chimney growth, hot, calcium-rich vent fluid mixes turbulently with cold, sulfate- and calcium-rich seawater, resulting in precipitation of a ring of calcium sulfate (anhydrite). Metal sulfides and oxides carried in the hot fluid also precipitate rapidly during the mixing process, forming a plume of dark particles above the vent. During Stage 2 of chimney growth, the initial chimney wall of anhydrite forms a surface on which chalcopyrite (copper-iron sulfide) begins to precipitate and plate the inner chimney wall. Mixing of seawater and hydrothermal fluid components across the porous wall by advection and diffusion results in the deposition of zinc, copper-iron, and iron sulfides in the interstices of the wall, which gradually makes the chimney less porous and more metal-rich.
In this schematic drawing of the TAG active hydrothermal mound, hydrothermal fluid rises rapidly and exits the mound at the Black Smoker Complex. Cold calcium- and sulfate-rich seawater is entrained into the mound, where it mixes with hydrothermal fluid. The mixing causes anhydrite, pyrite, and chalcopyrite to precipitate inside the mound. This precipitation increases the acidity of the hydrothermal fluid. Zinc and other elements, such as silver, gold, and cadmium, dissolve in this acidic fluid, allowing them to be carried by the white smoker fluid to the edges of the mound at the "Kremlin" area. Here the cooler temperatures within white smoker chimney walls cause the elements to precipitate.
Author Margaret Tivey examines the top of a spire of a black smoker chimney retrieved by Alvin from the East Pacific Rise at 17°S.
Thin-section samples of chimney wall specimens, examined under a microscope, reveal metal sulfide particles (black) embedded in and around anhydrite crystals (clear).