Abstract

The submersible Alvin was used to investigate three active hydrothermal discharge sites along the southern Juan de Fuca Ridge in September 1984. The hydrothermal zones occur within a 10- to 30-m-deep, 30- to 50-m-wide cleft marking the center of the axial valley. This cleft is the eruptive locus for the axial valley. The hydrothermal vents coincide with the main eruptive vents along the cleft. Each hydrothermal zone has multiple discharge sites extending as much as 500 m along the cleft. Sulfide deposits occur as clusters (15–100 m2 area) of small chimneys (≤2 m high) and as individual and clustered fields of large, branched chimneys (≤10 m high). Recovered sulfide samples are predominantly the tops of chimneys and spires and typically contain more than 80% sphalerite and wurtzite with minor pyrrhotite, pyrite, marcasite, isocubanite, chalcopyrite, anhydrite, and amorphous silica. The associated hydrothermal fluids have the highest chlorinity of any reported to date.

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