Active vents and massive sulfides at 26 degrees N (TAG) and 23 degrees N (Snakepit) on the Mid-Atlantic Ridge

Black smokers in differing geological settings occur at two active hydrothermal vent sites on the Mid-Atlantic Ridge at 26 degrees N (TAG) at 3700 m depth and at 23 degrees N (Snakepit) at 3500 m. TAG, the larger of the two and probably the older, is located on older sedimented crust a few km from the spreading axis, at the junction of the rift-valley floor and the east wall; Snakepit is on a volcanic ridge (40 km long, < or =600 m high) in the axial zone of the rift valley. Hydrothermal discharge from vents at both sites ranges from shimmering water, through white smokers (226 degrees C) to black smokers (300-350 degrees C). Hydrothermal solutions are similar in major-element composition to those from the East Pacific Rise. While minerals formed are similar to those on faster-spreading ridges -- the dominant sulphides are pyrite, pyrrhotite, chalcopyrite and sphalerite and anhydrite is the main sulphate -- the deposits differ from some on the east Pacific and Juan de Fuca ridges in having little or no baryte, very little amorphous silica, and in having abundant aragonite as a late-stage precipitate. Diagenesis and weathering, particularly at TAG, have produced abundant amorphous Fe oxides and hydroxides, goethite, hematite, atacamite, jarosite and sulphur. At Snakepit, the black smokers consist mainly of pyrrhotite but it is absent from the active chimneys at TAG. Zn sulphide is the predominant phase in the white smokers at both sites.