ACTIVE VENTS AND MASSIVE SULFIDES AT 26ON (TAG) AND 23ON (SNAKEPITI ON THE MID.ATLANTIC RIDGE

Two active hydrothermal vent sites on the Mid-Atlantic Ridge at 26'N (TAG) and 23"N (Snakepit) have recently been discovered at depths of 37@ and 3500 m, respectively. Although black smokers are present at both siles, their geological settings differ. The TAG area 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; the Snakepit site is atop a large volcanic ridge (40 km long, up to 600 m high) in the axial zone ofthe rift valley. The TAG site is the larger of the two and is probably older. Hydrothermal discharge from vents at both sites ranges from shimmering water, through white smokers Q26'C) to black smokers (335"C and 350'C). Hydrothermal solutions are similar in major-element composition to those from tlte East Pacific Rise. Mineralization is similar to that occurring on faster-spreading ridges, e.9., the dominant polymetallic sulfides are pyrite, pyrrhotite, chalcopyrite and sphalerite; anhydrite is the main sulfate phase. The deposits differ from some of those on the East Pacific and Juan de Fuca ridges in having little or no barite, very little "morphous silica, and in having abundant aragonite as a latestage precipitate. Diagenesis and weathering, particularly at the TAG site, have produced abundant amorphous iron oxides and hydroxyoxides, goethite, hematite, atacamite, jarosite and sulfur. At the Snakepit site the black smokers consist mainly ofpynhotite, but this sulfide phase is absent from the active chimneys at TAG. Zinc sulfide occurs as the predominant phase in the lower-temperature white smokers at both sites.

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