Chemistry of hydrothermal fluids at the TAG Active Mound, MAR 26°N, in 1998

Black smoker and clear fluids were sampled at the TAG active mound (26°08′N, 44°50′W) in August 1998. The major ion chemistry of the black smoker fluid appears to be stable over 12 years, indicating that the physico‐chemical condition of the deep reaction zone have not changed significantly within this period. The black smoker fluid chemistry is the same independent of the place of the venting on the mound, suggesting that the black smoker fluid is supplied from the single source. Two medium temperature clear fluid samples (256° and 219°C) are depleted in Ca 2+ , Fe 2+ , and H 2 S compared to the black smoker fluid due to the precipitation of anhydrite and iron sulfide caused by mixing with entrained seawater. These clear fluids are formed by the same mechanism as the white smoker and diffuse fluids.