Chronology of selected hydrothermal Mn oxide deposits from the transatlantic geotraverse “TAG” area, Mid-Atlantic Ridge 26°N

Radiometric ages on discrete layers in some hydrothermally deposited Mn-oxide (birnessite) samples from the TAG hydrothermal field on the Mid-Atlantic Ridge, range from 16,000 to 4,000 years, with marked increased activity noted about 15,000 and 7,000 years ago. These observations, along with previously published data and inferences on different kinds of hydrothermal activity in the TAG field, suggest episodic, multistage hydrothermal events ranging from high temperature black smokers precipitating sulfides to low temperature events precipitating Fe and Mn oxides over this entire time range. Black smokers are presently active. The location of the various deposits on the eastern wall of the median valley at this latitude suggests that the high temperature events occur at greater depths.