HYDROTHERMAL ACTIVITY ON A 10(5)-YEAR SCALE AT A SLOW-SPREADING RIDGE, TAG HYDROTHERMAL FIELD, MID-ATLANTIC RIDGE 26-DEGREES-N

New radiometric dates are presented for the hydrothermal deposits in the Alvin and Mir relict hydrothermal zones located on the lower east wall of the rift valley within the TAG hydrothermal field and placed in context of the evolution of the TAG field as a whole over the past 10(5) years. The field includes an active high-temperature sulfide mound situated near the base of the east wall and a low-temperature zone at middepth on the east wall all within a 5 by 5 kilometer area. The oldest hydrothermal deposits sampled in the Mir relict zone, the low-temperature zone, and an inactive sulfide mound in the Alvin relict zone are low-temperature manganese crusts dated at 140,000, 125,000 and 74,000 years, respectively. High-temperature activity began about 100,000 years ago in the Mir zone. A 50,000 year-high-temperature event took place at the active sulfide mound, in the Mir zone, and at the inactive sulfide mound in the Alvin zone. This event is followed by sporadic activity at the Mir zone and the active sulfide mound, until 10,000 and 5000 years ago, respectively. A reactivation of the system affected the Mir zone from about 2000 to 600 years ago. The presently active sulfide mound was reactivated only 60 years ago, after a quiescence of about 4000 years. The radiochronologic record reveals that the hydrothermal activity has been episodic with a major high-temperature event throughout the TAG field at 50,000 years and other more localized events.