Age dating of sulfide deposits from axial and off-axial structures on the East Pacific Rise near 12050'N

During several cruises (Clipperton, 1981; Cyatherm, 1982 and Geocyarise Leg 3, 1984), conducted between 12040'N and 12054'N on the East Pacific Rise, massive sulfide deposits have been dredged and recovered by the diving saucer ''Cyana''. These well localized and documented samples have been analyzed for their 210Pb/Pb ratio and uranium series disequilibrium in view of dating the hydrothermal phenomenon. Using samples which are presently forming, the initial 210Pb/Pb ratio has been established as 0.60+-0.06. Ten sulfide samples from the central graben (near 12050'N) have ages less than about 10 years and are younger than most of the deposits found at 210N on the East Pacific Rise. One sample from the eastern rim of the graben presents an age of about 150 years. Two off-axis structures with hydrothermal sulfide deposits were studied. The first is a semi-circular volcanic structure where low-temperature hydrothermal vents were observed at 500 m and 1000 m from the rise axis; sulfide samples collected from these sites yielded ages of about 2000 years. Further east, about 6 km from the rise axis, the second off-axis structure, called South Eastern (SE) Seamount presents thick inactive sulfide deposits as well as Mn-rich and nontronite deposits. An age around 20,000 years has been found for these deposits. These ages are discussed, and, taking into acount the asymmetrical structure of the ridge near 12043'N, it is concluded that the formation of the SE Seamount occurred 20,000 years ago, on a basement of about 100,000 years old, accompanied by an extensive hydrothermal event. This indicates that an important eastward shift (4-5 km from the graben) of the volcano-hydrothermal activity took place in this area (12043'N). (orig.)