A.-S. Andrieu,2 J.J. Honnorez,3 and J. Lancelot2


Lead isotope analyses were performed on sulfides from the Trans-Atlantic Geotraverse (TAG) hydrothermal site, located at 26°08´ N, on the unsedimented, slow-spreading Mid-Atlantic Ridge, which is characterized by a high Pb isotopic heterogeneity of the basalts erupting along its axis. Pb isotope ratios of TAG massive sulfides are very uniform. Reported in Pb-Pb diagrams, TAG sulfide data plot on the Northern Hemisphere Reference Line and define a small domain compared with the larger field of the Mid-Atlantic Ridge basalts, which suggests that the main lead sources for the TAG mineralization are the components of the upper oceanic crust. The means of the Pb isotopic compositions of TAG sulfides and neighboring basalts are similar, and identical to the lead composition of present TAG hydrothermal vent fluid. These data suggest that the Pb isotopic ratios of TAG sulfides reflect the average isotope composition of the upper part of the oceanic crust affected by the large-scale hydrothermal fluid circulation, and that the Pb isotopic compositions of the hydrothermal vent fluids recorded in recent and ancient sulfides remain constant during the active interval of a main hydrothermal vent field such as TAG.

1Herzig, P.M., Humphris, S.E., Miller, D.J., and Zierenberg, R.A. (Eds.), 1998. Proc. ODP, Sci. Results, 158: College Station, TX (Ocean Drilling Program).
2Laboratoire de Géochimie Isotopique, UMR 5567, CNRS-UMII, cc066, Université Montpellier II, Place E. Bataillon, 34095 Montpellier, cedex 05, France. Andrieu: andrieu@dstu.univ-montp2.fr
3Laboratoire de Géochimie de Surface, UPR-CNRS 6251, Université Louis Pasteur, 1 rue Blessig, 67084 Strasbourg cedex, France.