PB ISOTOPE STUDY OF BLACK-SMOKERS AND BASALTS FROM PITO SEAMOUNT SITE (EASTER MICROPLATE)

Abstract Previous studies of hydrothermal deposits on active sediment-starved spreading centers have shown that the lead isotope compositions of the oceanic crust are homogenized by the circulation of high temperature fluids. This averaging effect is confirmed by the homogeneity of the sulphide isotopic values at numerous sites in the Pacific ocean. Our study was undertaken at an active site located on a seamount (Pito Seamount) formed at the tip of a propagator on the northeast boundary of the Easter microplate near 23°19′S. Lead isotope analyses, combined with a mineral paragenesis study, were performed on hydrothermal phases of black-smokers as well as on the adjacent basalts. The field defined by hydrothermal samples departs unexpectedly from the pillow lava field. Furthermore, the hydrothermal deposits display unexpected heterogeneous isotopic values with respect to 207 Pb / 204 Pb and 208 Pb / 204 Pb ratios. Such a large lead isotope variability has not been encountered elsewhere on the Easter microplate. Hydrothermal sample values plot between an evolved EPR basalt and sediment and/or seawater, suggesting that the lead was not derived from a homogeneous source such as end-member hydrothermal fluids generated in a homogeneous reaction zone in the oceanic crust. The origin of high 207 Pb / 204 Pb and 208 Pb / 204 Pb ratios were tested for various mixing models. We made two major assumptions. (1) It is likely that hydrothermal fluids percolated through very heterogeneous volcanic sequences formed of both enriched and depleted MORBs. This suggests cyclic magmatism and a minor involvement of enriched mantle components with time, as the last magmatic events gave N-MORB-type magmas. (2) Near the seafloor, it is likely that other sources of radiogenic Pb are sediments and/or Mn-crusts in the underlying basaltic sequences that subsequently reacted with hydrothermal fluids.