Pb isotope study of black-smokers and basalts from Pito Seamount site (Easter microplate)

Previous studies of hydrothermal deposits on active sediment-starved spread ing centers have shown that the lead isotope compositions of the oceanic cr ust are homogenized by the circulation of high temperature fluids. This ave raging effect is confirmed by the homogeneity of the sulphide isotopic valu es at numerous sites in the Pacific ocean. Our study was undertaken at an a ctive site located on a seamount (Pito Seamount) formed at the tip of a pro pagator on the northeast boundary of the Easter microplate near 23 degrees 19'S. Lead isotope analyses, combined with a mineral paragenesis study, wer e performed on hydrothermal phases of black-smokers as well as on the adjac ent 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 Pb-207/Pb-204 and Pb-208/Pb-204 ratios. Such a large lead isotope variability has not been e ncountered elsewhere on the Easter microplate. Hydrothermal sample values p lot 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 Pb-207/Pb-204 and Pb-208/Pb-204 ratios were test ed for various mixing models. We made two major assumptions. (1) It is like ly 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-c rusts in the underlying basaltic sequences that subsequently reacted with h ydrothermal fluids. (C) 1999 Elsevier Science B.V. All rights reserved.