PB ISOTOPE STUDY OF MINERALIZATIONS AT OC EANIC HYDROTHERMAL VENT FIELDS AND HETEROGENEITIES IN THE NORTH FIJI BACK-ARC BASIN (SW PACIFIC)

Pb isotope compositions were determined on sulfides collected from hyd rothermal vent fields on the North Fiji Basin spreading ridge. On Pb-P b diagrams, the compositions are within both the MORB field and the le ast radiogenic part of the NFB basalt domain. Compared with the rather large variation of Pb isotope composition of the NFB basalts, our dat a indicate a remarkable homogeneity of Pb isotope ratios in the hydrot hermal sulfides studied, and suggest that sulfide lead reflects the av erage isotope composition of the upper part of the oceanic crust affec ted by hydrothermal processes. Thus, for the upper mantle below the NF B, sulfide lead indicates the presence of a DUPAL-type anomaly, or con tamination by recently subducted sediments. The data also reveal isoto pic heterogeneities at a ridge-segment scale that are correlated with petrological variations of NFB ridge basalts.