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Lead isotope systematics in Pacific hydrothermal sulfide deposits

Sulfide samples from 12 hydrothermal volcanogenic massive sulfide (VMS) sites around the Pacific Ocean were analysed for their lead isotope composition. They correspond to various geological environments, such as sediment-starved ridges (East Pacific Rise (EPR), Galapagos, Juan de Fuca), sediment-covered ridges, and back arc basins. Sulfides from all areas display rernarkably homogeneous lead isotope compositions caused by hydrolhermal circulation. Our results show that lead isotope compositions of sulfides have two major implications. The first aspect, at the scale of a volcanic segment, concerns metallogenic processes. Lead isotope compositions demonstrate that lead is leached from basalt in sediment-starved ridges and that part is derived from sediment at sediment-covered ridges. This conclusion confirms previous studies. At 13 o N on the EPR, results are compatible with two separate hydrothermal systems for axial and off-axial sulfide deposits. At this site, lead isotopes contribute to a better definition of the morphology of convective cells. Comparisons with the lead isotope composition of fossil VMS deposits open the discussion on the processes that produced highly homogeneous deposits in the same district. The second aspect, at a more regional scale, concerns the geodynamic and geochemical processes occurring at plate boundaries. Owing to homogenization through hydrothermal circulation, lead isotope compositions in VMS deposits can be considered as having the mean lead isotope composition of basalts from the same volcanic segment. In the Lau Basin, a slab with a Pacific lead isotope composition is subducted beneath the Indian Ocean lithosphere. The lead isotope composition of VMS samples illustrates the interaction of three contrasting end-member sources: basalts from Pacific Ocean and Indian Ocean plates and pelagic sediments. Our results show the progressive northward decrease of Pacific arc influence with increasing age of the ridge and distance to the island arc