LEAD-ISOTOPE SYSTEMATICS IN PACIFIC HYDROTHERMAL SULFIDE DEPOSITS

Sulfide samples from 12 hydrothermal volcanogenic massive sulfide (VMS ) sites around the Pacific Ocean were analyzed for their lead isotope composition. They correspond to various geological environments, such as sediment-starved ridges (East Pacific Rise (EPR), Galapagos, Juan d e Fuca), sediment-covered ridges, and back are basins, Sulfides from a ll areas display remarkably homogeneous lead isotope compositions caus ed by hydrothermal circulation. Our results show that lead isotope com positions of sulfides have two major implications. The first aspect, a t the scale of a volcanic segment, concerns metallogenic processes. Le ad isotope compositions demonstrate that lead is leached from basalt i n sediment-starved ridges and that part is derived from sediment at se diment-covered ridges. This conclusion confirms previous studies, At 1 3 degrees N on the EPR, results are compatible with two separate hydro thermal 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 fo ssil VMS deposits open the discussion on the processes that produced h ighly homogeneous deposits in the same district. The second aspect, at a more regional scale, concerns the geodynamic and geochemical proces ses occurring at plate boundaries. Owing to homogenization through hyd rothermal circulation, lead isotope compositions in VMS deposits can b e considered as having the mean lead isotope composition of basalts fr om the same volcanic segment. In the Lau Basin, a slab with a Pacific lead isotope composition is subducted beneath the Indian Ocean lithosp here. The lead isotope composition of VMS samples illustrates the inte raction 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 are influence with incre asing age of the ridge and distance to the island are.