Lead and osmium isotopic constraints on the oceanic mantle from single abyssal peridotite sulfides

Single sulfides from abyssal peridotites have been analyzed for Pb and Re-Os to constrain the evolution of oceanic mantle composition. These represent the first analyses of Pb and Os isotopic compositions in the same sulfide grain. The sulfides are from Gakkel and Southwest Indian ridge peridotites, occur at < 0.1% modal abundances, and contain 0.001-0.4 ppm Re, 0.003-5 ppm Os, and 0.12-12 ppm Pb. Sulfide Pb isotopic compositions extend from depleted (e.g., Pb-206/Pb-204=17.0) to enriched (19.6), covering a larger range than associated basalts. The Os isotopic range of sulfides is more restricted, but extends from depleted (Os-187/Os-188 = 0.116) to enriched (0.150). Pb and Os concentrations and isotopic compositions co-vary, with correlation coefficients of 0.76-0.94. Both Pb and Re-Os isotopic data follow similar to 2 Ga isochrons, with isotopic compositions varying down to small ( << 1 km) length-scales and some sulfides containing supra-chondritic Re-187/Os-188 and Os-187/Os-188. These observations are best explained by long-term recycling of oceanic lithosphere combined with melt extraction and refertilization at ancient ocean ridges, rather than a specific event at 2 Ga.