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%<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., 206Pb/204Pb=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 (187Os/188Os=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 ∼2Ga isochrons, with isotopic compositions varying down to small (⪡1km) length-scales and some sulfides containing supra-chondritic 187Re/188Os and 187Os/188Os. 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.The concentration of Pb in sulfides indicates that they host <4%<4% of the mantle Pb budget. A re-evaluation of the mass balance of Pb in peridotites indicates that most mantle Pb is stored in silicate phases. The Pb partition coefficient between sulfide melt and silicate melt is estimated to be ∼3∼3, based on the correlation of Pb–Os concentrations in this study and measured Os partition coefficients from the literature. These observations indicate that sulfides do not exert a strong control on the fractionation of Pb during mantle melting, but they can be used to constrain mantle Pb isotopic composition.Sulfides in this study, combined with literature data for Pb isotopes in peridotite whole rocks and pyroxene separates, provide evidence for ultra-depleted mantle, as 24% of peridotites are unradiogenic (i.e., 207Pb/204Pb <15.440<15.440 and 206Pb/204Pb <17.726<17.726), compared to only 3% of ridge basalts. This suggests that the mantle contains volumetrically significant reservoirs of ultra-depleted material, probably derived from recycled oceanic lithospheric mantle. These depleted reservoirs contribute in only small amounts to oceanic crust generation, both due to a limited ability to melt and to the dilution of any melt by more enriched melts during crust formation.

► We measure Pb and Re–Os in individual sulfides from abyssal peridotites.
► Sulfides store <4%<4% of the mantle Pb budget.
► The sulfide/silicate melt partition coefficient for Pb is ∼3∼3.
► Ancient melting/refertilization and mantle mixing can explain Pb–Os co-variation.
► Depleted peridotite reservoirs may store volumetrically significant amounts of the Earth's unradiogenic Pb.