Intellectual merit: Circumstantial evidence suggests that [1] most Pb in the silicate mantle of the deep Earth is contained in sulfide phases, [2] Pb strongly prefers to be in sulfide relative to silicate melt, and [3] rates of Pb mobility in sulfide (solid or melt) are likely to be very fast, and given the likelihood that low viscosity sulfide melt will form interconnected networks in mantle silicates. If true, sulfide phases may profoundly influence Pb behavior during mantle melting, and may hold the key to several long-standing and important Pb paradoxes and enigmas. For example, both ocean ridge and hotspot basalts have nearly constant Ce/Pb (near 25), implying similar partitioning for Ce and Pb during magmatic processes, yet silicate mineral/melt partition coefficients for Ce and Pb differ significantly. Also, both ridge and hotspot basalt trace element patterns exhibit large negative Pb anomalies relative to the bulk silicate earth, implying large Ce/Pb fractionations during melting, and/or deficiencies of Pb in the mantle sources for these basalts. Preliminary experiments suggest that Pb is indeed preferentially hosted by sulfide relative to the silicate melt (by a factor of ~50), and detailed modeling of melting in the presence of sulfide shows that constant Ce/Pb ratios can be produced over a wide range of extent of melting, with the melts directly reflecting the Ce/Pb of the mantle source. Thus the mantle may indeed be depleted in Pb, and long-term sequestering of sulfide (and Pb) into the deep mantle or core is probable . To more fully investigate the behavior of Pb in the mantle, a three year program is proposed to measure the partitioning of Pb between sulfide melt and silicate melt, and between solid sulfide and molten sulfide. The solidus and liquidus of Fe-Ni-Cu sulfide and the rate of Pb diffusion in solid sulfide (pyrrhotite) and in Fe-Ni-Cu sulfide melts will also be determined under high P-T conditions corresponding to mantle depths to ~100 km, and a range of temperatures and activities of oxygen and sulfur relevant to mantle melting. These experiments will help constrain the following: [1] origin of the Ce/Pb paradox in oceanic basalts, [2] Pb isotope evolution in terrestrial reservoirs, [3] the potential of sulfide networks as metasomatic agents, [4] the role of sulfide melts in redistributing Pb (by advection or diffusion) across otherwise isotopically heterogeneous mantle domains, and [5] broader understanding of mantle evolution and deep earth processes.
Broader impacts: Results of this study will be disseminated through the publication routes, and via a web site aimed at a more lay audience; they will be incorporated in courses taught by both in the WHOI/MIT Joint Program, and seminars in the public domain. Both PIs are active in the lab training of students and postdocs of all levels, and this research will provide many opportunities for teaching techniques of experimental petrology and trace element geochemistry.
