HR: 14:55h
AN: V23D-06    [Abstracts]
TI: Mantle Pb Paradoxes: The Sulfide Solution
AU: * Hart, S R
EM: shart@whoi.edu
AF: Woods Hole Oceanographic Institution, MS 25 360 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Gaetani, G A
EM: ggaetani@whoi.edu
AF: Woods Hole Oceanographic Institution, MS 25 360 Woods Hole Road, Woods Hole, MA 02543 United States
AU: Kelemen, P B
EM: peterk@ldeo.columbia.edu
AF: Lamont-Doherty Earth Obseratory, P.O. Box 1000 61 Route 9W, Palisades, NY 10964-1000 United States
AB: There is strong circumstantial evidence that the budget of Pb in mantle peridotites is largely contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt) are very fast. Given the likelihood that sulfide melt `wets' sub-solidus mantle silicates, and has very low viscosity, the implications for Pb behavior during mantle melting are profound. Yet there is literally no experimental data relating to Pb partitioning between sulfide and silicate, nor any data on Pb diffusion rates in sulfides. A full understanding of Pb behavior in sulfide may hold the key to several long-standing and important Pb paradoxes and enigmas. The classical Pb paradox arises from the fact that all known mantle reservoirs lie to the right of the Geochron, with no consensus as to the identity of the "balancing" reservoir. We propose that long-term segregation of sulfide (containing Pb) to the core may resolve this paradox. A second Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB is constant at a value of 25, implying similar partition coefficients for Ce and Pb during magmatic processes (Hofmann et al. 1986). Yet both MORB and OIB trace element patterns have large negative Pb anomalies, when normalized to a bulk silicate earth (BSE) Ce/Pb of 11. This implies large deficiencies of Pb in the mantle sources for these basalts (relative to bulk silicate earth). However, sulfide may create negative Pb anomalies during melting by sequestering Pb in residual mantle sulfide. Sulfide may play other important roles during magmagenesis: 1). advective/diffusive sulfide networks may form potent metasomatic agents (in both introducing and obliterating Pb isotopic heterogeneities in the mantle); 2). silicate melt networks may easily exchange Pb with ambient mantle sulfides (by diffusion or assimilation), thus `sampling' Pb in isotopically heterogeneous mantle domains differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an apparent `de-coupling' of these systems.
DE: 1025 Composition of the mantle
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005