Mantle Pb paradoxes: the sulfide solution
Stanley R. Hart,Glenn A. Gaetani +1 more
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In this paper, it was shown that long-term segregation of sulfide (containing Pb) to the core may resolve the Pb isotope paradox and enigmas.Abstract:
There is growing 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 possibility that sulfide melt “wets” sub-solidus mantle silicates, and has very low viscosity, the implications for Pb behavior during mantle melting are profound. There is only sparse experimental data relating to Pb partitioning between sulfide and silicate, and no 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 isotope 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. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB is greater than bulk earth, and constant at a value of 25. The constancy of this “canonical ratio” implies similar partition coefficients for Ce and Pb during magmatic processes (Hofmann et al. in Earth Planet Sci Lett 79:33–45, 1986), whereas most experimental studies show that Pb is more incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the sulfide melt/silicate melt partition coefficient for Pb has a value of ∼ 14. Modeling shows that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle sources for these basalts. 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.read more
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Copper systematics in arc magmas and implications for crust-mantle differentiation.
Cin-Ty A. Lee,Peter Luffi,Emily J. Chin,Romain A. Bouchet,Romain A. Bouchet,Rajdeep Dasgupta,Douglas M. Morton,Véronique Le Roux,Véronique Le Roux,Qing-Zhu Yin,Daphne Jin +10 more
TL;DR: The redox state of arc magmas is tracked from mantle source to emplacement in the crust on the basis of copper’s (Cu) affinity for reduced sulfur phases, suggesting that the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.
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Partitioning of platinum-group elements and Au between sulfide liquid and basalt and the origins of mantle-crust fractionation of the chalcophile elements
James E. Mungall,James M. Brenan +1 more
TL;DR: In this paper, the authors present direct experimental measurements of D PGE sul, based on in situ microanalysis of the sulfide and silicate melt, with values ranging from ∼4.5 × 105 (Ru) to ∼2.3 × 106 (Ir, Pt) and they use their new set of partition coefficients to develop a fully constrained model of PGE behavior during melting which accurately predicts the abundances of PME in mantle-derived magmas and their restites, including mid-ocean ridge basalts, continental picrites, and the parental magmas
Journal ArticleDOI
Sulfur K-edge XANES analysis of natural and synthetic basaltic glasses: Implications for S speciation and S content as function of oxygen fugacity
TL;DR: XANES analyses at the sulfur K-edge were used to determine the oxidation state of S species in natural and synthetic basaltic glasses and to constrain the fO2 conditions for the transition from sulfide (S 2� ) to sulfate (S 6+ ) in silicate melts as mentioned in this paper.
Journal ArticleDOI
Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism
TL;DR: In this article, the Mesozoic continental arcs in the North American Cordillera were examined to establish a baseline model for Phanerozoic continent formation, which allowed the petrogenesis and tectonics of Phanerzoic crust formation to be linked in space and time.
Compositions of HIMU, EM1, and EM2 from Global Trends between Radiogenic Isotopes and Major Elements in Ocean Island Basalts
TL;DR: In this article, the relationship between major elements and isotopes in ocean island basalts (OIBs) was analyzed based on the relationships between parent-daughter elements and radiogenic isotopes.
References
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