Abstract
Highly siderophile elements such as Pt and Re are important tracers for Earth's crust–mantle differentiation. The behavior of Pt and Re in oxidized arc magmas are thought to be controlled by their strong partitioning into sulfides and/or magnetite. Disequilibrium fractionation of sulfide liquid was proposed to explain the Pt systematics in magmas when using sulfide liquid–silicate melt partition coefficients of Pt (DPtSul/Sil) of 105. However, the segregating sulfides are coexisting monosulfide-solid-solution (MSS) and Cu-rich sulfide liquid in thin island arcs, but mainly as MSS in thick continental arcs. Here we experimentally determine the MSS–silicate melt partition coefficients of Pt (DPtMSS/Sil) and magnetite–silicate melt partition coefficients of Re (DReMag/Sil) at 0.4–0.5 GPa, 1020–1100°C, and oxygen fugacity (fO2) from below FMQ-1.7 to ∼FMQ+2.5, using a piston cylinder apparatus. The results show that DPtMSS/Sil range between 20–8700 and increase with increasing Pt concentration (20–530 μg/g) in MSS at a given fO2, which can be explained by the presence of Pt-rich nuggets in MSS and the non-Henrian law behavior of DPtMSS/Sil. This finding suggests that Pt in oxidized arc magmas is much less chalcophile than previously thought. The obtained DReMag/Sil increase from 0.5 to 2.9 with fO2 decreasing from ∼FMQ+2.5 to FMQ+1, suggesting that Re is incompatible to slightly compatible in magnetite in oxidized arc magmas. Re dissolves as Re4+ in magnetite through the substitution of Re4+ + Fe2+ for 2Fe3+. The application of our new Ds together with previous Ds to a magmatic differentiation model, which involves an equilibrium fractionation of MSS and/or sulfide liquid, magnetite, and silicate minerals, can fully reproduce the evolution trends of Pt, Re, and Cu in oxidized, sulfide-saturated arc magmas. Our model results demonstrate that equilibrium fractionation of sulfides predominantly controls the behavior of Pt and Re in oxidized arc magmas with sulfide saturation.
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