Processes controlling highly siderophile element fractionations in xenolithic peridotites and their influence on Os isotopes

Xenolithic peridotites having a similar range of major element compositions from two nearby localities in the Trans-North China Orogen, North China Craton, provide a rare opportunity to explore effects resulting from both primary partial melting and secondary processes on Os isotopes and highly siderophile element (HSE) abundances. HSE patterns of peridotites from Hannuoba are similar to those of orogenic peridotite massifs worldwide, but are rare for xenolithic peridotites. These patterns can be explained by relatively low degrees of melt depletion, coupled with long-term preservation of sulfides. By contrast, peridotites from Yangyuan have major element compositions similar to or slightly more depleted than Hannuoba xenoliths, but are characterized by distinct, highly fractionated HSE patterns with lower total HSE abundances and Os, Pd and Re depletions relative to Ir. Some of the latter HSE characteristics must reflect secondary processes. The low S and Se contents of Yangyuan peridotites, coupled with scarcity of observable sulfides, suggest that they experienced sulfide breakdown, possibly as a result of interaction with a S-undersaturated melt/fluid. This may have occurred under oxidizing conditions, as suggested by the somewhat higher ƒO2 recorded in the Yangyuan peridotites compared to the Hannuoba peridotites, as well as the metal-deficient composition of rare, mono-sulfide-solid solution (mss) sulfides within the Yangyuan peridotites. We speculate that under such conditions, Os, Pd, and possibly Re, more strongly partition into a sulfide liquid, or the oxidizing medium (melt or fluid), than Ir and Pt and, thus, become depleted. These effects would have been imposed on original patterns that were similar to those in the Hannuoba suite. The good correlation between 187Os/188Os and major element indices of melt depletion in the Yangyuan rocks, coupled with the poor correlation between 187Os/188Os and 187Re/188Os, suggests that the S, Os, Pd and Re removal was recent. Hence, the long-term Re–Os isotopic systematics of these rocks would not have been affected, and Re depletion model ages, based on Os isotopes, remain viable to constrain the timing of melt deletion in these peridotites. The similarity of model age distributions between Yangyuan and Hannuoba peridotites (TRD = 0 to 1.7 and 0 to 1.5 Ga, respectively) is consistent with this, and further indicates that these peridotites formed in the Paleoproterozoic.