Re-Os isotopic evidence for ancient melt depletion in refertilized Neo-Tethyan suboceanic mantle domain

Ancient mantle domains with unradiogenic Os isotopes have been well documented in the Yarlung-Tsangpo ophiolites (YTO), southern Tibet. Nonetheless, previous Os isotope studies have yielded relatively young Re-depletion model (TRD) ages of <1.0 Ga for mantle peridotites of the Zedong ophiolite in the eastern segment of the YTO. In this study, a comprehensive petrological and geochemical study has been conducted on a suite of mantle samples including harzburgites, lherzolites and one pyroxenite. Our results indicate that the Zedong harzburgites have been subjected to low to moderate degrees (∼6–20 %) of partial melting and late-stage melt-rock interaction with basaltic melts. In contrast, both lherzolites and pyroxenite were generated by melt refertilization. The Zedong lherzolites display flat patterns of highly siderophile elements (HSE; Os, Ir, Ru, Pt, Pd, Re), whereas the harzburgites show depletion in Pt, Pd and Re relative to Os, Ir and Ru. The Re-Os isotopes of the Zedong lherzolites have been modified by melt refertilization and thus cannot provide reliable age information. Three Zedong harzburgites with the lowest PdN/IrN and ReN/IrN ratios have the least radiogenic 187Os/188Os of 0.11497–0.11537, giving the old TRD ages of 1.89–1.98 Ga. These ages are considerably older than previously reported TRD ages for the Zedong peridotites (< 1.0 Ga), but comparable to the oldest TRD age for the Luobusa peridotites. This supports the occurrence of similarly old mantle domains in both Zedong and Luobusa ophiolites in the eastern part of the YTO. The Zedong pyroxenite has a radiogenic 187Os/188Os ratio of 0.158 that is similar to the 187Os/188Os ratios (0.156–0.180) of previously reported for the YTO pyroxenites, which can be explained by selective assimilation of interstitial sulfides during the percolation of migrating melts through the mantle lithosphere. Grain boundary sulfides may play a significant role in the high 187Os/188Os of the YTO pyroxenites.