Local Os isotope mantle heterogeneities preserved during melt transport processes in supra-subduction mantle at Voikar Ophiolite (Polar Urals)

We report a detail study of Highly Siderophile Elements (HSE) and Re-Os isotopes of lithologies from the Voykar Ophiolite (Polar Urals) which formed in a suprasubduction tectonic environment. Our data suggest that refractory harzburgite exhausted by melting more than 2 Ga ago was then reactivated by suprasubduction melts at about 0.6 Ga. These melts have formed dunite reaction channels and pyroxenite veins and locally redistributed the HSE in the surrounding harzburgite. In dunite channels and pyroxenite veins numerous sulfide globules transported by silicate melts have been observed. These sulfide globules make up the HSE budget of the pyroxenites and they are characterized by low concentration of Os, Ir and Ru compared to Pt, Pd and Re. High Re abundance in the pyroxenite (up to 2 ppb) suggest contribution from subduction slab. HSE composition of dunites results from reaction between percolating melt and host peridotite. Close to the contact, residual peridotites become enriched in Pt, Pd and Re. But primary HSE features of the harzburgite, such as Pd, Pt, Re depletion relative to Ir, Os and Ru appear to be preserved in a distance of about 50 cm away from the contact with dunite channel. The more important that even ancient Os isotope signatures (2 Ga model ages) are preserved in the harzburgite at 50 cm from the contact with dunite. Also relatively small blocks of harzburgite relicts within network of replacive dunites preserve similar ancient Os isotope signatures. These suggest that local heterogeneity in HSE concentrations and Os isotopes in the convecting mantle could survive melting and focused melt transport events.