Os Isotopic Composition and Highly Siderophile Elements: Tracers of Mantle Melting and Melt Percolation Processes (Voykar Complex, Polar Ural Ophiolites)

The Voykar complex is located in the Northern part of the Uralian ophiolite belt and represents Early Devonian lithosphere formed in a suprasubduction environment (e. g. (1)). It is unique in that the mantle peridotites are very well preserved and virtually free of serpentine, and it provides excellent exposures. The mantle section of Voykar complex is composed of harzburgite which is intruded by numerous dunite and pyroxenite channels and contains chromitite ore deposits. The distribution of highly siderophile elements (HSE) in harzburgite and dunite channels can be explained by melt-peridotite reaction processes. The harzburgites (Cr/(Cr+Al) of cr-spinel = 0.3-0.4) show two types of HSE patterns. One is depleted in Pd, Pt and Re relatively to Os, Ir, Ru with (Pt/Ir)N=0.3, the second pattern is flat with (Pt/Ir)N=0.9. The first type of harzburgite has lower ratios of 187Os/188Os (0.1149) compared to the second (0.1236). Dunite and associated orthopyroxenite are significantly enriched in 187Os (187Os/188Os=0.1279 - 0.1327), hence shifted towards lava compositions. Their HSE patterns also show the influence of mantle melts, because they are enriched in Pd ((Pt/Ir)N=5-6). The clinopyroxenite has the highest ratio of 187Os/188Os up to 32.38 and shows even stronger depletion of Os, Ir, Ru relative to Pt, Pd, Re ((Pt/Ir)N=20). They could represent crystallization products of percolating melts. Chromitites have variable 187Os/188Os=0.1244-0.1352, and their HSE concentrations overlap with the range observed in chromitites world-wide. Osmium isotope data show evidence for at least two significant events in the magmatic history of the Voykar ophiolite mantle section: an ancient (2.1-1.9 Ga) melting event formed the depleted spinel harzburgite; a younger melt percolation event (0.6-0.5 Ga) led to the formation of pyroxenite veins, dunite channels and chromitites. The age of melt percolation events based on Os isotopic data coincides with the U-Pb age of zircons found in chromitites (2). The conservation of old ages (older than 1.9 Ga) in mantle sections more recently infiltrated by mantle melts indicate the long-time preservation of local-scale (less than100 m) isotope heterogeneities. References: 1. G. N. Savelieva, R. W. Nesbitt, Journal of the Geological Society 153, 525 (1996). 2.G. N. Savelieva, P. V. Suslov, A. N. Larionov, N. G. Berejnaya, Doklady Akademii Nauk in press (2006).