Preservation of ancient Os isotope signatures in the Yungbwa ophiolite (southwestern Tibet) after subduction modification Chuan-Zhou Liu ⇑ , Fu-Yuan Wu, Zhu-Yin Chu, Wei-Qiang Ji, Liang-Jun Yu, Ji-Liang Li State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China article info Article history: Available online 13 September 2011 Keywords: Re–Os isotopes Highly siderophile elements The Yungbwa ophiolite Indus-Yarlung Zangbo Suture The Tibetan Plateau abstract Both spinel and plagioclase harzburgites from the Yungbwa ophiolite, southwestern Tibet, have been studied for highly siderophile elements (HSEs) and Re–Os isotopes. The spinel harzburgites can be sub- divided into low-Cr# and high-Cr# groups, according to the spinel Cr#. The low-Cr# harzburgites have been estimated to experience with ca. 13% degrees of partial melting, whereas the high-Cr# harzburgites have been subjected to 16–18% degrees of melting. Clinopyroxenes in the low-Cr# harzburgites show depleted REE patterns, indicating they have been negligibly modified by melts. In contrast, both high- Cr# and plagioclase harzburgites have been metasomatized to different extents, as evidenced by the variable enrichment of LREE in their clinopyroxenes. Compositions of the metasomatic agents are of affin- ities to the subduction-related magmas. The low-Cr# harzburgites have equilibrium temperatures higher than both high-Cr# and plagioclase harzburgites. We explain that the low-Cr# harzburgites represent mantle residues after melt extraction at mid-ocean ridge (MOR), whereas both high-Cr# and plagioclase harzburgites were originated from the supra-subduction zone (SSZ) settings. These different types of mantle peridotites were tectonically juxtaposed during the emplacement of the Yungbwa ophiolite. The low-Cr# harzburgites display consistent highly siderophile element (HSE) patterns with suprachon- dritic Ru/Ir and Pd/Ir ratios, whereas both high-Cr# and plagioclase harzburgites show fractionated HSE patterns. A high-Cr# harzburgite, GHP-86, is strongly depleted in both Pd and Re. The Yungbwa harzburg- ites have variable 187 Os/ 188 Os ratios ranging from 0.12228 to 0.12876. Sample GHP-86 has the less radio- genic 187 Os/ 188 Os ratio, giving a depletion age (T RD ) of 1 Ga. The Re–Os isotope compositions of different types of the Yungbwa harzburgites suggest that their 187 Os/ 188 Os ratios have not been remarkably mod- ified by subduction-related melts. Furthermore, subduction modification has not erased the ancient Os signatures in the Yungbwa peridotites. The reason is probably because the primary unradiogenic Os in the Yungbwa harzburgites had not been significantly reduced during the metasomatism. The astheno- spheric mantle beneath the Neo-Tethys Ocean, from which the Yungbwa ophiolite was derived, had heterogeneous Os isotope compositions and contained some ancient mantle domains. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Both rhenium and osmium are siderophile elements and form a long-lived isotope system, in which 187 Re decays to 187 Os with a decay constant (k) of 1.666  10 11 year 1 (Shirey and Walker, 1998). Osmium behaves like a compatible element during partial melting of mantle peridotites, whereas Re behaves as a moderately incompatible element with a bulk partition coefficient similar to aluminum (Reisberg and Lorand, 1995). Melt extraction, thus, leads the melting residues to unradiogenic 187 Os/ 188 Os ratios. Osmium in mantle peridotites is dominantly controlled by sulfides, and some sulfide grains are trapped as inclusions within the silicate minerals (Alard et al., 2000, 2005; Morgan and Baedecker, 1983). Therefore, Os isotopic equilibrium among the sulfide inclusions and other minerals is hard to be achieved through diffusion (Burton et al., 1999), which leads to the Os isotope heterogeneity in the mantle. On the other hand, the 187 Os/ 188 Os ratios of mantle peridotites are robust to late metasomatic processes, because mantle perido- tites commonly contain Os contents two orders of magnitude high- er than the basaltic melts (Rudnick and Walker, 2009). This feature makes the Re–Os isotope system a good tracer for heterogeneity that is preserved in the mantle. Previous Re–Os studies on abyssal peridotites have demonstrated that the modern asthenospheric mantle is highly heterogeneous in Os isotopes (Brandon et al., 2000; Harvey et al., 2006; Liu et al., 2008; Meibom and Frei, 2002; Meibom et al., 2002). Ancient mantle with Os model ages up to 2 billion years (Ga) can be preserved through the convective stirring in the asthenosphere (Harvey et al., 2006; Liu et al., 2008). Ophiolites represent sections of oceanic lithosphere that were generated by sea-floor spreading and later emplaced onto 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.08.010 ⇑ Corresponding author. Tel.: +86 10 82998547; fax: +86 10 62010846. E-mail address: chzliu@mail.iggcas.ac.cn (C.-Z. Liu). Journal of Asian Earth Sciences 53 (2012) 38–50 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes