Widespread Os-isotopically ultradepleted mantle domains in the Paleo-Asian oceanic upper mantle: evidence from the Paleozoic Tianshan ophiolites (NW China)

The scale and extent of isotopic heterogeneities in the Paleo-Asian oceanic mantle remain poorly constrained. Here, we report Os isotopic data for mantle peridotites from the Paleozoic Gangou and Kumishi ophiolites, which are preserved in the western Chinese Tianshan sutures (Xinjiang, China) and considered to sample the Paleo-Asian oceanic upper mantle. These heavily serpentinized peridotites display positive correlations between Al2O3, Ti, and Yb contents, indicating variable degrees of melt extraction from fertile mantle sources. However, enriched light rare-earth element patterns in the peridotites are inconsistent with simple residues of partial melting, thus reflecting the effect of post-melting melt percolation. Peridotites from both ophiolites show wide variation of Os isotopic compositions, several of which are characterized by highly unradiogenic 187Os/188Os values (0.117–0.118), reflecting Proterozoic melt depletion. Such feature is common in modern oceanic peridotites and most Phanerozoic ophiolites, meaning that Os-isotopically ultradepleted domains should be widespread in the Paleo-Asian oceanic mantle. The overall similarity of Os-isotope distribution between the Paleozoic Tianshan ophiolites and modern oceanic peridotites suggests that they derive from the same heterogeneous upper mantle reservoir. This might support the hypothesis that isotopically ultradepleted domains are inherent components of the convecting upper mantle and have not been sufficiently homogenized by convection. Alternatively, Re-depletion ages (TRD) for these ancient domains are broadly coincident with the major peaks in crustal zircon ages that are thought to reflect episodes of continental crust growth in the adjacent Yili–Central Tianshan microcontinents. This suggests that the ultradepleted mantle domains in some of the Tianshan ophiolites may derive from ancient subcontinental lithospheric mantle, which detached and were exhumed to form the substrate of ocean basins.