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The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.

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Detailed knowledge of lithospheric structure is essential for understanding the long-term evolution and dynamics of continents. We present an image of lithospheric structure across the central and western North China Craton (NCC), derived using S and P receiver functions from a dense seismic array. A negative velocity discontinuity is identifi ed at ~80-100 km depth within the thick lithosphere (~160-200 km), similar to that observed in many other cratonic regions and roughly at the same depth as the base of the lithosphere in the eastern NCC. The intralithospheric discontinuity may indicate an ancient, mechanically weak layer within the overall strong cratonic lithosphere, and probably also existed beneath the eastern NCC before the Mesozoic. The presence of such a weak layer could have facilitated simultaneous lithospheric modifi cation at the base and the middle of the lithosphere in the eastern NCC, especially under the strong infl uence of the Mesozoic Pacifi c subduction, leading to the severe lithospheric thinning and destruction recorded in this region. The weak layer probably did not strongly affect the stability and evolution of the central and western NCC and other cratonic regions where effects from plate boundary processes were weak. Our seismic images, integrated with geological data, provide new insights into structural heterogeneities in the subcontinental lithospheric mantle and their roles in the dynamic evolution of continents.

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