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Earth science

Mixing it up in the mantle

Nature volume 517, pages 275–276 (2015)Cite this article

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Analysis reveals that the uranium isotopic composition of oceanic crust that is being subducted into Earth's interior is distinctive, allowing the development of chemical heterogeneity in the mantle to be tracked. See Letter p.356

It is generally accepted that recycled materials have a key role in the generation of compositional heterogeneity in Earth's mantle3, and indeed, evidence to this effect continues to appear4. By contrast, the question of when the mantle became modified in this way has proved remarkably intractable. For example, the abundances of the isotopes of lead (Pb) — derived from slow decay of long-lived uranium (U) and thorium (Th) parent nuclei — in OIBs form linear correlations that suggest a broad, model-dependent age range (about 2.5 billion to 1 billion years) for the establishment of isotopic heterogeneity in their mantle source5. Another temporal constraint is provided by the unusually low abundance ratios of thorium to uranium (Th/U) observed in basaltic lavas erupted at Earth's ocean ridges, known as mid-ocean-ridge basalts (MORBs). These ratios are lower than those estimated for the bulk Earth and have been explained6 as resulting from uranium recycling into the mantle at subduction zones, perhaps starting about 2.4 billion years ago, coincident with the rise of atmospheric oxygen (and hence the availability of water-soluble hexavalent uranium, U(VI)). Beyond these few, rather imprecise estimates, we have scant information on the timescales of crustal-recycling phenomena.

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Figure 1: Crustal recycling.

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  1. Jon Woodhead is in the School of Earth Sciences, University of Melbourne, Victoria 3010, Australia.,

    Jon Woodhead

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Correspondence to Jon Woodhead.

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Woodhead, J. Mixing it up in the mantle. Nature 517, 275–276 (2015). https://doi.org/10.1038/517275a

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