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Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts

Abstract

‘Recycled’ crustal materials, returned from the Earth's surface to the mantle by subduction, have long been invoked to explain compositional heterogeneity in the upper mantle1. Yet increasingly, problems have been noted with this model2,3. The debate can be definitively addressed using stable isotope ratios, which should only significantly vary in primitive, mantle-derived materials as a consequence of recycling. Here we present data showing a notable range in lithium isotope ratios in basalts from the East Pacific Rise, which correlate with traditional indices of mantle heterogeneity (for example, 143Nd/144Nd ratios). Such co-variations of stable and radiogenic isotopes in melts from a normal ridge segment provide critical evidence for the importance of recycled material in generating chemical heterogeneity in the upper mantle. Contrary to many models, however, the elevated lithium isotope ratios of the ‘enriched’ East Pacific Rise lavas imply that subducted ocean crust is not the agent of enrichment. Instead, we suggest that fluid-modified mantle, which is enriched during residency in a subduction zone, is mixed back into the upper mantle to cause compositional variability.

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Figure 1: Li isotopic compositions of the mantle and important recycled components.
Figure 2: Variations of δ 7 Li with other geochemical parameters in northern EPR MORB.

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Acknowledgements

Analytical work was supported by a Philip Leverhulme Prize awarded to T.E. A.J. was supported by an NERC studentship. Reviews by B. Leeman were appreciated. We also thank J. Blundy, C. Hawkesworth and D. Vance for comments on versions of the mansucript.

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Correspondence to Tim Elliott.

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Supplementary Table 1

Table of all EPR MORB Li isotope data (DOC 40 kb)

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Elliott, T., Thomas, A., Jeffcoate, A. et al. Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts. Nature 443, 565–568 (2006). https://doi.org/10.1038/nature05144

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