Unradiogenic lead in Earth’s upper mantle

Abstract

The mantle and continental crust—Earth’s main silicate reservoirs—have a lead isotope composition that is too radiogenic to have evolved from primitive Solar System material over 4.57 billion years1. To account for this imbalance, it has been suggested that unradiogenic lead may have partitioned into the metallic core2,3,4 or lower continental crust5. Alternatively, radiogenic lead could have been added to Earth later by meteorite impacts6. Unradiogenic lead was discovered in fragments of mantle rocks exhumed in the Horoman massif, Japan, implying that the mantle itself may provide a complementary reservoir of unradiogenic lead7. However, it is unclear why this unradiogenic component is not sampled by the melting that generates oceanic basalts8. Here we present double-spike lead isotope data for abyssal peridotite rocks, considered to represent suboceanic mantle, exposed on the Atlantic Ocean floor. We find that sulphides dated at about 1.83 billion years old and trapped as inclusions in silicate minerals preserve extremely unradiogenic lead isotope compositions. This unradiogenic lead could have been prevented from adding significantly to oceanic basalts if either the silicates shield the sulphide inclusions or if the sulphides reside in refractory mantle rocks that are rarely sampled during melting. We conclude that the lead isotope composition of the silicate Earth could be largely balanced by unradiogenic lead in sulphide in the mantle.

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Figure 1: 187Re–187Os isotope diagram for sulphide grains from a mid-Atlantic abyssal peridotite.
Figure 2: 207Pb–206Pb isotope diagram for sulphide grains from a mid-Atlantic abyssal peridotite.

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Acknowledgements

This research used samples provided by the Ocean Drilling Program, sponsored by the US National Science Foundation and participating countries under the management of the Joint Oceanographic Institutions. Financial support for this research was provided by Ocean Drilling Program UK, NERC (grant ref. NER/A/S/2001/00538) and the EC Marie Curie Research and Training Network EUROMELT (HPRN-CT-2002-00211). A.G. acknowledges financial support from the Laboratory of Excellence ClerVolc. We would like to thank B. Wood, A. Halliday and J. Wade for discussions and W. M. White for a constructive and thoughtful review.

Author information

K.W.B., A.G. and J.H. prepared the samples for analysis. I.J.P., B.C-T. and F.M. designed the analytical procedure. B.C-T., F.M. and O.A. carried out the chemistry and measurements. K.W.B. and I.J.P. wrote the main paper and the Supplementary Information. All authors discussed the results and commented on the manuscript atall stages.

Correspondence to Kevin W. Burton or Ian J. Parkinson.

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The authors declare no competing financial interests.

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Burton, K., Cenki-Tok, B., Mokadem, F. et al. Unradiogenic lead in Earth’s upper mantle. Nature Geosci 5, 570–573 (2012). https://doi.org/10.1038/ngeo1531

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