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Continental mantle signature of Bushveld magmas and coeval diamonds

Abstract

The emplacement of the 2.05-billion-year-old Bushveld complex, the world’s largest layered intrusion and platinum-group element (PGE) repository1, is a singular event in the history of the Kaapvaal craton of southern Africa2,3,4, one of Earth’s earliest surviving continental nuclei. In the prevailing model for the complex’s mineralization, the radiogenic strontium and osmium isotope signatures of Bushveld PGE ores are attributed to continental crustal contamination of the host magmas5,6,7,8,9,10,11. The scale of the intrusion and lateral homogeneity of the PGE-enriched layers1, however, have long been problematical for the crustal contamination model, given the typically heterogeneous nature of continental crust. Furthermore, the distribution of Bushveld magmatism matches that of seismically anomalous underlying mantle3,12, implying significant interaction before emplacement in the crust. Mineral samples of the ancient 200-km-deep craton keel, encapsulated in macrodiamonds and entrained by proximal kimberlites, reveal the nature of continental mantle potentially incorporated by Bushveld magmas13,14. Here we show that sulphide inclusions in 2-billion-year-old diamonds from the 0.5-billion-year-old Venetia and 1.2-billion-year-old Premier kimberlites (on opposite sides of the complex) have initial osmium isotope ratios even more radiogenic than those of Bushveld sulphide ore minerals6,15. Sulphide Re–Os and silicate Sm–Nd and Rb–Sr isotope compositions indicate that continental mantle harzburgite and eclogite components, in addition to the original convecting mantle magma, most probably contributed to the genesis of both the diamonds and the Bushveld complex. Coeval diamonds provide key evidence that the main source of Bushveld PGEs is the mantle rather than the crust.

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Figure 1: Sketch map of the Kaapvaal and Zimbabwe cratons (light blue) and intervening Limpopo belt (dark blue) of southern Africa (following ref. 33).
Figure 2: Re–Os isochron diagram for sulphide inclusions in Premier and Venetia diamonds.
Figure 3: Isotope correlation diagrams comparing Bushveld rocks and PGE sulphide ore minerals 6, 8, 9, 15 to potential mantle source components 2.05 Gyr ago.

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Acknowledgements

We thank De Beers for donation of rare diamond specimens; R. W. Carlson and J. W. Harris for collaboration on related diamond projects; R. J. Hemley, C. Yan and J. Lai for access to diamond cutting and polishing facilities; and T. Mock, M. Horan and C. Hadidiacos for analytical laboratory help. This work was supported by the Carnegie Institution of Washington and NSF grant EAR-0310059 to S.B.S.

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Richardson, S., Shirey, S. Continental mantle signature of Bushveld magmas and coeval diamonds. Nature 453, 910–913 (2008). https://doi.org/10.1038/nature07073

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