Sm-Nd isochron ages coupled with C-N isotope data of eclogitic diamonds from Jwaneng, Botswana

M. U. Gress*, J. M. Koornneef, E. Thomassot, I. L. Chinn, K. van Zuilen, G. R. Davies

*Corresponding author for this work

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Abstract

Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>100′s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (δ13C < −21.1‰) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth's long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.

Original languageEnglish
Pages (from-to)1-17
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume293
Early online date16 Oct 2020
DOIs
Publication statusPublished - 15 Jan 2021

Funding

The authors sincerely thank Debswana and De Beers for providing access to run of mine production at DTCB and the generous supply of diamonds. MUG was funded through NWO project 824.14.004. Carbon isotope analyses were partly funded by Europlanet 2020 RI that received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654208. We thank Nordine Bouden for his technical assistance during SIMS analytical sessions as well as Etienne Deloule and Johan Villeneuve for the fruitful discussions while setting up the analytical condition for C-N isotope measurements by SIMS. Travel for sampling was sponsored by Stichting Dr. Schürmannfonds. Special thanks go to Hielke Jelsma for his advice on regional geology, Richard Smeets for assistance in the clean lab, to Pieter Ouwerkerk and Gassan Diamonds BV for assistance in polishing and to Suzan Verdegaal-Warmerdam, Ciaran Kelly, Evalien van der Valk and Anna Pals for their contributions in C isotope analyses. Sergei Matveev and Tilly Bouten are thanked for support with EPMA in Utrecht, Frank Peeters for providing access to the micro-balance and microscopy.

FundersFunder number
Horizon 2020 Framework Programme871149, 654208
Nederlandse Organisatie voor Wetenschappelijk Onderzoek824.14.004

    Keywords

    • Carbon and nitrogen isotope
    • Craton
    • Diamond
    • Eclogite
    • Inclusion dating
    • Nitrogen aggregation

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