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Reply to: Evidence for two blue (type IIb) diamond populations

Nature volume 570, pages E28–E29 (2019)Cite this article

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The Original Article was published on 12 June 2019

replying to: Moore, A. E. & Helmstaedt, H. Nature https://www.nature.com/articles/s41586-xxx-xxxx-x (2019)

In our earlier examination of boron-bearing (type IIb) diamonds1 we reported inclusion assemblages comparable to those described previously in sublithospheric diamonds from transition-zone and lower-mantle depths2,3,4, and ascribed them to subducted oceanic lithosphere. The boron that characterizes these unusual diamonds was proposed to come from seawater-serpentinized oceanic lithosphere. In the accompanying Comment5, Moore and Helmstaedt reinterpret the data in our paper and propose that the type IIb diamonds actually constitute two distinct populations, one being sublithospheric as originally described and the other being a population of much shallower, lithospheric origin from websteritic host rocks. Although the observation that our largest diamonds contained only Ca-silicates (with or without ferropericlase) is interesting, we argue that this does not require the division of the type IIb samples into two populations. Even though such a division is conceivable, there is no evidence for a second, lithospheric type IIb diamond paragenesis. Instead, it remains more likely that those type IIb diamonds containing Ca-silicates alone are part of the firmly established sublithospheric type IIb diamond paragenesis, associated with deeply subducted slabs.

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Fig. 1: Additional features consistent with a sublithospheric origin for the large 24.18-carat blue type IIb diamond (sample 110208093607, from the Premier kimberlite pipe at the Cullinan mine).

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Acknowledgements

We thank T. Stachel for constructive peer review and B. Luth for discussion on mineral stability.

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Authors and Affiliations

  1. Gemological Institute of America, New York, NY, USA

    Evan M. Smith & Wuyi Wang

  2. Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, USA

    Steven B. Shirey & Jianhua Wang

  3. Department of Geological Sciences, University of Cape Town, Rondebosch, South Africa

    Stephen H. Richardson

  4. Department of Geosciences, University of Padova, Padua, Italy

    Fabrizio Nestola

  5. Geophysical Laboratory, Carnegie Institution for Science, Washington, DC, USA

    Emma S. Bullock

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  1. Evan M. Smith
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Contributions

E.M.S. prepared the manuscript with contributions from S.B.S., S.H.R. and F.N., and expert approval from E.S.B., J.W. and W.W. All authors were involved in this Reply.

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Correspondence to Evan M. Smith.

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Smith, E.M., Shirey, S.B., Richardson, S.H. et al. Reply to: Evidence for two blue (type IIb) diamond populations. Nature 570, E28–E29 (2019). https://doi.org/10.1038/s41586-019-1246-8

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