Mineralogy and Petrology

, Volume 112, Supplement 2, pp 583–596 | Cite as

Cr-rich megacrysts of clinopyroxene and garnet from Lac de Gras kimberlites, Slave Craton, Canada – implications for the origin of clinopyroxene and garnet in cratonic lherzolites

  • Yannick BussweilerEmail author
  • D Graham Pearson
  • Thomas Stachel
  • Bruce A. Kjarsgaard
Original Paper


Kimberlites from the Diavik and Ekati diamond mines in the Lac de Gras kimberlite field contain abundant large (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) crystals. We present the first extensive mineral chemical dataset for these megacrysts from Diavik and Ekati and compare their compositions to cratonic peridotites and megacrysts from the Slave and other cratons. The Diavik and Ekati Cr-diopside and Cr-pyrope megacrysts are interpreted to belong to the Cr-rich megacryst suite. Evidence for textural, compositional, and isotopic disequilibrium suggests that they constitute xenocrysts in their host kimberlites. Nevertheless, their formation may be linked to extensive kimberlite magmatism and accompanying mantle metasomatism preceding the eruption of their host kimberlites. It is proposed that the formation of megacrysts may be linked to failed kimberlites. In this scheme, the Cr-rich megacrysts are formed by progressive interaction of percolating melts with the surrounding depleted mantle (originally harzburgite). As these melts percolate outwards, they may contribute to the introduction of clinopyroxene and garnet into the depleted mantle, thereby forming lherzolite. This model hinges on the observation that lherzolitic clinopyroxenes and garnets at Lac de Gras have compositions that are strikingly similar to those of the Cr-rich megacrysts, in terms of major and trace elements, as well as Sr isotopes. As such, the Cr-rich megacrysts may have implications for the origin of clinopyroxene and garnet in cratonic lherzolites worldwide.


Kimberlite Megacrysts Cr-rich Megacrysts Cratonic lherzolites 



This study forms part of Y.B.’s PhD funded through D.G.P’s Canada Excellence Research Chair. Yuri Kinakin and Gus Fomradas at Diavik Diamond Mine are thanked for access to drill core. Juanita Bellinger at Rio Tinto is thanked for sending concentrate samples. Y.B. is grateful for lab assistance from Andrew Locock (EPMA), Yan Luo (LA-ICP-MS), and Chiranjeeb Sarkar (TIMS). B.A.K. acknowledges support and funding from the Geological Survey of Canada, and EPMA lab assistance from Katherine Venance. We thank Maya Kopylova and an anonymous reviewer for detailed and constructive comments and Phil Janney for the efficient editorial handling. Y.B. is grateful for a University of Alberta Doctoral Recruitment Scholarship and a Society of Economic Geologists student research grant. At the University of Münster, Y.B. acknowledges support through a Marie Skłodowska-Curie Fellowship (Project ID 746518).

Supplementary material

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Copyright information

© Springer-Verlag GmbH Austria, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Earth and Atmospheric SciencesUniversity of AlbertaEdmontonCanada
  2. 2.Institute for MineralogyUniversity of MünsterMünsterGermany
  3. 3.Geological Survey of CanadaOttawaCanada

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