Diamond, subcalcic garnet, and mantle metasomatism

Kimberlite sampling patterns define the link

V. G. Malkovets*, W. L. Griffin, S. Y. O'Reilly, B. J. Wood

*Corresponding author for this work

    Research output: Contribution to journalArticle

    88 Citations (Scopus)


    A genetic relationship between diamond and subcalcic Cr-pyrope garnet, both being produced by a metasomatic process, can be inferred from the sampling patterns of kimberlites in the Daldyn-Alakit province, Yakutia, Russia. Pressure-temperature estimates for xenoliths and xenocrysts show a strong concentration of highly depleted rocks in a well-defined zone 140-190 km deep; diamond inclusions and diamond-bearing xenoliths show that most diamonds come from harzburgites within this layer. Xenocryst distribution curves indicate that diamondiferous kimberlites have sampled both garnet and chromite from the harzburgitic layer, but low-grade pipes have sampled only chromite. Diamond formation probably is due to the oxidation of methane-rich, silica-bearing fluids: Fe2O3 (in chromite) + CH4 → C + H2O + FeO (in chromite), accompanied by another reaction: chromite ± olivine ± orthopyroxene + Si, Ca (in fluid) → low-Ca, high-Cr garnet. The presence or absence of diamond in kimberlites thus reflects the distribution of metasomatized fluid conduits in a lithosphexic mantle that originally consisted of highly refractory harzburgites containing neither garnet nor diamond.

    Original languageEnglish
    Pages (from-to)339-342
    Number of pages4
    Issue number4
    Publication statusPublished - Apr 2007

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