Diamond, subcalcic garnet, and mantle metasomatism: Kimberlite sampling patterns define the link
- 1Geochemical Evolution and Metallogeny of Continents, Australian Research Council National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia and Institute of Mineralogy and Petrography, Siberian Branch Russian Academy of Sciences, Novosibirsk 630090, Russia
- 2Geochemical Evolution and Metallogeny of Continents, Australian Research Council National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
Abstract
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 lithospheric mantle that originally consisted of highly refractory harzburgites containing neither garnet nor diamond.
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- Accepted 27 November 2006.
- Received 14 June 2006.
- Revision received 19 November 2006.
- Geological Society of America
