Geology

Diamond Diversity

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Science  23 Mar 2007:
Vol. 315, Issue 5819, pp. 1638
DOI: 10.1126/science.315.5819.1638a

The chemistry of diamonds brought up from Earth's mantle—notably their widely ranging nitrogen contents and nitrogen and carbon isotope values—has complicated understanding of their origins. It is commonly thought that many diamonds form from the movement of carbon-rich fluids into deep mantle rocks of a contrasting composition, thereby inducing diamond precipitation. To better constrain these fluids and sources, Thomassot et al. studied in detail nearly 60 diamonds contained within one small (<30 cm3) mantle sample carried to the surface in a kimberlite volcano in South Africa. Surprisingly, the nitrogen contents and isotopic values of these diamonds in this one sample spanned a large part of the ranges observed from all diamonds worldwide. The covariations of the data imply that these diamonds formed from a methane-rich fluid, not a more oxidized fluid as commonly assumed. The wide variation can be produced by the fractionation of nitrogen and carbon during growth of the diamonds over time. Such fluids may also account for the variable oxidation state of the mantle beneath Earth's most ancient crust. — BH

Earth Planet. Sci. Lett. 10.1016/j.epsl.2007.02.020 (2007).

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