Eclogitic Diamond Formation at Jwaneng: No Room for a Recycled Component

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Abstract
Eclogitic diamonds have a large range of δ 13 C values, whereas peridotitic diamonds do not. Paired δ 15 N-δ 13 C-N variations in 40 eclogitic diamonds from the Jwaneng kimberlite in Botswana show that neither the influence of recycled biogenic carbon nor the global and primordial heterogeneity of mantle carbon are likely for the origin of the large δ 13 C range; the data instead support a fractionation process. It is proposed that carbonatitic mantle melts from which diamonds crystallize undergo different evolutions before diamond precipitation, when percolating through either a peridotite or an eclogite. These different evolutions, reflecting the presence or absence of olivine, can account for their respective δ 13 C distributions.