DIAMOND GENESIS, MANTLE FRACTIONATIONS AND MANTLE NITROGEN CONTENT: A STUDY OF δ13C-N CONCENTRATIONS IN DIAMONDS

Abstract

A compilation of more than 1200 δ13C–N data from well-characterised diamonds show a correlation of the maximum diamond nitrogen content (i.e. a limit sector) with δ13C over the full diamond δ13C range (i.e. more than 30‰). Diamonds with low δ13C values are characterised by rather low N contents (~0 ppm at δ13C<−30‰), whereas diamonds with high δ13C have more variable nitrogen contents, with a much higher upper limit (~3500 ppm at δ13C=−4.5‰). This correlation defines a concave trend that is therefore incompatible with a mixing relationship, such as would be produced by the admixture of subducted and primordial components. The limit sector more likely reflects the evolution of mantle melts (or fluids) during differentiation. Nitrogen uptake is seen as a kinetic process, depending mostly on the diamond rate of growth; at a given δ13C value, as a result of slow growth conditions, diamonds with nitrogen contents lower than the maximum value are interpreted as having fractionated the N/C ratio relative to their growth medium. The limit sector is applicable to every diamond paragenesis (peridotitic, eclogitic and fibrous) suggesting that every diamond type may derive from a similar isotopic source. Assuming a mantle δ13C value of −4.5‰, we deduce that the initial C/N ratio of mantle melts (i.e. the diamond growth medium) from which diamonds crystallise ranges between 200 and 500, which is surprisingly similar to that of mid-ocean ridge basalts. Therefore, in spite of their different context and age, it appears that subcontinental and oceanic mantles give samples with similar δ13C, δ15N and C/N, suggesting an overall homogeneity of volatiles within these parts of the Earth since the Archaean. Diamonds also demonstrate that carbon and nitrogen do not behave similarly during the evolution of the diamond growth medium. Accordingly, mantle nitrogen concentration cannot be deduced in a simple way. If N behaved as an incompatible element during partial melting, a mantle nitrogen concentration of about 2 ppm could be expected, provided that the mantle carbon content is about 400 ppm. However, from several lines of evidence presented in this study, nitrogen is not regarded as a totally incompatible element, and a higher mantle nitrogen concentration (perhaps up to 40 ppm) is preferred.