Diamond genesis, mantle fractionations and mantle nitrogen content: a study of delta C-13-N concentrations in diamonds

A compilation of more than 1200 delta C-13-N data from well-characterised d iamonds show a correlation of the maximum diamond nitrogen content (i.e. a limit sector) with delta C-13 over the full diamond delta C-13 range (i.e. more than 30 parts per thousand). Diamonds with low delta C-13 values are c haracterised by rather low N contents (similar to0 ppm at delta C-13 < -30< parts per thousand>), whereas diamonds with high delta C-13 have more varia ble nitrogen contents, with a much higher upper limit (similar to 3500 ppm at delta C-13= -4.5 parts per thousand). This correlation defines a concave trend that is therefore incompatible with a mixing relationship, such as w ould be produced by the admixture of subducted and primordial components. T he limit sector more likely reflects the evolution of mantle melts (or flui ds) during differentiation. Nitrogen uptake is seen as a kinetic process, d epending mostly on the diamond rate of growth; at a given delta C-13 value, as a result of slow growth conditions, diamonds with nitrogen contents low er than the maximum value are interpreted as having fractionated the N/C ra tio relative to their growth medium. The limit sector is applicable to ever y diamond paragenesis (peridotitic, eclogitic and fibrous) suggesting that every diamond type may derive from a similar isotopic source. Assuming a ma ntle delta C-13 value of -4.5 parts per thousand, we deduce that the initia l C/N ratio of mantle melts (i.e. the diamond growth medium) from which dia monds 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 s amples with similar delta C-13, delta N-15 and C/N, suggesting an overall h omogeneity 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 nit rogen concentration cannot be deduced in a simple way. If N behaved as an i ncompatible 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 h igher mantle nitrogen concentration (perhaps up to 40 ppm) is preferred. (C ) 2001 Elsevier Science B.V. All rights reserved.