In order to study the geochemistry of diamonds formed during similar growth conditions in a very localised environment in the mantle, we carried out a detailed study of the variation in delta N-15, delta C-13, N content, and N aggregation state of 35 diamonds in a single eclogite xenolith from the Kaalvallei kimberlite in South Africa. Diamond nitrogen contents determined by infrared spectroscopy range from 239 ppm to 1272 ppm, and are positively correlated with nitrogen aggregation states which vary from 11.5% to 43.7% of laB defects. Modelling of these parameters using second order reaction kinetics suggests that the diamonds likely represent a single population which has been resident in the mantle at temperatures of 1090 degrees C to 1190 degrees C. delta C-13 values of the diamonds analysed range from -6.0 parts per thousand to -4.2 parts per thousand, while delta N-15 values vary from -8.9 parts per thousand to -4.1 parts per thousand, with no correlation between Omega C-13 and delta N-15. These values account for 5% and 15%, respectively, of the worldwide isotopic range for diamonds. The limited variability of C and N isotopic compositions for the diamonds analysed are compatible with a model of metasomatic diamond formation from a single, homogeneous fluid, and this is also supported by the infrared data. Although the absence of any clear correlation between delta N-15, delta C-13 and diamond N content precludes the accurate identification of the fluid species involved in diamond growth, the lack of correlation may indicate the involvement of a carbonate- or CO2-type fluid. The observed range in negative delta N-15 values for all the diamonds analysed are within the limits of the so-called mantle range (delta N-15(mantle) = -5 +/- 2 parts per thousand) which is consistent with a mantle origin for these diamonds. Negative delta N-15 is inconsistent with diamond formation from recycled (crustal) material which is enriched in N-15 (i.e. positive delta N-15). In contrast, positive europium and strontium anomalies in silicate minerals of the host xenolith, as well as oxygen isotopic data which deviate from the mantle range, are consistent with a protolith consisting of recycled oceanic crust. It is therefore concluded that our data supports a model of metasomatic diamond crystallisation from a mantle-carbon source. (C) 2009 Elsevier B.V. All rights reserved.
Palot, Mederic Cartigny, Pierre Viljoen, Fanus 9th International Kimberlite Conference AUG 10-15, 2008 Frankfurt, GERMANY Part 2 Sp. Iss. SI