The origin and formation of metamorphic microdiamonds from the Kokchetav massif, Kazakhstan: a nitrogen and carbon isotopic study

This study reports delta C-13. delta N-15 and N-content values for microdia monds from ultrahigh-pressure metamorphic rocks of the Kokchetav massif in Kazakhstan. Both alluvial diamonds and in-situ diamonds from a garnet-clino pyroxene rock and a marble (i.e. a garnet-pyroxene dolomitic rock) were inv estigated. In-situ diamonds were analysed in batches, because of their smal l size (average 40 mum). whereas the larger alluvial diamonds were analysed individually. The latter group has delta C-13-values ranging from - 15.92 parts per thousand to - 10.57 parts per thousand, delta N-15 from - 1.8 par ts per thousand to + 1.1 parts per thousand and N-contents from 2300 to 365 0 ppm. Diamonds from the garnet-clinopyroxene rock yield mean values of - 1 0.50 parts per thousand for delta C-13, +5.9 parts per thousand for delta N -15 and a high average nitrogen content of 11,150 ppm. Values for diamonds in marble are -10.19 parts per thousand. +8.5 parts per thousand and 2650 p pm, respectively. For diamonds from garnet-clinopyroxene rock and marble, there is more nitro gen released by bulk combustion than estimated by infrared (IR) spectroscop y, the differences being of about 7000 and 1500 ppm, respectively. These di fferences suggest that a significant quantity of nitrogen is IR-inactive an d may be present as fluid inclusions. Their carbon and nitrogen isotopic co mpositions are compatible with an in-situ crystallisation of diamond from d ominantly metasedimentary sources, suggesting that sedimentary nitrogen can be subducted to very high pressures. Carbon isotopic fractionation between coexisting carbonate and diamond suggests crystallisation temperatures bef ore the peak of metamorphism at temperatures probably below 700 degreesC an d deduced pressures of 3 GPa. Relative to the isotopic data reported for se diments, metasediments and in-situ diamonds, the slightly N-15-depleted com positions of alluvial diamonds is striking. These values suggest that the c ontribution of any metasedimentary source is unlikely and may point toward a mafic/ultramafic protolith. (C) 2001 Elsevier Science B.V. All rights res erved.