Diamond growth during ultrahigh‐pressure metamorphism of the Kokchetav Massif, northern Kazakhstan

Abstract

Characteristic features of in situ diamonds can be used to retrace diamond formation during ultrahigh pressure (UHP) metamorphism of the Kokchetav Massif, Kazakhstan. These features include the nitrogen aggregation state in diamond, dissolution features observed on diamond surfaces, and the carbon and nitrogen isotopic composition of the diamonds. The minerals in which the diamonds are included provide additional information and support the view that at least some of the Kokchetav microdiamonds are the products of prograde or peak UHP metamorphism. The coexistence of diamond and graphite is evaluated within this framework.

Discrimination of metamorphic diamond populations by Raman spectroscopy (Kokchetav, Kazakhstan)

“…Comparing this band with our spectra, we can conclude that there is clearly an offset of about 50 cm −1 with the C O peak; the Raman band which is reported here is at a different position and is much broader. Among all different diamond types, diamonds from garnet-pyroxene rocks were shown to bear up to 11,000 ppm of nitrogen, the highest amount recorded [19][20][21]. The 1700 cm −1 band may be associated with the very high content of double bounded N N or N C groups [35,36].…”

“…These microdiamonds also have much lower nitrogen content than the diamonds included in Grt-Cpx rocks [20]. They appear as clean yellow octahedral diamonds and twins (Fig.…”

“…Previous studies demonstrated that the diamond morphology and the content and aggregation state of its nitrogen impurities are related to the lithology [19][20][21]. Relying on this information, intragranular diamond included in garnets from representative samples of garnet-biotite gneiss, garnet-zoisite gneiss, carbonate rock (marble) and garnet-clinopyroxene (Grt-Cpx) rock were selected.…”

“…Diamonds in the carbonate rocks of Kokchetav are included in garnet, clinopyroxene and zircon [18,20,24] and occur as skeletal crystals or as cuboids (Fig. 1h).…”

“…Kokchetav microdiamonds have been widely investigated by other analytical methods and it has been shown that their morphology, nitrogen impurities and inclusion contents change depending on the lithology [18][19][20][21]. Assuming that these changes in their characteristic properties should also be reflected in their Raman spectra, it is the second aim of this work to attempt to discriminate distinct diamonds from different rock types of the Kokchetav massif, by means of Raman spectroscopy.…”