Carbon isotopic composition of diamonds from the Archangelsk diamond province

Abstract

The first data are reported on the carbon isotopic composition of diamond crystals from the Grib pipe kimberlite deposit of the Archangelsk diamond province (ADP). The δ13C value of the crystals ranges from −2.79 to −9.61‰. The isotopic composition of carbon was determined in three zoned crystals (δ13C of −5.8 −6.96 ‰, −5.64/ −5.85 ‰, and −5.94/ −5.69 ‰), two “diamond in diamond” samples (diamond inclusion with δ13C of −4.05 and −6.34 ‰ in host diamond crystals with δ13C of −8.05 and −7.54 ‰, respectively), and two samples of coated diamonds (cores with δ13C of −6.98 and −6.78‰ and coats with δ13C of −7.51 and −8.01 ‰, respectively). δ13C values were obtained for individual diamond crystals from bort-type aggregates (δ13C of −4.24/ −4.05 ‰, −6.58/ −7.48 ‰, and −5.48/ −6.08 ‰). Correlations were examined between the carbon isotopic composition of diamonds and their crystal morphology; the color; the concentration of nitrogen, hydrogen, and platelet defects; and mineral inclusions content. It was supposed that the observed δ13C variations in the crystals are most likely related to the fractionation of carbon isotopes rather than to the heterogeneity of carbon sources involved in diamond formation. The isotopic characteristics of diamonds from the Grib pipe were compared with those of previously investigated diamonds from the Lomonosov deposit. It was found that diamonds from these relatively closely spaced kimberlite fields are different; this also indicates the existence of spatially localized peculiarities of isotope fractionation in processes accompanying diamond formation.