Evidence for Large Scale Fractionation of Carbon Isotopes and of Nitrogen Impurity during Crystallization of Gem Quality Cubic Diamonds from Placers of North Yakutia

The spatial distribution of carbon and nitrogen isotopes and of nitrogen concentrations is studied in detail in three gem quality cubic diamonds of variety II according to Orlov's classification. Combined with the data on composition of fluid inclusions our results point to the crystallization of the diamonds from a presumably oxidized carbonate fluid. It is shown that in the growth direction delta C-13 of the diamond becomes systematically lighter by 2-3% (from -13.7 to -15.6% for one profile and from -11.7 to -14.1% for a second profile). Simultaneously, we observe substantial decrease in the nitrogen concentration (from 400-1000 to 10-30 at ppm) and a previously unrecognized enrichment of nitrogen in light isotope, exceeding 30%. The systematic and substantial changes of the chemical and isotopic composition can be explained using the Burton- Prim-Slichter model, which relates partition coefficients of an impurity with the crystal growth rate. It is shown that changes in effective partition coefficients due to a gradual decrease in crystal growth rate describes fairly well the observed scale of the chemical and isotopic variations if the diamond-fluid partition coefficient for nitrogen is significantly smaller than unity. This model shows that nitrogen isotopic composition in diamond may result from isotopic fractionation during growth and not reflect isotopic composition of the mantle fluid. Furthermore, it is shown that the infra-red absorption at 1332 cm(-1) is an integral part of the Y-defect spectrum. In the studied natural diamonds the 1290 cm(-1) IR absorption band does not correlate with boron concentration.