Effect of diamond growth rate on carbon isotope fractionation in Fe–Ni–C system

The carbon isotope fractionation factor between diamond and carbon dissolved in the Fe–Ni metal melt varies from 4.1‰ down to 2.4‰ in HTHP crystallization experiments with the average diamond growth rates ranging from 1 to 12.5 mg/h. The observed fractionation of heavy carbon isotope is in agreement with the Burton–Prim–Slichter model for the compatible impurity distribution associated with crystallization. The variations in the carbon isotope composition within a single diamond crystal can result from appropriate variations of the crystal growth rate. The scale of the observed fractionation effect is comparable with the internal isotope inhomogeneity of natural stones.

► Carbon isotope effect is decreasing with rising of HTHP diamond growth rate.
► Heavy carbon isotope behaves as a compatible impurity in diamond.
► When crystallizing together, graphite is always heavier than diamond.
► Carbon isotope behavior fits well with the Burton–Prim–Slichter model.
► The scale of effect is comparable with isotope heterogeneity of natural stones.