Abstract:
Diamonds containing nitrogen impurities in the form of singly substituted C centres (type Ib diamond) are rare in nature because lb nitrogen atoms aggregate to form A centres (type IaA diamond) during the long-term (∼0.5 to 3 Gyr) upper mantle residence period experienced by most diamonds. Preservation of N-rich diamond with a type Ib component requires a young age of crystallization, or unusually cool conditions of mantle storage, or both. Quantitative constraints on the origin of Ib diamonds are possible if the kinetics of nitrogen aggregation are accurately known. However, previous experimental studies have produced significantly different values for the activation energy of Ib → IaA conversion (Ea) which is at least partly due to sector dependence of the aggregation rate in the starting materials. We report a series of high-pressure (P), temperature (T) experiments on synthetic and natural type Ib diamond in which we have used infrared (IR) microspectrometry techniques to unravel the sector dependency. The results show that cube sectors have an Ea of 6.0 ± 0.2 eV, which is significantly greater than the Ea of 4.4 ± 0.3 eV determined for the octahedral sectors. Both values are in excellent agreement with a recent theoretical study in which the sector dependency is ascribed to different mechanisms of nitrogen migration. The higher Ea is most relevant to natural Ib diamond which are invariably of cube or cuboid habit. Application to the Kokchetav microdiamonds of metamorphic origin indicate that their aggregation state is consistent with peak temperatures of 950°C and a burial to exhumation history of ∼17 Myr. Application to yellow cubes and “coat” from Yakutia having ∼20% Ib component, indicate this diamond generation grew Myr before kimberlite eruption for mantle but did not grow directly from the transporting kimberlite magma.