Kinetics of Ib to IaA nitrogen aggregation in diamond

Diamonds containing nitrogen impurities in the form of singly substituted C centres (type Ib diamond) are rare in nature because Ib nitrogen atoms aggregate to form A centres (type IaA diamond) during the long-term (similar to 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 --> laA conversion (E(a)) 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 E(a) of 6.0 +/- 0.2 eV, which is significantly greater than the E(a) 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 E(a) 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 degrees C and a burial to exhumation history of similar to 17 Myr. Application to yellow cubes and ''coat'' from Yakutia having similar to 20% Ib component, indicate this diamond generation grew less than or equal to 7 Myr before kimberlite eruption for mantle T greater than or equal to 950 degrees C but did not grow directly from the transporting kimberlite magma.