Nitrogen-bearing nanoinclusions in diamonds are known for more than 30 years, but their content and mode of formation is debated. Molecular nitrogen and amonia were suggested with a range of possible crystalline structures. We suggest that the nanoinclusions were formed by exsolution of nitrogen from the diamond lattice and the formation of small (~20 nm), octahedral inclusions of solid molecular nitrogen in diamonds. We also suggest that this is the ultimate stage in the aggregation of nitrogen in diamonds. IR spectroscopy of four diamonds from Juina, Brazil, revealed high concentrations of fully aggregated nitrogen (average of 900 ppm, all in B centers). The platelets which accompanied the formation of B centers have all been degraded and disappeared. Bimodal population of inclusions comprised microinclusions (average size: 150 nm, average concentration: 100 ppm) and nanoinclusions (20-30 nm, 350 ppm), suggesting two distinct nucleation events. Raman spectroscopy indicated the presence of solid, cubic d-N2 at 10.9±0.2 GPa (Density=1900 kg/m3). Transmission electron microscopy also indicates the presence of a fully crystalline phase and EELS detects the presence of nitrogen. Atomic force microscopy revealed up-warping of small areas (~150 nm in size) on the polished surface of one diamond. The ~2 nm rise is explained by a pressurized shallow subsurface microinclusion, with P>10 GPa.
Using equations of state for nitrogen and diamond and assuming no plastic relaxation and formation at near geotherm conditions, we estimate origin at the deepest part of the transition zone of the earth`s mantle at pressures of ~22 GPa (630 km) and temperatures of ~1640°C. We suggest that both generations of inclusions are the result of exsolution of nitrogen from B centers and that growth took a few million years. The microinclusions nucleated first, followed by the nanoinclusions. Shortly after the exsolution events, the diamonds were trapped in a plume or an ascending melt and were transported to the base of the lithosphere and later to the surface. This conclusion joins earlier studies that advocated a fast ascent from such great depth to the surface.
