Effect of growth rate on diamond composition

Carbon isotope and nitrogen impurity variations within natural diamond crystals reflect the growth system composition. Here we describe in detail an additional mechanism responsible for the generation of a gradual change of 613C and nitrogen content in the growth direction of diamond single crystals. A decrease in the linear growth rate with an increase of the crystal size changes partition coefficients of impurities and isotopes. This process may result in gradual changes of 613C and of N content in direction of crystallization, with magnitudes reaching several permil and hundreds of ppm, respectively. Importantly, these changes can occur at constant extensive parameters such as temperature, pressure and fluid composition. The effect is experimentally demonstrated at linear growth rates from 10-8 to 10-5 m/s, but can occur at lower rates if longer carbon diffusion distances are encountered. The growth rate effect generates 613C and Nppm trends opposite to the closed-system Rayleigh depletion under the same redox conditions. In some cases, it gives better consistency of the carbon isotope profile with the composition of microinclusions. The influence of the diamond growth kinetics on the isotope ratio of nitrogen impurity is not yet obvious.