High Precision FTIR Mapping Reveals the Nitrogen-Related Characteristic of Multi-Stage Growth Diamond from Mengyin

Influenced by multi-stage growth events, diamonds exhibit complex internal growth zoning, indicating the different characteristics of the carbon-bearing melt/fluid resulting from various mantle metasomatic processes. The analytical method of Fourier transform infrared spectroscopy(FTIR) has undergone the development from point test, line scan to mapping, making FTIR mapping the most reliable method to research on the nitrogen-related characteristics of diamonds, and providing with more spatial information. In this study, four primary diamonds with inclusions from the Mengyin kimberlite in the Luxi region of the North China Craton were selected. Using high-precision FTIR mapping tests, we analyzed the distribution of A and B center nitrogen content (N_A, N_B), total nitrogen content (N_T), and B center conversion rate (Ⅰ aB %). The results revealed that all four samples are unique "mixed-type diamonds", exhibiting a sharp contrast in carbon-bearing melt/fluid sources before and after the intermediate Ⅱ a growth zone. The nitrogen impurity characteristics suggest that the ancient lithospheric mantle in the Luxi region underwent at least three stages of carbon-bearing melt/fluid metasomatism. Furthermore, the analysis of a special sample suggested that the apparent two-stage growth structure might be due to fluid erosion which destroyed the outer layer growth stage, resulting in a "pseudo two-stage" growth structure. Therefore, when using the FTIR mapping method to analyze diamond growth structures, it is essential to combine observations of surface features and morphological characteristics to avoid misinterpretation.