Correlations between the structural and impurity features reveal fluctuant growth conditions of natural fibrous diamonds

Abstract

Abstract Fibrous diamonds containing mantle fluid inclusions have been studied to explore their unique growth patterns and the evolution of mantle fluids. However, the growth of fibrous diamonds is not a completely homogeneous process, and there has been limited focus on the growth fluctuations of the diamonds themselves. This gap in research hinders our understanding of mantle fluid evolution. To address this issue, polarizing microscope, energy-dispersive spectroscopy (EDS), cathodoluminescence (CL), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) were used to investigate the distribution and composition of fluid micro-inclusions, growth structure, plastic deformation, residual stress, and impurities in coated diamonds from the Democratic Republic of the Congo (DRC). It was found that VN3H defects controlled the luminescence of diamond growth layers, and plastic deformation and residual stress were mainly related to nitrogen content. However, the relationship between VN3H defects and nitrogen content in the core varied, leading to contrasting correlations with plastic deformation and residual stress. In the fibrous coat, there are growth layers with varying densities of micro-inclusions. Interestingly, both the dark fibrous layers with the highest density of micro-inclusions and the light-yellow fibrous layers with the lowest density of micro-inclusions showed positive anomalies of plastic deformation, tensile residual stress, nitrogen concentration, and VN3H defects. According to the micro-inclusion composition and the effect of growth conditions on the growth rate and impurity incorporation of diamonds, it was speculated that growth media with high water contents led to the formation of dark layers with the highest growth rate, and reduced growth pressures were the major reason causing the crystallization of light-yellow fibrous layers. This study demonstrates that the nucleation of natural fibrous diamonds could be subjected to the influence of multiple factors, and the systematic correlations between the structural and impurity features of diamonds have the potential to reflect variations of mantle conditions under which they crystallized.