Zhuoyuan Li

M.Sc. Thesis

Micro-morphology and Resorption features of Diamonds from Snap Lake and Ekati Kimberlites (Canada) as an Indicator of Kimberlite Emplacement Conditions

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Volatiles play a significant role in the emplacement mode and the resulting geology of kimberlites, however, the volatile composition and concentrations are poorly constrained. Previous experimental studies have suggested that diamond resorption features are sensitive to the presence and composition of C-O-H fluids. Atomic force microscopy (AFM) can be used to quantify individual features on diamonds to place robust constraints on their resorption conditions and the composition of kimberlitic fluid.

This study examines resorption features on diamonds from kimberlite localities with presumably different fluid and emplacement histories - Snap Lake kimberlite dyke and four kimberlite pipes (Fox, Panda, Koala, Misery) from Ekati Mine. The morphologies of 251 diamonds from Snap Lake were examined and 91 crystals with octahedral or tetrahexahedral forms were selected, which were further divided via optical microscopy and Scanning Electron Microscopy (SEM) into 12 morphological groups based on the resorption features on {111} faces. Related sub-groups were identified based on the edge types. The internal properties (N content and aggregation state) of 82 diamonds selected from different morphological groups were also examined by Fourier Transform Infrared Spectroscopy (FTIR). The lack of correlation between the internal properties of diamonds and resorption morphologies suggests a limited effect of internal factors on resorption morphologies.

Selected diamonds from Snap Lake and Ekati kimberlites were studied with Atomic Force Microscopy (AFM) to provide quantitative data on the geometry of diamond micro-features. The results were compared to the AFM data for diamonds etched in experiments at 0.1 MPa in H2-CO2 gas mixture and at 1-3 GPa in CHO fluid with various CO2/(H2O+CO2) ratios. Snap Lake diamonds show widespread development of positive trigons in addition to the common negative trigons, indicating late-stage etching at near-surface conditions superimposed on earlier stage (deeper) etching. Ekati diamonds show only negative trigons implying deeper resorption (at pressures ≥ 1 GPa). The AFM data show significant variation in CO2/H2O ratio between the volumetrically dominant coherent facies (HKM) and the subordinate phlogopite-rich coherent facies (HKP) in Snap Lake kimberlite, and among the four studied Ekati kimberlites. The differences in the emplacement of Snap Lake dyke and Ekati kimberlite pipes can be explained by variation in fluid composition. The study shows that the new AFM-based method of examining trigonal pits on diamonds can be applied to reconstruct conditions during kimberlite emplacement and the nature of kimberlitic fluids.

Keywords: Diamonds, Surface features, kimberlite, Snap Lake Mine, Ekati Mine, Volatiles
Pages: 216
Supervisor: Yana Fedortchouk