Micro-morphology and Resorption of Diamonds from Snap Lake and Ekati Mine Kimberlites (Canada) as an Indicator of the Fluid and Emplacement History

English Abstract:

Natural diamonds develop resorption features due to partial dissolution in kimberlites and mantle metasomatism. Experimental data shows that diamond resorption features strongly depend on the presence and composition of kimberlitic fluid. Atomic force microscopy (AFM) can be used to quantitatively examine individual features on diamond to put robust constraints on their resorption conditions. Thus, diamond morphology can be a proxy of magmatic and mantle fluids and the emplacement conditions of kimberlite magma. This study examines dissolution features on diamonds from kimberlite localities with presumably different fluid and emplacement histories - Snap Lake kimberlite dyke and pipes with volcaniclastic and coherent kimberlite facies from Ekati Mine. The results are compared to the dissolution features produced in controlled experiments. Optical microscopy and SEM study of resorption on 251 micro-diamonds from Snap Lake and several hundred of micro-diamonds from six Ekati kimberlites, allowed dividing them into morphological groups. Selected crystals studied with AFM provided quantitative data on the geometry of diamond micro-features. The results were compared to the AFM data for diamonds after resorption in experiments at 0.1 MPa in H₂ – CO₂ gas mixture and at 1 – 3 GPa in CHO fluid with H₂O:CO₂ = 1, 0.5, 0.35, 0.1, 0. Snap Lake diamonds show widespread development of positive trigonal pits in addition to the common negative trigons, indicating severe etching at the near-surface conditions and evolved composition of the reacting fluid during the early (deep) and late (shallow) resorption. Ekati diamonds show only negative trigons implying deeper resorption. The AFM data show significant variation in H₂O:CO₂ ratio between the kimberlites. Application of the new AFM-based method to examine conditions during kimberlite emplacement using diamond resorption helps to better constraint of the nature of kimberlitic fluids and evaluating diamond preservation.