Emplacement conditions of some Lac de Gras kimberlites and their effect on the resorption of diamonds




Fedortchouk, Yana

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Crystallization temperatures (T) and oxygen fugacities (fO2) of kimberlite magma estimated from oxides included in olivine phenocrysts from eight kimberlite pipes in the central Slave Province, Canada, are compared to the degree and character of resorption observed in diamonds recovered from these kimberlites. The mechanism of diamond oxidation in kimberlite melts and the rate-controlling parameters for this reaction are explored in oxidation experiments. The T and maximum fO2 recorded by olivine - chromite pairs at an assumed pressure of 1 GPa are 970° -- 1070°C and 2.2 - 3.1 log units below the nickel - nickel oxide (NNO) buffer. This mineral assemblage crystallized from a magma with 1 1 to 28 mol% of liquid, 10 mol% of earlier-precipitated olivine phenocrysts and 62 to 79 molc7o of mantle xenocryst olivine. The T - fO2 values vary between kimberlites from Northwest and Southeast clusters within 150°C and one log unit, respectively, and form a trend of decreasing fO2 and increasing crystallization T in the southeast direction. This trend corresponds to substantial differences in the diamond populations. A detail description of morphological forms and surface resorption features for five diamond parcels (> 7000 stones) show an increase in diamond resorption with increase in kimberlite crystallization T and more extensive surface etching in more oxidized kimberlites. The surface etch features on diamonds are determined by the conditions in the kimberlite melt, whereas some of the volume resorption occurs in the mantle and its relationship with the melt conditions is obscure. The diamond grade is higher in kimberlites with lower fO2 confirming the effect of the melt conditions on diamond preservation. Diamond oxidation experiments at 1350°C to 1500°C and 1 GPa produced only surface graphitisation, and no diamond resorption in volatile undersaturated melts. In contrast, volatile oversaturated conditions produce resorption features seen in diamonds recovered from kimberlites, suggesting that the process of diamond resorption is its reaction with the fluid and not with the melt. Both CO2 and H2O oxidize diamonds at a similar rate, but produce very different surface features. Therefore, the surface features of natural diamonds may provide information on the H2O/CO2 ratio in the kimberlitic fluid. The morphologies of diamonds from this study imply high H2O/CO2. The scarcity of surface graphitisation and presence of highly resorbed diamonds in kimberlites suggest presence of free fluid phase in kimberlite magmas for the most of their history. The diamond oxidation is not affected by the physical properties of diamonds.



kimberlite, diamonds, Northwest Territories