Records of mantle metasomatism in the morphology of diamonds from the Slave craton

Abstract

Every population of kimberlitic diamonds contains crystals with remnants of mantle-derived resorption features that carry information about diamond-destroying mantle metasomatic events. We investigated 109 octahedral and tetrahexahedral diamonds (with {111} faces) representing unresorbed, kimberlite-induced (KIM), and mantle-derived (MR) resorption morphologies selected from 630 crystals from Grizzly, Misery, Leslie, and Koala kimberlites (Ekati mine, Northwest Territories, Canada). The MR category is further subdivided into morphologies with complex features (CM-1/2/3) and with resorption-derived step-faces (SM-1/2/3a, b/4) and classification criteria are proposed. In order to test if the different morphological styles have a common residence history in the mantle we obtained cathodoluminescence (CL) images on polished surfaces of each diamond and measured content and aggregation state of nitrogen defects using Fourier transform infrared (FTIR) spectroscopy. Zoning patterns on CL images also helped to distinguish between growth and resorption for step-faced morphologies. Nitrogen data of diamond rims form two clusters: (I) total nitrogen above ~400 ppm and low state of aggregation (<30 % of B centres); (II) total nitrogen <1000 ppm (mostly <400 ppm) and high state of aggregation (~20–98 % of B centres, mostly above 30 %). The study shows a clear correlation between the mantle resorption styles and the internal properties – nitrogen data and zoning patterns on CL images. Comparison of our results with the existing experimental data on diamond dissolution at high pressure and with the published datasets of diamonds recovered from eclogitic and peridotitic xenoliths shows that, in the Slave craton, metasomatism in peridotite develops complex resorption features (CM-1) most likely in the presence of CO2-rich fluid and step-faced (SM-2) diamonds in the presence of H2O fluid, whereas metasomatic fluid in eclogite is probably H2O-rich.