In-situ analysis of diamonds and their inclusions from the Diavik Mine, Northwest Territories, Canada: Mapping diamond growth

Abstract

Results are presented here of an in-situ study of diamonds from the A154 South kimberlite pipe at the Diavik diamond mine, Northwest Territories, Canada. One hundred and ten diamonds were selected from run of mine production on the basis of morphology and visible inclusions. Diamonds that crystallized as cubes have a higher incidence of fluorescence in response to UV light than those that have crystallized in the octahedral primary form. Fifty-one diamonds were cut and polished to expose included mineral grains and to allow for imaging of internal structure. Mineral inclusions were analysed in-situ for major element composition using electron microbeam methods. Internal zonation of the diamonds was imaged using cathodoluminescence in thirteen P-type diamonds, two U-type diamonds, nine E-type diamonds, and two diamonds with only sulfide inclusions of indeterminate paragenesis. Inclusions of Fe ± Cu ± Ni sulfide, magnesian chromite, ferropericlase, chromian diopside, forsteritic olivine, omphacite, and enstatite occur in order from most to least abundant. The chromite inclusions are lower in Mg than the worldwide average. At least six of the sulfide inclusions are significantly heterogeneous. Equilibration temperature and pressure conditions for the diopside inclusions indicate that the diamonds equilibrated in a region of geothermal gradient with an equivalent surface heat flow of 42 mW/m 2 over a range from 51–57 kbar and 1170–1260 °C.