Diamonds and their inclusions from Snap Lake diamond mine (Northwest Territories, Canada)

  • Author / Creator
    Graversen, Kelsey
  • This research was motivated by a current lack of systematic information, pairing inclusion chemistry (major,minor, and trace element) to host diamond isotopic data (δ13C, δ15N, N abundance) regarding the Snap Lake kimberlitedyke. The Snap Lake kimberlite dyke is located on the southern Slave Craton, approximately 220 km northeast ofYellowknife, NWT. Previous studies (Pokhilenko et al. 2001, 2004) have inferred, through the observation of rarehigh-Cr majorite garnet inclusions, the presence of an unusually thick lithospheric mantle root (>200 km), which atdepths corresponding to the diamond stability field was mostly harzburgitic. This thesis characterizes the variousdiamond substrates which make up the lithospheric mantle beneath Snap Lake, in addition to the conditions ofdiamond formation and mantle residence.Based on a set of 87 inclusion-bearing diamonds it is inferred that diamond formation occurred primarily inperidotitic mantle (93%), with a more minor component forming in eclogitic mantle (6%) and a single studied diamondshows rare mixed paragenesis (1%). The peridotitic inclusion suite consists of garnet, olivine, chromite, clino- andorthopyroxene, sulphide and rare native iron-wustite, while the eclogitic inclusion suite consists of garnet, SiO2, N-poorsulphide and potassium feldspar. From the single mixed-paragenesis diamond, a peridotitic garnet, an eclogiticcoesite and an eclogitic sulphide were recovered, indicating the two sub-populations do not necessarily representdistinct environments beneath Snap Lake. Sinusoidal and weakly sinusoidal patterns of REE depletion for the majorityof peridotitic (harzburgitic and lherzolitic) garnet indicate enrichment through similar styles of metasomaticfluids/melts. Geothermobarometry results indicate diamond formation occurred along a cool 36 – 38 mW/m2 paleogeothermat temperatures ranging from 920 to 1230 °C, between 120 and at least 210 km in depth.Overall, diamond fragments showed FTIR nitrogen abundances from below the limit of detection (< 10at.ppm) to 1190 at.ppm with a large variation in nitrogen aggregation state ranging from 0 to 95 %B. Nitrogen-basedmantle residence temperatures agree well with geothermometry results ranging from 1050 – 1280 °C. SIMS analysisresulted in δ13C values in a tight range from – 8.0 to – 1.3 ‰ for Snap Lake diamonds, with a median of – 4.2 ‰ andan average of – 4.3 ‰. The δ13C values of the peridotitic and eclogitic suites differed very little with ranges from –8.0 to – 1.3 ‰ (median = - 4.2 ‰, average = - 4.3 ‰) and – 6.2 to – 3.9 ‰ (median = - 4.7 ‰, average = - 4.7 ‰),respectively. Snap Lake diamonds showed a wide range in δ15N values ranging from – 20 to +11 ‰ reflecting a highlikelihood of involvement of subducted sediments contributing to the nitrogen signature and supporting the idea of aheterogeneous nitrogen reservoir accounting for the extreme depletion in 15N observed in peridotitic diamonds.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.