Mineral inclusions in diamonds from Chidliak (Nunavut, Canada): constraining the diamond substrates

  • Author / Creator
    Xia, Xinchen
  • The Chidliak kimberlite pipe is located on the Hall Peninsula on Baffin Island in Nunavut. The first kimberlite in Chidliak was discovered in 2008. Previous studies (e.g., Hogberg et al., 2016) on Chidliak inclusion-free diamonds have suggested that 1) two diamond growth stages are recorded by carbon isotopic ratios and nitrogen concentrations; and 2) diamonds resided at temperatures from ~ 980 to 1350 ℃ corresponding to depths of ~ 150 to 200 km. Seventy-four diamonds from the CH-7 kimberlite pipe at Chidliak were broken to release their mineral inclusions. The characteristics of major and minor elements of inclusions in diamond suggest a source dominated by eclogite, with a minor peridotitic component. The trace-element compositions of eclogitic garnet and clinopyroxene diamonds inclusions measured by laser ablation ICP-MS, and the presence of both negative and positive europium anomalies indicate low-pressure oceanic crustal protoliths. Temperatures calculated from non-touching mineral inclusions reflect diamond formation at ~ 1050 to 1150 ℃ (derived from garnet-olivine associations and the single olivine thermometer). This result compares very well with the nitrogen-based mantle residence temperatures (~ 1000 to 1200 ℃) of the host diamonds, equivalent to a depth range of ~ 160 to 200 km when projected on the xenolith/xenocryst based local paleo-geotherm of 36 mW/m2 (Pell et al., 2013). Fragments of inclusion-bearing diamonds were measured via SIMS. The overall carbon isotopic data (δ13C range of -25 to -1.6 ‰) shows a principal mode in class bin -5 to -4 ‰ (close to the typical mantle value: -5 ‰; Deines, 1980; Cartigny, 2005) with a secondary mode in class bin -17 to -16 ‰, which is consistent with previous research (Hogberg et al., 2016) and an eclogite-dominated source determined from the suite of diamond inclusions. The δ13C values of peridotitic and eclogitic diamonds range from -5.8 to -1.6 ‰ and 24.8 to -3.2 ‰, respectively. The δ15N of peridotitic diamonds ranges from -6.9 to +4.7 ‰, whilst eclogitic diamonds have a broader distribution from -5.1 to +9.8 ‰. The narrow range in δ13C of peridotitic diamonds is in agreement with a mantle-derived carbon source. For eclogitic diamonds, the characteristics and correlations of δ13C and δ15N indicate that the δ13C-depleted group (< -10 %) has a subduction origin, whereas the group with δ13C > 10 ‰ results from mixing of predominantly mantle-derived fluids and subducted carbon. A lack of systematic correlations of δ13C-[N] and δ15N-[N] has been identified and is possibly due to the mixing of sources with distinct C and N, or the presence of more than one diamond population in the studied bulk sample. Combining the SIMS data, CL imaging, and the presence of yellow-coated diamonds, at least two growth episodes of diamonds are revealed, with a resorption event between each stage. On the scale of individual diamonds, multiple pulses of fluids/melts are represented by the internal CL characteristics and associated variation in δ13C values.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Master of Science
  • DOI
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