Age, Mineralogy and Geochemistry of the Mel and Dharma Kimberlites, Northern Canada
Creators: Kepezhinskas, Nikita 
Publisher: name: University of Alberta Library
Year: 2022

<p>This work presents new petrography, mineral major and trace element chemistry, whole rock major and trace element chemistry, whole rock Sr-Nd-Hf isotope composition, and phlogopite Rb-Sr isotope geochronology for the Mel kimberlite, Nunavut and Dharma kimberlite, Northwest Territories. This data provides an opportunity to confirm the classification of the Mel and Dharma bodies as true archetypal kimberlite, characterize the source region and intrusion/eruption history of the Mel and Dharma kimberlites, trace the magmatic evolution of the Mel and Dharma kimberlite, and provide thermal characteristics of the source mantle beneath the Mel and Dharma.</p>

<p>The diamondiferous Mel kimberlite is composed of north-south oriented hypabyssal and volcaniclastic kimberlite dikes exposed in float at the surface and intersected by diamond drilling at depth. Major element chemistry of phlogopite, ilmenite, and spinel from the Mel kimberlite indicate that the intrusion shares compositional characteristics with regional and world-wide kimberlites. This is further corroborated by whole rock major and trace element chemistry. Rb-Sr phlogopite geochronology of the kimberlite establishes a kimberlite emplacement age of 555.6 ± 2.7 Ma with an initial 87Sr/86Srinitial ratio of 0.704414 ± 0.000012. Uncontaminated samples of the Mel kimberlite present 87Sr/86Srinitial (0.70538 to 0.70686), εNdinitial (2.35 to 3.04), and εHfinitial (- 15.44 to -1.05) ratios similar to that of Eoarchean to Cambrian primitive kimberlites in eastern Canada. Garnet mineral chemistry identified predominantly lherzolite and eclogite garnet groups. Chondrite-normalized REE patterns from lherzolite garnets record predominantly LREE-depleted patterns indicating melt-metasomatism. Ni-thermometry in most peridotite garnets from the Mel kimberlite records median mantle temperatures of 1250°C. Extrapolation of these temperatures to the West Central Rae Geotherm indicates that Mel kimberlite peridotite garnet was sampled from a depth of 105 km to 185 km with 86% of mantle garnet sampled within the diamond stability field.</p>

<p>The diamondiferous Dharma kimberlite is part of a larger field of at least two discrete kimberlite bodies, Dharma and Dharma Uttar. Diamond drilling indicates that the Dharma kimberlite is composed of kimberlite tuff overlying kimberlite tuff-breccia that host at least two discrete domains of hypabyssal kimberlite. Major element chemistry of olivine, phlogopite, ilmenite, spinel, and rutile from the Dharma kimberlite indicate that the intrusion shares compositional characteristics with regional and world-wide kimberlites. Whole rock major and trace element chemistry of hypabyssal kimberlite further demonstrate evidence for the kimberlite classification. Rb-Sr phlogopite geochronology of the kimberlite establishes a kimberlite emplacement age of 225.3 ± 0.8 Ma with an initial 87Sr/86Srinitial ratio of 0.707081 ± 0.00005. A single, high Rb/Sr, phlogopite macrocryst records an errorchron age of 222 ± 2 Ma regardless of its 87Sr/86Srinitial ratio. Uncontaminated samples of the Dharma kimberlite present 87Sr/86Srinitial (0.703902 to 0.704666), εNdinitial (2.21 to 2.45), and εHfinitial (1.4 to 6.8) ratios similar to that of Jurassic kimberlite from Jericho and perovskite from the Triassic to Jurassic Churchill kimberlite field. Garnet mineral chemistry identified lherzolite and harzburgite garnet groups in the Dharma kimberlite. LREE depleted chondrite-normalized REE patterns are found in most lherzolite and all G10B Ti harzburgite garnet indicating melt-metasomatism. Sinusoidal chondrite-normalized REE patterns typically associated with diamond-forming fluids are observed in a subset of lherzolite garnet and in all G10 harzburgite garnets. Ni-thermometry of peridotite garnet indicates sampling at mantle temperatures of 1100°C for lherzolite garnets, 1100°C for G10B Ti harzburgite garnet, and 1300°C for G10A harzburgite garnet. These temperatures are extrapolated to the Victoria Island geotherm providing a depth sampling profile of 177 km to 182 km for lherzolite garnet and 176 km to 181 km for harzburgite garnet. These extrapolations are supported by the CaO and Cr2O3 content of G10A garnet which plot on the 47 kbar isobar (approx. 174 km). As a result, the Dharma kimberlite sampled 49% of mantle garnet from within the diamond stability field.</p>