Petrology, geochemistry, and Triassic eruption age of the Dharma kimberlite, Northwest Territories, Canada

Abstract

The Dharma kimberlite, northeast of Great Bear Lake, is one of a few kimberlites erupted through Proterozoic cratonic regions west of the Slave Craton. The Dharma and nearby Dharma Uttar kimberlites consist dominantly of volcaniclastic kimberlite, with hypabyssal sills and dikes. Phlogopite, spinel, and ilmenite compositions plus whole-rock major and trace elements indicate the intrusions and volcanic bodies are archetypal kimberlites. A phlogopite Rb – Sr isochron yielded 225.3 ± 0.4 Ma, interpreted as the emplacement age, making it a rare example of Triassic kimberlite volcanism in Canada. This age helps define a corridor of magmatism that displays a broad younger age progression from northwest to southeast that could be interpreted as an extension of Great Meteor hot-spot, although the trend is scattered in the northwest. Whole-rock Dharma kimberlite samples have ⁸⁷Sr/⁸⁶Sr_initial 0.7039 to 0.7047, εNd_initial + 2.2 to + 2.5, and εHf_initial + 1.4 to + 6.8, with the isotopic characteristics of the least-contaminated rocks lying on the global temporal trend of mildly depleted source compositions defined by kimberlite. The compositions of peridotite micro-xenoliths and garnet xenocrysts indicate the presence of a heterogenous lithospheric mantle beneath Dharma. Garnet harzburgite compositions indicate a thick lithospheric mantle profile: minimum garnet Cr-saturation pressures are between 4.7 and 5.1 GPa. Ni-in-garnet thermometry for lherzolitic and harzburgitic garnets provide evidence for a stratified mantle, with a broad distribution of lherzolite and two harzburgite horizons. Extrapolation of these Ni-in-garnet thermometry temperatures to a regional geotherm indicates that approximately half of the mantle load sampled by the Dharma kimberlite originated within the diamond stability field.