A new occurrence of kimberlite located on Somerset Island in the Canadian Arctic is described. The intrusion is in the form of a diatreme showing evidence of fluidization and is emplaced in Ordovician sediments. Kimberlitic breccia containing rounded xenoliths of country rock and massive kimberlite are the dominant rock types. The massive kimberlite contains phenocrysts of olivine (Fo90–Fo93), chrome-pyrope (7–12 mol.% uvarovite), and phlogopite together with xenocrysts of pyrope-almandine (28–36 mol.% pyrope, 58–64 mol.% almandine) set in a groundmass of olivine (Fo89.5), magnetite, perovskite, and serpentine. Abundant dolomitic carbonate, which replaces the silicate groundmass, is considered to be primary and to represent a late stage immiscible phase. Emplacement of the diatreme is thought to have been controlled by a crustal lineament defined by the Boothia Uplift. An Upper Ordovician or Lower Silurian age is postulated on the basis of the lack of country rock xenoliths older than Upper Ordovician age within the diatreme.