Mineralogy of the Elwin Bay kimberlite, Somerset Island, N.W.T., Canada

The Elwin Bay kimberlite is a small post-Silurian diatreme characterized by the presence of ultrabasic xenoliths and chrome-poor garnet megacrysts [Mg/(Mg+Fe) = 0.79-0.83; Cr₂O₃ = 0.12-2.3%]. Olivine [Mg/(Mg+Fe) = 0.89-0.94] occurs as large rounded “phenocrysts” and as smaller groundmass crystals. Chemically and texturally the “rounded phenocrysts” are identical to olivines found in garnet lherzolite xenoliths and as xenocrysts in this kimberlite. Phlogopite occurs as rounded phenocrysts (0.6-3.2% TiO₂) and as euhedral groundmass laths (0.2-0.8% ТiO₂). The groundmass mineral assemblage is composed of calcite, monticellite (3.2-7.5% FeO), perovskite (1.3-1.6% FeO), serpentine, chlorite, and spinel. Pre-fluidization spinels are rounded Ti-poor aluminous-magnesian-chromites. Post-fluidization spinels are euhedral titaniferous-magnesian-aluminous-chromites (1.5-7.1% ТiO₂). Calcite and primary serpentine-filled ocelli are considered to represent separation of a late-stage immiscible carbonate fluid.