The Sequoia kimberlite complex, central Slave Craton – A new superdeep diamond location

We present the first study of 73 micro- and macro-diamonds and their inclusions from the Sequoia kimberlites, Central Lac de Gras kimberlite field, NWT. Nitrogen aggregation systematics show the presence of a diamond population that resided in the mantle at high temperatures (> 1250 ºC; 17% of all Type I diamonds), plus a high proportion of Type II diamonds (26%). Raman analysis of mineral inclusions in 24 diamonds reveals that the Sequoia kimberlites sampled both lithospheric (19 diamonds) and sublithospheric (5 diamonds) mantle sources. Olivine crystals coexisting with minerals characteristic of retrogressed sublithospheric assemblages display an extreme shift in Raman peak positions, indicating high pressure of entrapment. In one diamond, we observed the typical X-ray diffraction reflections of preserved ringwoodite, indicating the coexisting olivine identified by Raman is retrogressed. The preserved ringwoodite was stable in the transition zone. Lithospheric diamonds are of peridotitic affinity and the presence of clinopyroxene inclusions indicates sampling of possibly younger, less depleted portions of the mantle beneath Lac de Gras. Eight percent of the studied diamonds are fibrous and trap mineral micro-inclusions derived from peridotitic substrates. The diamond C and N isotope compositions are generally within typical mantle values, but the δ¹³C variability significantly exceeds the mantle range to both lower and higher values. Diamond internal variations in δ¹³C-δ¹⁵N exceeding the mantle range document the influence of subducted fluids in diamond formation. This first look at Sequoia diamonds underlines the need for exploration approaches that assess the presence of lherzolitic and sublithospheric-derived diamonds during indicator mineral assessment.