UBC Undergraduate Research
The Diamond Potential of the Tuwawi Kimberlite (Baffin Island, Nunavut). Cross, Jodi
Ten samples of kimberlite and associated mantle xenoliths were studied to constrain the diamond potential of the Tuwawi kimberlite of Baffin Island, Nunavut. The Tuwawi kimberlite, one in a cluster of 3 kimberlites, is located at the northwestern end of Baffin Island on the Brodeur Peninsula. Baffin Island is underlain by Archean crust of the Rae craton with Paleoproterozoic reworking, and is known to contain several kimberlites, possibly, one at least, of Cretaceous age. These include the Freightrain and Cargo kimberlites. Petrographic analyses established that both hypabyssal and volcaniclastic kimberlitic types are present among the four kimberlite samples. Hypabyssal kimberlite is the predominant type in Tuwawi, consisting of olivine macrocrysts set in a carbonate-serpentine groundmass with olivine microphenocrysts, phlogopite and spinel. Volcaniclastic kimberlite is characterized by the presence of 1) irregularly-shaped juvenile lapilli; 2) two semi-intermixed dark cryptocrystalline matrix materials; 3) olivine grains with a restricted size distribution and angular shapes. These features suggest mild sorting of the kimberlite, a possible incorporation of mud to the matrix, an epiclastic origin and formation in the crater facies. Petrographic analyses established that the six mantle xenolith samples include two garnet lherzolites, a spinel and a garnet-spinel harzburgites, a dunite, and a clinopyroxenite. Both coarse and deformed (porphyroclastic and mosaic-porphyroclastic) textures are present within the peridotite xenoliths. Garnet is present in all but one sample, whereas spinel occurs only in coarse peridotites. Electron microprobe geochemical analyses of the mantle xenoliths provided important information regarding the equilibrium compositions of the major minerals present: olivine, orthopyroxene, clinopyroxene, garnet, and spinel. All minerals, except spinel, show chemical homogeneity between and within grains. Cr-diopside from deformed xenoliths shows higher TiO2 (0.16 wt%) content than in coarse peridotites. All garnets present are pyropes (Mg81-84), and spinels are magnesiochromites showing strong chemical heterogeneity. This is controlled by random intra-grain compositional changes in FeO (from 12 to 16 wt%), MgO, Al2O3 and Cr2O3 (from 43 to 57 wt% ). Olivine andorthopyroxene in all xenoliths are very magnesian (Fo91-92 and En92-93), slightly more so in coarse peridotites. Pressures and temperatures of mineral equilibria for the mantle xenoliths were estimated using various two-pyroxenes, garnet-pyroxene, olivine-garnet, and olivine-spinel geothermobarometers. Deformed garnet lherzolite and garnet clinopyroxenite were formed at 1103-1209oC and 53.8–60 kb. Deformed peridotites are equilibrated at higher temperatures and pressures than coarse peridotites. Garnet peridotites and pyroxenites show higher temperatures than spinel peridotites. These patterns match the commonly observed mantle lithological columns below cratons. In comparison to temperature and pressure data from kimberlites of Somerset Island, xenoliths from Tuwawi plot farther into the diamond stability field and at a lower geothermal gradient (~42 mW/m2). The majority of mantle xenoliths from Cretaceous Somerset Island kimberlites plot in the graphite stability field along a geotherm of ~44 mW/m2. Several factors were identified that give a positive outlook on the diamond potential of the Tuwawi kimberlite. These factors include 1) a preservation of the crater facies kimberlite, and 2) kimberlite sampling of the deep diamondiferous mantle. The diamond potential is reduced by the estimated 42 mW/m2 geothermal gradient that is hotter than the desired low geotherm for Archean cratons. In addition, a relatively narrow “diamond window” (i.e. the range of temperatures where diamond is stable in the asthenosphere), 1050-1100 °C, lowers the diamond potential of the Tuwawi kimberlite.
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