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Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton, Canada

University of British Columbia

Petrography, mineralogy and thermobarometry are reported for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 15% coarse spinel peridotite, 4% coarse spinel-garnet peridotite, 4% coarse garnet peridotite, 9% porphyroclastic peridotite, 60% websterite and 8% orthopyroxenite. Peridotites are composed primarily of forsteritic olivine (Fo 89-94), enstatite (En 89-94), Cr-diopside, Cr-pyrope garnet and chromite spinel. Pyroxenites are composed primarily of enstatite (En 90-92), Cr-diopside and Cr-pyrope garnet. Thermobarometric estimates were made using two-pyroxene, garnet-clinopyroxene and Ca-in-orthopyroxene thermometers and garnet-orthopyroxene barometer. Results suggest that coarse peridotites equilibrated at 650-1220 °C and 23-63 kbar; porphyroclastic peridotites equilibrated at 1200-1350 °C and 57-70 kbar; pyroxenites equilibrated at 1030-1230 °C and 50-63 kbar. Muskox xenoliths are compared with xenoliths recovered from the neighboring Jericho kimberlite, erupted 15 km away from and at the same time as Muskox. Contrasts in the characteristics of these two suites of mantle samples include: 1) higher levels of depletion throughout the Muskox mantle column based on the contents of MgO in olivine and orthopyroxene and Cr₂O₃ in garnet; 2) the presence of a shallow zone of metasomatism in the spinel stability field in the Muskox mantle; 3) a higher proportion of pyroxenitic versus peridotitic rock types at the base of the mantle column beneath the Muskox kimberlite and higher Cr₂O₃ in all minerals in pyroxenites; and 4) lower levels of deformation in the Muskox mantle. We interpret these contrasts as representing small scale heterogeneities in the bulk composition of the mantle, as well as the local effects of kimberlite formation and ascent. If percolation of asthenosphere-derived pre-kimberlitic fluids in the less permeable Muskox mantle was impeded, localization of this fluid may have resulted in higher proportions of pyroxenitic rock types here, as well as lower degrees of deformation of the peridotitic mantle.

Text

2015-08-24

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/54596

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/