LITHOLOGICAL COLUMN OF THE MANTLE BELOW THE MUSKOX KIMBERLITE, N SLAVE CRATON

This work reports petrography, geochemistry and thermobarometry of 52 mantle peridotite and pyroxenite xenoliths recovered from the 170 Ma Muskox kimberlite in the northern Slave craton. The Muskox suite includes 59% pyroxenite, 11% coarse spinel peridotite, 4% coarse spinel-garnet peridotite, 14% coarse garnet peridotite and 10% sheared garnet peridotite. Sheared peridotites are minimally deformed with a maximum of 10% neoblasts. Xenolith mineral compositions match those recovered from other kimberlites in the Slave craton and other cratons worldwide. Olivines from pyroxenites and sheared peridotites generally have Mg# at around 88-90, those in spinel peridotites are Fo 92-94. Enstatites and Cr-diopsides are more ferrous (Mg# 90-93) in pyroxenites and sheared peridotites than in spinel peridotites (Mg# 92-95). The rocks may also contain Cr-pyrope and spinel or chromite with 17-55 wt. % Cr2O3. Thermobarometric estimates were made using six methods, which include Brey & Kohler (1990) combined thermobarometry, two-pyroxene thermometry and Ca-in-Opx thermometry, Nakamura (2009) Cpx-Gar thermometry, Girnis & Brey (1999) Spl-Gar thermobarometry and O’Neill (1981) Spl-Gar barometry. Resulting P-T estimates constrain the thermal state of the mantle identical to that of the contemporaneous Jericho kimberlite 15 km from Muskox, i.e. the 40 mW/m2 heat flow. The Muskox mantle has an upper layer of depleted (Ol Mg# 92-94) spinel peridotite to 100-130 km depth, spinel-garnet peridotite to 170 km, more fertile garnet peridotite (Ol Mg# 89-91) to 190-210 km, the asthenosphere boundary at 200 km and sheared peridotite below. Pyroxenite pods coexist with peridotite between 170-220 km depth, and dominate the sample suite in this depth range. The Muskox chemically stratified mantle lithosphere, with a stronger depletion at shallow level, is similar to that at other locations on the Slave craton. The Muskox mantle, in contrast to the adjacent Jericho mantle, contains spinel peridotite to greater depth, multiple pyroxenite intrusions at a shallower level and shows a thicker lithosphere.