Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere

Abstract

We present the first data on the petrology of the mantle lithosphere of the Southeastern (SE) Slave craton, Canada. These are based on petrographic, mineralogical and geochemical studies of mantle xenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlite cluster. Major types of mantle xenoliths include altered eclogite, coarse garnet or spinel peridotite, and deformed garnet peridotite. The peridotites belong to the low-temperature suite and formed at T=600–1300°C and P= 25–80 kbar in a thick (at least 220–250 km), cool lithosphere. The SE Slave mantle is cooler than the mantle of other Archaean cratons and that below other terranes of the Slave craton. The thick lithosphere and the relatively cool thermal regime provide favourable conditions for formation and preservation of diamonds beneath the SE Slave terrane. Similar to average Archaean mantle worldwide, the SE Slave peridotite is depleted in magmaphile major elements and contains olivine with forsterite content of 91–93·5. With respect to olivine composition and mode, all terranes of the Slave mantle show broadly similar compositions and are relatively orthopyroxene-poor compared with those of the Kaapvaal and Siberian cratons. The SE Slave spinel peridotite is poorer in Al, Ca and Fe, and richer in Mg than deeper garnet peridotite. The greater chemical depletion of the shallow upper mantle is typical of all terranes of the Slave craton and may be common for the subcontinental lithospheric peridotitic mantle in general. Peridotitic xenoliths of the SE Slave craton were impregnated by kimberlitic fluids that caused late-stage recrystallization of primary clinopyroxene, spinel, olivine and spinel-facies orthopyroxene, and formation of interstitial clinopyroxene. This kimberlite-related recrystallization depleted primary pyroxenes and spinel in Al. The kimberlitic fluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introduced serpentine, chlorite, phlogopite and spinel into peridotites at P < 35 kbar.