Composition and thermal evolution of cratonic mantle beneath the central Archean Slave Province, NWT, Canada

Abstract

The composition and thermal evolution of the upper mantle lithosphere beneath the central Archean Slave Province has been studied using mineral chemical and petrographic data from mantle xenoliths entrained in the Torrie kimberlite pipe. Coarse-, granuloblastic-, and porphyroclastic- textured harzburgite, lherzolite, and pyroxenite xenoliths yield equilibration temperatures ranging between 850 and 1350 °C. Thermobarometry of these samples requires a minimum lithospheric thickness of approximately 180 km at the time of kimberlite magmatism. The distribution of pressures and temperatures of equilibration for the xenoliths lie on a calculated 42 mWm⁻² paleogeotherm, ∼10 mWm⁻² lower than the present heat flow measured at Yellowknife, near the SW margin of the Slave Province. The Mg# [Mg/(Mg + Fe)] of olivine in peridotites varies between 0.906 and 0.938 with an average of 0.920. The Torrie xenolith suite shows variable degrees of serpentinization and/or carbonation with the rim compositions of many clinopyroxene grains showing Ca enrichment, but in general, the xenoliths are homogeneous at all scales. The Torrie xenoliths are rich in orthopyroxene similar to low temperature (<1100 °C) peridotites from southern Africa, and Siberia. Estimates of bulk rock composition based on mineral chemical and modal data reveal a negative correlation between Si and Fe, similar to peridotite xenoliths from Udachnaya. The similarity of olivine Mg#s with other cratons combined with the negative correlation of Fe and Si suggest that the lithosphere beneath the Slave craton has experienced a evolution similar to other cratons globally.