Constraints on the distribution, composition, and lithology of eclogite and pyroxenite from the central Slave craton

We investigate the petrogenesis of a newly collected suite of 225 barren and one diamond-bearing eclogite and pyroxenite microxenoliths from the A154, Panda, and DO-27 kimberlites from the Lac de Gras region, Northwest Territories, Canada. All samples contain low-Cr garnet (Cr₂O₃ < 1 wt%) and were analysed for major-element compositions of garnet and clinopyroxene. Trace-element compositions were determined for 140 mineral pairs, and δ¹⁸O values for garnets from 160 samples. Geothermometry indicates that eclogite and pyroxenite xenoliths from A154 are distributed throughout the subcratonic lithospheric mantle (SCLM), from ~ 80 km to as deep as ~ 200 km. Eclogites/pyroxenites from A154 that equilibrated shallower than ~ 170 km are derived from a variety of protoliths including basaltic- and cumulate-type oceanic crust. The majority of these rocks have δ¹⁸O values (average + 5.54 ± 0.89‰, 2σ) within or below the canonical mantle range. The deeper suite of eclogites (≥ 170 km) typically have Ca-rich oceanic cumulate-type protoliths, with garnet δ¹⁸O values (+ 4.95 ± 0.58‰) mostly below the mantle range. Eclogite/pyroxenite microxenoliths from Panda and DO-27 have protoliths that include basaltic- and cumulate-type oceanic crust. Oxygen isotope compositions for low-Cr garnets from DO-27 have a similar distribution to A154 whereas those from Panda are largely within or near the mantle range. Our expanded dataset shows that eclogites are key components in mantle root assembly, and reinforces previous findings of a lithological, mineralogical, and compositional boundary at approximately 170 km depth in the Slave craton lithosphere.