Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

Abstract

Peridotites that sample Archean mantle roots are frequently incompatible trace element enriched despite their refractory major element compositions. To constrain the trace element budget of the lithosphere beneath the Canadian craton, trace element and rare earth element (REE) abundances were determined for a suite of garnet peridotites and garnet pyroxenites from the Nikos kimberlite pipe on Somerset Island, Canadian Arctic, their constituent garnet and clinopyroxene, and the host kimberlite. These refractory mantle xenoliths are depleted in fusible major elements, but enriched in incompatible trace elements, such as large ion lithophile elements (LILE), Th, U and light rare earth elements (LREE). Mass balance calculations based on modal abundances of clinopyroxene and garnet and their respective REE contents yield discrepancies between calculated and analyzed REE contents for the Nikos bulk rocks that amount to LREE deficiencies of 70–99%, suggesting the presence of small amounts of interstitial kimberlite liquid (0·4–2 wt %) to account for the excess LREE abundances. These results indicate that the peridotites had in fact depleted or flat LREE patterns before contamination by their host kimberlite. LREE and Sr enrichment in clinopyroxene and low Zr and Sr abundances in garnet in low-temperature peridotites (800–1100°C) compared with high-temperature peridotites (1200–1400°C) suggest that the shallow lithosphere is geochemically distinct from the deep lithosphere beneath the northern margin of the Canadian craton. The Somerset mantle root appears to be characterized by a depth zonation that may date from the time of its stabilization in the Archean.