Constraints on the trace element composition of the Archean mantle root beneath Somerset Island, Arctic Canada

Peridotites that sample Archean mantle roofs are frequently incompatible tr ace element enriched despite their refractory major element compositions. l o constrain the trace element budget of the lithosphere beneath the Canadia n craton, trace element and rare earth element (REE) abundances were determ ined for a suite of garnet peridotites and garnet pyroxenites from the Niko s kimberlite Pipe on Somerset Island, Canadian Arctic, their constituent ga rnet and clinopyroxene, and the host kimberlite. These refractory mantle xe noliths are depleted infusible major elements, but enriched in incompatible trace elements, such as large ion lithophile elements (LILE), Th, U and li ght rare earth elements (LREE). Mass balance calculations based on modal ab undances of clinopyroxene and garnet and their respective REE contents yiel d discrepancies between calculated and analyzed REE contents for the Nikos bulk racks that amount to LREE deficiencies of 70-99%, suggesting the prese nce of small amounts of interstitial kimberlite liquid (0.4-2 wt %) to acco unt for the excess LREE abundances. These results indicate that the peridot ites had in fact depleted or flat LREE patterns before contamination by the ir host kimberlite. LREE and Sr enrichment in clinopyroxene and low Zr and Sr abundances in garnet in low-temperature peridotites (800-1100 degreesC) compared with high-temperature peridotites (1200-1400 degreesC) suggest tha t the shallow lithosphere is geochemically distinct from the deep lithosphe re 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.