Sr-Nd-Pb isotope systematics of mantle xenoliths from Somerset Island kimberlites: Evidence for lithosphere stratification beneath Arctic Canada

Sr, Nd, and Pb isotopic compositions were determined for a suite of Archean garnet peridotite and garnet pyroxenite xenoliths and their host Nikos kim berlite (100 Ma) from Somerset Island to constrain the isotopic character o f the mantle root beneath the northern Canadian craton. The Nikos peridotit es are enriched in highly incompatible trace elements (La/Sm-N = 4-6), and show Nd-143/Nd-144((t)) (0.51249-0.51276) and a large range in Sr-87/Sr-86( (t)) (0.7047-0.7085) and Pb (Pb-206/Pb-204((t)) = 17.18 to 19.03) isotope r atios that are distinct from those estimated for "depleted mantle" composit ions at the time of kimberlite emplacement. The Nd isotopic compositions of the peridotites overlap those of the Nikos kimberlite, suggesting that the xenoliths were contaminated with kimberlite or a kimberlite-related access ory phase (i.e., apatite). The highly variable Sr and Pb isotopic compositi ons of the peridotites, however, indicate that kimberlite contribution was restricted to very small amounts (similar to1 wt % or less). The high-temperature peridotites (> 1100 degreesC) that sample the deep Som erset lithosphere trend toward more radiogenic Sr-87/Sr-86((t)) (0.7085) an d unradiogenic Pb-206/Pb-204((t)) (17.18) isotopic ratios than those of the low-temperature peridotites (< 1100 degreesC). This is in agreement with S r isotopic compositions of clinopyroxene from the low-temperature peridotit es (Sr-87/Sr-86((t)) = 0.7038-0.7046) that are significantly less radiogeni c than those of clinopyroxene from the high-temperature peridotites (Sr-87/ Sr-86((t)) = 0.7052-0.7091). The depth correlation of Sr isotopes for clino pyroxene and Sf and Pb isotopic compositions for the Nikos whole-rocks indi cate that the deep Somerset lithosphere (> 160 km) is isotopically distinct from the shallow lithospheric mantle. The isotopic stratification with dep th suggests that the lower lithosphere is probably younger and may have bee n added to the existing Archean shallow mantle in a Phanerozoic magmatic ev ent. The radiogenic Sr isotope ratios of the high-temperature peridotites a nd their clinopyroxenes suggest that the underplated deep lithosphere conta ined recycled (altered oceanic crust and sedimentary component?) material i ntroduced during earlier subduction. Copyright (C) 2001 Elsevier Science Lt d.