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

摘要

Sr, Nd, and Pb isotopic compositions were determined for a suite of Archean garnet peridotite and garnet pyroxenite xenoliths and their host Nikos kimberlite (100 Ma) from Somerset Island to constrain the isotopic character of the mantle root beneath the northern Canadian cration. The Nikos peridotites are enriched in highly incompatible trace elements (La/Sm_N = 4 - 6), and show ~(143)Nd/~(144)Nd_(t) (0.51249 - 0.51276) and a large range in ~(87)Sr/~(86)Sr_(t) (0.7047-0.7085) and Pb (~(206)Pb/~(204)Pt_(t) = 17.18 to 19.03) isotope ratios that are distinct from those estimated for "depleted mantle" compositions 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 accessory phase (i.e., apatite). The highly variable Sr and Pb isotopic compositions of the peridotites, however, indicate that kimberlite contribution was restricted to very small amounts (～1 wt % or less). The high-temperature peridotites ~(87)Sr/~(86)Sr_(t) (0.7085) and unradiogenic ~(206)Pb/~(204)Pb_(t) (17.18) isotopic ratios than those of the low-temperature peridotites (< 1100 ℃). This is in agreement with Sr isotopic compositions of clinopyroxene from the low-temperature peridotites (~(87)Sr/~(86)Sr_(t) = 0.7038 - 0.7046) that are significantly less radiogenic than those of clinopyroxene from the high-temperature peridotites (~(87)Sr/~(86)Sr_(t) = 0.7052 - 0.7091). The depth correlation of Sr isotopes for clinopyroxene and Sr and Pb isotopic compositions for the Nikos whole-rocks indicate that the deep Somerset lithosphere (> 160 km) is isotopically distinct from the shallow lithospheric mantle. The isotopic stratification with depth suggests that the lower lithosphere is probably younger and may have been added to the existing Archean shallow mantle in a Phanerozoic magmatic event. The radiogenic Sr isotope ratios of the high-temperature peridotites and their clinopyroxenes suggest that the underplated deep lithosphere contained recycled (altered oceanic crust and sedimentary component?) material introduced during earlier subduction.