Probing Archean lithosphere using the Lu-Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada

摘要

A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth's mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites (< 1100 ℃), which represent the shallow Somerset lithosphere, are characterized by higher ~(176)Lu/~(177)Hf ratios (0.03-0.05) and Hf isotopic values (~(176)Hf/~(177)Hf_((0.1 Ga)) = 0.28296-0.28419) than the deep-seated high-temperature peridotites (> 1100 ℃; 0.004-0.03, 0.28265-0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu-Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpetation that the shallow Somerset lithosphere (to depths of ～150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic ~(176)Lu/~(177)Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths > 80 km). The Hf isotope compositions for the Somerset low-temperature peridotites indicate that part of the mantle root beneath the North American craton is characterized by a more radiogenic Hf isotope signature than that estimated for a typical 'depleted' mantle.