Stefanie S. Schmidberger

Nature of the mantle roots beneath the North American craton: mantle xenolith evidence from Somerset Island kimberlites

Abstract The recently discovered Nikos kimberlite on Somerset Island, in the Canadian Arctic, hosts an unusually well preserved suite of mantle xenoliths dominated by garnet–peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, with minor garnet–pyroxenite. The whole rock and mineral data for 54 Nikos xenoliths indicate a highly refractory underlying mantle with high olivine forsterite contents (ave. Fo=92.3) and moderate to high olivine abundances (ave. 80 wt.%). These characteristics are similar to those reported for peridotites from the Archean Kaapvaal and Siberian cratons (ave. olivine Fo=92.5), but are clearly distinct from the trend defined by oceanic peridotites and mantle xenoliths in alkaline basalts and kimberlites from post-Archean continental terranes (ave. olivine Fo=91.0). The Nikos xenoliths yield pressures and temperatures of last equilibration between 20 and 55 kb and 650 and 1300°C, and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive paleogeotherm corresponding to a surface heat flow of 44 mW/m2. Paleogeotherms based on xenolith data from the central Slave province of the Canadian craton require a lower surface heat flow (∼40 mW/m2) indicating a cooler geothermal regime than that beneath the Canadian Arctic. A large number of kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (ave. Kaapvaal 32 wt.%, Siberia 20 wt.%). The calculated modal mineral assemblages for the Nikos peridotites show moderate to low contents of orthopyroxene (ave. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons is not a feature of the cratonic upper mantle beneath Somerset Island.

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

Abstract 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°C; 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 interpretation 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 176Lu/177Hf 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.

Small-scale Sr isotope investigation of clinopyroxenes from peridotite xenoliths by laser ablation MC-ICP-MS-implications for mantle metasomatism

Although numerous studies have shown that mantle xenoliths are isotopically heterogeneous on a hand specimen scale, experimental data imply that isotopic equilibrium should be attained within single mineral grains in the upper mantle. Until recently, this prediction has not been verified because of the difficulty of analyzing individual grains. We report in situ 87 Sr/ 86 Sr compositions of clinopyroxenes in garnet peridotite xenoliths from the Nikos kimberlite, Somerset Island (Arctic Canada) obtained by laser ablation MC-ICP-MS. Results for five different peridotites indicate the existence of large Sr isotope variations within individual xenolith samples, varying from 0.5xto as much as 1.1x(3 to 8 parts in 7000). This study is the first to document isotopic heterogeneity in peridotite xenoliths at the scale of individual grains. Multiple analyses of the same grain, however, indicate intra-grain Sr isotopic equilibrium. The Sr isotopic ratios for individual clinopyroxene grains correlate with major element abundances such as SiO2 and TiO2, and trend towards the composition of the host Nikos kimberlite. It thus appears likely that the Sr isotope heterogeneities recorded by the clinopyroxenes are the result of metasomatic interaction with the host kimberlite magma. The preservation of Sr isotopic variability in clinopyroxene separates from different peridotite samples and between clinopyroxene grains of individual xenoliths suggests that metasomatism occurred just prior to or during kimberlite transport. The Sr isotope heterogeneities documented here for a single peridotite phase suggest that isotopic compositions obtained on mineral separates from peridotite xenoliths likely represent weighted averages and could be less homogenous than previously assumed. D 2003 Elsevier Science B.V. All rights reserved.

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 kimberlite (100 Ma) from Somerset Island to constrain the isotopic character of the mantle root beneath the northern Canadian craton. The Nikos peridotites are enriched in highly incompatible trace elements (La/SmN = 4–6), and show 143Nd/144Nd(t) (0.51249–0.51276) and a large range in 87Sr/86Sr(t) (0.7047–0.7085) and Pb (206Pb/204Pb(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 (>1100°C) that sample the deep Somerset lithosphere trend toward more radiogenic 87Sr/86Sr(t) (0.7085) and unradiogenic 206Pb/204Pb(t) (17.18) isotopic ratios than those of the low-temperature peridotites ( 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.