Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

Abstract

Peridotites that sample Archean mantle roots are frequently in- INTRODUCTION compatible trace element enriched despite their refractory major The mantle underlying many Archean cratons has anelement compositions. To constrain the trace element budget of the omalously high seismic velocities to depths of 350–400 km lithosphere beneath the Canadian craton, trace element and rare ( Jordan, 1988; Grand, 1994), indicating the presence of earth element (REE) abundances were determined for a suite of cold refractory roots, depleted in the fusible major elegarnet peridotites and garnet pyroxenites from the Nikos kimberlite ments compared with fertile mantle (Boyd & Mertzman, pipe on Somerset Island, Canadian Arctic, their constituent garnet 1987; McDonough, 1990). These deep residual peridotite and clinopyroxene, and the host kimberlite. These refractory mantle roots probably contribute to the stability of Archean xenoliths are depleted in fusible major elements, but enriched in continental lithosphere because of their lower density and incompatible trace elements, such as large ion lithophile elements higher viscosity compared with that of the surrounding (LILE), Th, U and light rare earth elements (LREE). Mass asthenospheric mantle (Boyd & McCallister, 1976; Jorbalance calculations based on modal abundances of clinopyroxene dan, 1979; Pollack, 1986). Mantle xenoliths that are and garnet and their respective REE contents yield discrepancies hosted by kimberlites and alkaline basalts are our only between calculated and analyzed REE contents for the Nikos bulk window into the subcontinental lithosphere. They provide rocks that amount to LREE deficiencies of 70‐99%, suggesting essential evidence on the chemical composition and evoluthe presence of small amounts of interstitial kimberlite liquid tion of the upper mantle to depths of >200 km. Studies (0·4‐2 wt %) to account for the excess LREE abundances. These of these mantle xenoliths enable us to characterize the results indicate that the peridotites had in fact depleted or flat abundance and distribution of major, minor and trace LREE patterns before contamination by their host kimberlite. LREE elements in peridotites and between their constituent and Sr enrichment in clinopyroxene and low Zr and Sr abundances minerals. Most subcratonic peridotite samples have in garnet in low-temperature peridotites (800‐1100°C) compared undergone a complex history of melt extraction that has with high-temperature peridotites (1200‐1400°C) suggest that the changed their chemical composition and resulted in shallow lithosphere is geochemically distinct from the deep lithosphere depletion of the residual mantle in fusible major elements beneath the northern margin of the Canadian craton. The Somerset such as Fe, Al and Ca (e.g. Nixon, 1987; Herzberg, 1993; mantle root appears to be characterized by a depth zonation that Boyd et al., 1997). In contrast to their depletion in may date from the time of its stabilization in the Archean. incompatible major elements, however, many peridotite