Compositional constraints on the continental lithospheric mantle from trace elements in spinel peridotite xenoliths

DATA for niobium in mid-ocean-ridge basalts (MORBs) and ocean-island basalts (OIBs) have provided new constraints on global geochemical models of continental growth and mantle differentiation. Until now, there were no such data in the concentration range of parts per 10⁹ (p.p.b.) for peridotite samples. Here we report new measurements of Nb and other trace elements in fertile peridotite xenoliths. Our data show that the Nb/Ta ratio is chondritic, whereas Nb/Th (and presumably Nb/U) ratios are higher than chondritic. High Nb/Th ratios are characteristic of sources and/or residues of divergent-margin (for example, MORB) and intraplate (for example, OIB) magmatism. Compared with primitive mantle, these peridotites show a systematic and uniform depletion of the highly inompatible elements. The depleted patterns are apparently inconsistent with these peridotites being sources or residua from convergent-margin (that is, island-arc) magmatism. These data may indicate that the continental lithospheric mantle does not possess a Nb enrichment to complement the depletion found in the continental crust and island-arc basalts.