Trace element composition of mantle end‐members: Implications for recycling of oceanic and upper and lower continental crust

Abstract

Recycling of oceanic crust together with different types of marine sediments has become somewhat of a paradigm for explaining the chemical and isotopic composition of ocean island basalts. New high‐precision trace element data on samples from St. Helena, Gough, and Tristan da Cunha, in addition to recent data from the literature, show that the trace element and isotope systematics in enriched mantle (EM) basalts are more complex than previously thought. EM basalts have some common characteristics (e.g., high Rb/La, Ba/La, Th/U, and Rb/Sr and low Nb/La and U/Pb) that distinguish them from HIMU basalts (high μ = 238U/204Pb). The isotopically distinct EM‐1 and EM‐2 basalts, however, cannot be clearly distinguished on the basis of incompatible trace element ratios. Ultimately, each suite of EM basalts carries its own specific trace element signature that must reflect different source compositions. In contrast, HIMU basalts show remarkably uniform trace element ratios, with a characteristic depletion in incompatible trace elements (Rb, Ba, Th, U, and Pb) and enrichment in Nb and Ta relative to EM basalts. Compositional similarities between HIMU and EM basalts (e.g., Nb/U, La/Sm, La/Th, Sr/Nd, Ba/K, and Rb/K) suggest that their sources share a common precursor, most likely recycled oceanic lithosphere. The compositional differences between HIMU and EM basalts, on the other hand, can only be explained if the EM sources contain an additional heterogeneous component. Parent‐daughter ratios in subducted marine sediments have a unimodal distribution. Recycling of sediments alone can therefore not account for the isotopic bimodality of EM basalts. The upper and lower continental crust have similarly variable trace elements ratios but are systematically distinct in their Rb/Sr, U/Pb, Th/Pb, and Th/U ratios. Thus the upper and lower continental crust evolve along two distinct isotopic evolution paths but retain their complex trace element characteristics, similar to what is observed in EM basalts. We therefore propose that recycling of oceanic lithosphere together with variable proportions of lower and upper continental crust, which are introduced into the mantle together with the oceanic lithosphere via subduction erosion and/or subduction of marine sediments, respectively, provides a plausible explanation for the trace element and isotope systematics in ocean island basalts.