Formation of enriched mantle components by recycling of upper and lower continental crust

Abstract

The origin of enriched mantle (EM) sources remains an unsolved problem for constraining the composition and chemical evolution of the Earth's mantle, because a wealth of different, often mutually exclusive models has been suggested. To address this predicament and to re-investigate the origin of EM sources on a global scale, this study is based on combined chemical and isotopic literature data for more than 530 samples from 16 key locations from worldwide ocean islands. The combined Sr, Nd, Pb isotope and trace element systematics of global ocean island basalts suggest that each EM source contains a unique enriched additive. Systematic variations between Th/Nb, K/La, Rb/La, and Ce/Pb ratios and 87Sr/ 86Sr ratios in all EM basalts suggest that all EM-type end-members share a common heritage from the continental crust. The observed coupling of relative Eu enrichments or deficits with 87Sr/ 86Sr isotope ratios further indicates that the inferred compositional differences of EM-type sources are caused by the addition of different proportions of lower and upper continental crust. Recycling of marine sediment and oceanic lithosphere in subduction zones accounts for the isotopic and chemical composition of EM sources with high 87Sr/ 86Sr and relatively constant 206Pb/ 204Pb ratios (e.g. Samoa), which have a high affinity for the upper continental crust. Sources with a coupled 87Sr/ 86Sr– 206Pb/ 204Pb isotope evolution that extend to low 206Pb/ 204Pb but less radiogenic 87Sr/ 86Sr ratios (e.g. Pitcairn) are dominated by lower continental crust. Transfer of the lower continental crust into the mantle can occur either by subduction erosion or by crustal delamination. Here we propose that one common process, the recycling of upper and lower continental crust and oceanic lithosphere at destructive plate margins and their subsequent re-melting as part of the mantle sources of ocean island basalts, can account for the entire range of chemical and isotopic signatures in EM-type oceanic basalts. This implies that the compositional heterogeneity in the Earth's mantle is induced by, and intrinsic to the recycling process and not principally dependent on intra-mantle stirring of a limited number of originally distinct and physically separate mantle reservoirs.