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

Abstract

[1] 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 m = 238 U/ 204 Pb). 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.

Mantle convection and evolution with growing continents

“…Parman [2007] shows that the MORB contribution to CC generation dominates in the Phanerozoic while the OIB contribution prevails in the main preceding part of the mantle's evolution. This is the background for why we based our convection -fractionation model, particularly the geochemical part, on the more recently developed reservoir models by Hofmann [2003], Stracke et al [2005], and Willbold and Stracke [2006] (cf. section 1.1).…”

“…For example, Stracke et al [2005] and Willbold and Stracke [2006] propose a new FOZO similar to the traditional FOZO except that this new FOZO is a small-scale component, ubiquitously dispersed throughout the entire mantle. The exact definition of FOZO can be found in Hart et al [1992].…”

Nd, Pb, and Sr isotope composition of Late Mesozoic to Quaternary intra-plate magmatism in NE-Africa (Sudan, Egypt): high-μ signatures from the mantle lithosphere

“…rocks from enriched mantel sources ( Fig. 7; eFig.5 a-l; e.g., Hofmann et al 1986;Weaver, 1991;Willbold and Stracke 2006). Nb/U ratios of the samples with radiogenic 206 Pb/ 204 Pb are systematically displaced from DM and PUM values to higher values known from some OIB and show no relation to the low crustal values of average crust (eFig.5 m-o) and local basement.…”

“…The variable canonical element ratios plot mainly in the broad range of DM-PUM-OIB compositions and could reflect source variability of the enriched sources as known for OIB (e.g. Willbold and Stracke 2006). A systematic and substantial contamination by reworked cratonic crust is not detected.…”

The geochemistry and petrogenesis of the Paleoproterozoic du Chef dyke swarm, Québec, Canada

“…By using these degrees of partial melting and olivine fractionation, we can investigate the possible mantle sources for the du Chef dykes. basalts from the Tristan da Cuhna, Gough, Samoan and Society islands (Willbold and Stracke 2006). The HIMU reservoir is similarly estimated from inverse modelling using the compositions of basalts from Tubuai, Mangaia and Rurutu (Chauvel et al 1992).…”