Constraints on the composition of the continental lithospheric mantle

Abstract

Major and trace element data for 375 spinel lherzolite and harzburgite xenoliths from continental basalts are used to constrain the nature and composition of the continental lithospheric mantle (CLM). The major and compatible trace elements (e.g., Ni, Co) have similar average and median values, whereas median values of the highly incompatible trace elements (e.g., LREE, Rb) are systematically lower than their average values. Both the average and median compositions have LREE-enriched patterns, with La abundances at about 10 and 3 times C1 chondrites, respectively, and a relatively flat HREE pattern at about 2 times C1 chondrites. Relative to the primitive mantle, highly incompatible trace elements are enriched in the average and median, whereas the mildly incompatible elements (e.g., the middle and heavy REE, Hf, Ti, V, Sc, Al, Ca, etc.) are depleted. The enrichment pattern of incompatible elements in these peridotites is comparable to that in ocean island basalts, although at lower absolute abundances. For most elements the average composition provides an estimate of the bulk composition of the CLM, however for those elements with a strong degree of skewedness in their data the median composition may provide a more reasonable estimate. A comparison of geochemical data for spinel and garnet peridotites reveals many similarities, however, garnet peridotites have, on average, lower concentrations of elements with bulk distribution coefficients close to 1.0 (eg., HREE, Sc, Ca, Mn, Fe, Cr). These differences may be attributed to regional variation or age of the lithosphere sampled. It is proposed that the values reported here may be best viewed as a compositional estimate of the post-Archaean CLM. The composition of the CLM is consistent with a growth and stabilization model involving the underplating of refractory peridotite diapirs produced during magmatism, and to a lesser extent by advective thickening of the lithosphere. The incompatible element enriched character of the CLM has been developed in intraplate and/or divergent-margin tectonic environments. The absence of a convergent-margin chemical signature in peridotite xenoliths, as characterized by a Nb depletion or enrichment, argues against incompatible element-enrichment of CLM in this environment. The incompatible element-enriched character of the CLM does not possess the necessary source composition for continental flood basalts, therefore alternative crustal and/or mantle sources need to be considered for these large volume incompatible element-enriched melts. Although the CLM has an incompatible element-enriched character, it does not appear to represent a significant geochemical reservoir for these elements. Based on the median composition, the CLM has less than or equal to 5% of the highly incompatible elements relative to the primitive mantle, and this is about a factor of 5 less than that in the continental crust for such elements.