The metasomatic alternative for ocean island basalt chemical heterogeneity

Abstract

Subduction of oceanic lithosphere is thought to be responsible for producing heterogeneity in Earth's mantle; the heterogeneity is most clearly preserved in the compositions of ocean island basalts (OIB). The variation of trace element and isotopic ratios in OIB is commonly explained by recycling of ancient oceanic crust associated with terrigenous or pelagic sediment. However a variety of chemical and physical arguments seem to indicate that subducted oceanic crust is not the source of OIB. In particular, experimental petrologic studies indicate that the most plausible source for OIB is the partial melting of peridotite in the presence of CO 2 or silica-deficient pyroxenites. Alternative hypotheses for the source of OIB are subducted oceanic basal lithosphere enriched by metasomatic liquids and delaminated metasomatised continental lithosphere. Lithospheric metasomatism is common to both hypotheses and could produce silica-deficient pyroxenite; however, the exact chemical and physical nature of the process and how it leads to chemical variations in recycled lherzolite that produce the various OIB isotopic end-member “signatures” is unclear. Chemical variations observed in the Cantal basalt (France) are interpreted as the result of a lithospheric metasomatic mechanism and provide important new constraints on the nature of this metasomatic process. The Cantal basalts, similar to OIB in composition, show unusual variations in Nb / Th, Nb / U, La / Nb and Ce / Pb ratios from the first (13–9 Ma) to the last (9–3 Ma) emitted basalt. The basalts are homogeneous with respect to their Sr, Nd, and Pb isotopic composition, ruling out variable sediment contamination of their mantle sources. We postulate that these trace element variations result from an evolution of metasomatic vein compositions present in a vein plus enclosing lithospheric mantle source. Cantal basalt compositions may be used to constrain the trace element variation generated by the evolution of a metasomatic agent within the lithosphere. Reconstruction of compositions in different parts of the lithosphere indicates significant heterogeneities in Th / Pb, U / Pb and Sm / Nd resulting from the metasomatic process. Subduction and isolation of this metasomatic lithosphere generates over 1–1.8 Ga, Nd and Pb isotopic variations similar to those observed in oceanic island basalts. Moreover, these isotopic variations are correlated with variations in trace element ratios commonly interpreted as the product of mixing between different end-member mantle components (HIMU and EM). The model calculations suggest that isotopic and trace element variations observed in OIB may more likely be the result of melting metasomatised subducted oceanic lithosphere rather than a mixture of chemically distinct mantle sources that formed and evolved independently from one another.