Geochemical and isotopic systematics in carbonatites and implications for the evolution of ocean-island sources

Abstract

Geochemical and Sr, Nd, Pb, O and C isotopic data are reported for 13 carbonatites from Africa, Australia, Brazil, Europe and the United States. The carbonatites possess generally high Ba, Th, LREE, Sr and low Cs, Rb, K and HREE abundances. Some examples have low Ti, Nb, Ta, P, Zr, Hf and U concentrations which are consistent with variable fractionation of sphene, apatite, perovskite, monazite or zircon. The samples range in age from Proterozoic to Tertiary and possess a range of initial Sr isotopic compositions between 0.7020 and 0.7054, initial ϵ Nd values of −0.4 to +3.8 and (with the exception of the Brazilian Jacupiranga carbonatite) generally radiogenic initial Pb isotopic compositions. δ 18O SMOW compositions of the intrusive carbonatites range from +5.5 to +12.4‰ Higher δ 18O SMOW values of +14 and +17%. are found in the volcanically-emplaced Proterozoic Goudini complex of South Africa, suggesting the involvement of secondary alteration processes. δ 13C PDB ranges from −0.5 to −6.6‰ with samples having near-primary δ 18O SMOW (between +5.5 and +8%.) possessing δ 13C PDB between −2.9 to −6.6‰. On the initial Sr-Nd isotope diagram, most carbonatites plot below the mantle array and below or within the field of many ocean-island basalts. The Pb isotopic compositions of carbonatites generally lie along the array defined by oceanic basalts. The characteristics of carbonatites from a number of continents and their isotopic similarity to some ocean-island basalts favour an asthenospheric mantle “plume” origin. This conclusion suggests that some ocean-island alkali basalts may have been derived from trace-element-depleted mantle sources which have been re-fertilised by low-viscosity, trace-element-rich carbonatitic melts. The common close spatial and temporal association and the overlap in trace-element geochemistry and isotopic characteristics of Group 1 (basaltic) kimberlites and carbonatites argues strongly for a genetic relationship. Although late-stage melt/vapour fractionation may play some role, the extreme LREE-enrichment typical of carbonatites requires their derivation by small degrees of melting (< ≈ 1%) from a garnet-rich eclogitic source. This source may originally have been CO 2- and volatile-rich subducted oceanic lithosphere.