Nb/Ta and Zr/Hf in ocean island basalts — Implications for crust–mantle differentiation and the fate of Niobium

Abstract

Variations of high-field strength element (HFSE) ratios in terrestrial reservoirs, in particular Zr/Hf and Nb/Ta, are critical for understanding crust–mantle differentiation. Growing experimental and observational evidence shows that these ratios are fractionated during magmatic processes, despite their very similar geochemical characteristics. Here we present new high-precision Nb, Ta, Zr, Hf and Lu measurements for a variety of ocean island basalts determined by isotope dilution MC-ICPMS together with Hf isotope compositions in order to constrain OIB source characteristics and HFSE fractionation during mantle melting and crystal fractionation. Observed variations in Zr/Hf are larger than expected from fractional crystallisation alone. Partial melting of garnet and/or spinel peridotite assemblages can produce the observed range in Zr/Hf and Nb/Ta ratios, but require the presence of grossular-rich garnet, i.e. of recycled eclogite or garnet pyroxenite in the source of OIBs. This is consistent with Lu/Hf ratios that are lower in OIBs than expected from partial melting of pure garnet peridotite sources.Nb/Ta ratios in terrestrial reservoirs can be used to place constraints on crust–mantle differentiation and mantle evolution since the Archean. The average Nb/Ta in the OIB source region (15.9±0.6 (1σ)) is identical to values observed in many MORB suites, but higher than the ratio of the bulk silicate Earth (∼14) and the estimate for the continental crust (∼12–13). Despite the inferred presence of recycled eclogite in OIB sources, which had previously been postulated to be a potential reservoir with superchondritic Nb/Ta ratios, their Nb/Ta ratios are invariably subchondritic and therefore provide no evidence for the existence of a silicate reservoir with superchondritic Nb/Ta in the Earth's mantle, and also exclude significant contributions from core material with superchondritic Nb/Ta ratios. The complementary Nb/Ta ratios in the Earth's crust and mantle with respect to bulk silicate Earth can be explained by partial melting of amphibolite bearing slabs with bulk DNb/Ta>1 during crust–mantle differentiation. As melting of subducted amphibolites was probably most intense during the Archean, major portions of the continental crust may have formed early in Earth's history. Such a model is consistent with Nb/Ta ratios in Archean rocks and with 142Nd and 176Hf/177Hf evidence for early Earth differentiation.