A possible high Nb/Ta reservoir in the continental lithospheric mantle and consequences on the global Nb budget - Evidence from continental basalts from Central Germany

Compared to chondrites the accessible silicate reservoirs on Earth (i.e., mantle and continental crust) are depleted in Nb as expressed in their relatively low Nb/Ta. Although it was postulated that the missing Nb may be stored within a hidden reservoir in the mantle or within the Earth's core, the role of the subcontinental lithospheric mantle in balancing the global Nb budget remains unclear. Continental basalts are pooled melts that tap the compositional spectrum of the subcontinental lithospheric mantle, and alkaline basalts from Central Germany are typical representatives of such melts. Here we present high-precision concentration data of Zr, Hf, Nb, Ta, and Lu determined by isotope-dilution MC-ICPMS along with Hf isotope compositions in a variety of intracontinental volcanic rocks from different locations in Central Germany. These rocks display Nb/Ta ratios (15.0-19.1) that still lie below the chondritic value (19.9 +/- 0.6) but are distinctly higher than in ocean-island basalts (15-16) that share similarly enriched mantle sources. They are also higher than Nb/Ta in the continental crust (12-13) and in the bulk-silicate Earth (BSE: similar to 14), and therefore imply that the subcontinental lithospheric mantle also has high Nb/Ta and could potentially balance the Nb deficit observed in most terrestrial silicate reservoirs. Trace element modelling indicates that the HFSE composition of the continental basalts cannot be explained by simple melting of asthenospheric garnet or spinel peridotite sources, but requires the presence of metasomatised mantle domains that have been re-enriched by low-degree melts. Positively correlated Nb/Ta and Lu/Hf along with low Zr/Nb and Zr/Sm provide strong evidence that these low-degree melts may have a carbonatitic affinity and that the slight Nb-excess observed in continental basalts results primarily from carbonatite assimilation within the subcontinental lithospheric mantle. This is consistent with the evidence for carbonatite metasomatism found in xenoliths from Central Germany. Hafnium isotope modelling indicates that carbonatite metasomatism occurred long before the onset of Cenozoic magmatism (>100 Ma). Our results suggest that the lithospheric mantle may host some of the missing Nb, but high-Nb/Ta domains are likely restricted to regions that have been affected by carbonatite metasomatism. Although Nb concentrations can be extremely high in carbonatites, such domains are probably a more local phenomenon and volumetrically too small to account as a whole for the global Nb deficit. Model calculations, however, indicate, that up to similar to 30% of the missing Nb may be hosted in the subcontinental lithospheric mantle. The Earth's core or any other hidden reservoir in the deep mantle remains significant in balancing the global Nb budget, but their role may be less important than previously thought. (C) 2011 Elsevier Ltd. All rights reserved.