Abstract
The first part of this study deals with the distribution of Nb and Ta at microscopic scale in mantle rocks. Their concentration was measured by EPMA in titanium-oxides to constrain the role of these microphases in the Nb-Ta fractionation.<br /> The second part adresses the distribution of Nb and Ta at the scale of whole Earth. ICP-MS analyses were performed in representative samples to constrain the variations of Nb/Ta, Nb/Th and Ta/La ratios in the main geochemical reservoirs. These data indicate that most of the Silicate Earth is characterized by sub-chondritic Nb/Ta ratios (< 17.5). Only a thin lithospheric layer including the lower continental crust and shallow mantle would be characterized by super-chondritic Nb/Ta ratios ascribed to the evolution of volatile-rich small melt fractions. However, this layer is too thin and too depleted in Nb-Ta to counterbalance the sub-chondritic Nb/Ta ratio of the other reservoirs.<br /> Our data do not support the idea that the subchondritic Nb/Ta ratio of the Silicate Earth would be balanced by a super-chondritic, subduction-related material, recycled and « hidden » in deep-seated mantle. Evidence against this hypothesis include : (1) the observation that the Nb/Ta value of arc volcanism is not significantly lower than the oceanic basalt value, on average, (2) the fact that the low Nb/Ta signature of the continental crust is not inherited from subduction, but merely the reflection of within-crust differenciation processes, and (3) our data on oceanic basalts do not reveal the existence of a high Nb/Ta component in their source.<br /> Finally, whole Earth inversion of Nb, Ta, Th and La data indicates that the sub-chondritic Nb/Ta signature of the Silicate Earth may be accounted for by incomplete segregation of Nb from the core in the early evolutionnary stages of the Earth. The distribution coefficient of Nb between core and primitive mantle would be 0.25 ± 0.08, a value which is well consistent with recent metal/silicate experimental data.