Earth processes cause Zr–Hf and Nb–Ta fractionations, but why and how?

Abstract

Zr–Hf and Nb–Ta are two elemental twins, each of which has the same valence (i.e. 5⁺ for Nb and Ta, and 4⁺ for Zr and Hf) and same ionic size at a given coordination number (e.g. R_Nb/R_Ta = 1.000 and R_Zr/R_Hf = 1.006 to ∼1.026 for coordination numbers of 6, 7, 8 and 12). As a result, it has been the view that Zr does not fractionate from Hf and Nb does not fractionate from Ta in the formation of minerals and in all Earth processes. Recent studies, however, have shown that this traditional view is in error. Up to 2 orders of magnitude fractionations have been observed in rocks from the world ocean floor and also in the seawater on various scales. I discuss some possible processes that may help explain such large fractionations, but further work is needed to test the validity of these interpretations. Perspectives from the chemistry community are in compelling need to help address this basic problem of scientific significance in understanding the chemical differentiation of our planet.