Structural constraints to Nb and Ta incorporation in minerals

Nb and Ta are geochemically important trace elements whose crystal-chemical behaviour has not yet fully understood. Albeit these HFS elements have the same oxidation state and very similar ionic radii, values of the Nb/Ta ratio measured in crustal and mantle rocks are highly variable. Subchondritic Nb/Ta values are found in continental crust and astenospheric MORB-type mantle, and the presence and location in the silicate Earth of reservoirs with superchondritic Nb/Ta values is under debate. It is therefore important to know which mineral phases may control the Nb and Ta budget, and which are the mechanisms for their incorporation in the various minerals. New partitioning data and present crystal-chemical knowledge are combined to discuss the mechanisms ruling Nb and Ta incorporation in some silicates (amphibole, clinopyroxene, mica, titanite) and Ti-rich oxides (perovskite, Ti-spinel). A crystal-chemical prerequisite for Nb and Ta compatibility is the availability of one (or more) site suitable to host Ti and of a one (or more) exchange vector allowing local neutralisation of the extra positive charge provided by Nb and Ta with respect to Ti. However, sites in which Ti is the dominant cation do not allow S/LDNb/S/LDTa>1, because Ta is smaller than Nb and therefore is more similar to the ideal site dimension. On this ground, a site hosting Ti and larger cations is needed to have S/LDNb/S/LDTa>1. Both perovskite and titanite have a site in which Ti is dominant, and also a suitable charge balance mechanism. Accordingly, Nb and Ta are highly compatible in these minerals, and measured S/LDTa/S/LDNb values are by far >1. Also, incorporation of the smaller Al3+ in titanite further increases measured S/LDTa/S/LDNb values. In Ti-spinel, Nb and Ta are less abundant due to the difficulties to balance the extra charge, which is limited to the presence of divalent trace elements. Ta is still compatible, whereas S/LDNb is generally <1. In calcic clinopyroxenes, both Nb and Ta are highly incompatible. This feature may be explained by the low [6]Ti contents; the S/LDNb/S/LDTa values are significantly higher than in the above Ti-rich minerals in agreement with measured <M1-O> longer than expected for <Ti-O>. In amphiboles (especially in calcic amphiboles from mantle environments) the frequent need for high charged cations at the M1 site balancing for (partial) dehydrogenation allows Nb and Ta to behave as compatible elements. The S/LDNb/S/LDTa value is controlled by the dimensions of the M1 site, so that decreasing Mg# values (i.e., larger <M1-O>) allow this ratio to approach and even to exceed 1. The same behaviour is observed in micas, which however provide similar mechanisms for Ti incorporation.