Ta/Nb Fractionation in the Earth's Crust: Evidence from Columbite Solubility Experiments

R. L. Linnen Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany


H. Keppler Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany

The solubilities of fully ordered synthetic MnTa2O6 and MnNb2O6 in water saturated haplogranitic melts have been determined at 800° to 1035°C, 800 to 4000 bars for Al/[2Mn+Na+K] (A.S.I.) compositions of 0.4 to 1.2. Three types of experiments were conducted: simple (equilibrium) dissolution of crystals into melt, dissolution of crystals into melt doped with MnO, Ta2O5 or Nb2O5 (to determine whether or not the solubility products and hence the activity coefficients are constant), and reversal (crystallization) experiments.

At 800°C and 2 kbar columbite solubility increases dramatically with increasing peralkalinity and weakly with increasing Al content in peraluminous compositions. Greater than 3 wt% Nb2O5 and >5 wt% Ta2O5 are required for columbite saturation in strongly peralkaline granitic melts (A.S.I. 0.55), whereas >1000 ppm Nb2O5 and >5000 ppm Ta2O5 required for saturation in subaluminous to peraluminous granitic melts. Consistency between the dissolution, Mn-, Ta-, and Nb-doped and the crystallization experiments indicate not only that equilibrium was attained but that the activity coefficients of Nb and Ta exhibit Henry's law behaviour at concentrations up to ~5 wt%. If the excess number of moles of Ta or Nb is plotted against the excess of Na+K, relative to the subaluminous composition, there is a strong correlation, with one mole of excess Ta or Nb corresponding to two moles of excess Na+K. This implies that two non-bridging oxygens are required for Ta or Nb to dissolve into the melt, and in this respect Ta and Nb behave similar to other HFSE such as Zr. The solubility of Ta is evidently higher than that of Nb when expressed as a wt%. However, the solubility products of MnTa2O6 (Ksp-Ta) and MnNb2O6 (Ksp-Nb) are roughly equal in peralkaline melts (250x10-4 and 200x10-4 mol2/kg2 ,respectively, for A.S.I. 0.55). By contrast, Ksp-Ta (3x10-4 mol2/kg2) is two times higher than that of Ksp-Nb (1.4x10-4 mol2/kg2) in subaluminous
(A.S.I. 1.0) melts and similar values are obtained for peraluminous (A.S.I. 1.2) melts, 4x10-4 and 1.6x10-4 mol2/kg2, respectively. In the latter melts the activity coefficient of Ta2O5 is thus lower than that of Nb2O5, and with crystal fractionation the Ta/Nb ratio of the residual melt should increase. This is consistent with observed increase of Ta/Nb in natural peraluminous suites and the fractionation of Ta/Nb in the continental crust. The similar activity coefficients in peralkaline melts may also explain the lack of Nb depletion (relative to Ta) generally observed in peralkaline granites and oceanic crust.

Columbite solubility is strongly dependent on temperature in subaluminous to peraluminous melts, e.g. for the water-saturated A.S.I. 1.0 composition at 1035°C and 2 kbar, Ksp-Ta increasing to 35x10-4 mol2/kg2 and Ksp-Nb to 26x10-4 mol2/kg2. At these conditions Ksp-Ta is only ~1.3 times higher than Ksp-Nb, thus decreasing temperature also promotes Ta/Nb fractionation in subaluminous and peraluminous suites. If the data for the subaluminous composition are extrapolated to the crystallization conditions of pegmatites or highly evolved granite (600°C, 2 kbar), where columbite occurs as a magmatic mineral, and assuming a melt with 0.05 wt% MnO, 1400 ppm Ta or 330 ppm Nb are required for columibite saturation. Although these values are high, natural columibite invariably contains Fe, and Fe concentrations in melts are >>0.05 wt%. The extrapolation of the solubility data to 600°C thus yields geologically reasonable results. By contrast to solubility in peralkaline melts is only weakly temperature dependent , for the A.S.I. 0.55 composition Ksp-Ta increases to 460x10-4 mol2/kg2 and Ksp-Nb to 500x10-4 mol2/kg2. Several wt% Ta or Nb are required for Mn-columbite saturation in peralkaline melts
at 600°C, but this is consistent with the lack of primary columbite in peralkaline rocks. Finally, the effect of pressure has been determined for Ksp-Nb at H2O-saturated conditions. In the A.S.I. 1.0 composition Ksp-Nb increases from 0.7 x10-4 mol2/kg2 at 800 bars to 2.2 x10-4 mol2/kg2 at 4000 bars. The opposite trend is observed in the A.S.I. 0.6 composition, with Ksp-Nb decreasing from 245 x10-4 mol2/kg2 at 800 bars to 90 x10-4 mol2/kg2 at 4000 bars.