COLUMBITE SOLUBILITY IN GRANITIC MELTS - CONSEQUENCES FOR THE ENRICHMENT AND FRACTIONATION OF NB AND TA IN THE EARTHS CRUST

The behaviour of niobium and tantalum in magmatic processes has been i nvestigated by conducting MnNb2O6 and MnTa2O6 solubility experiments i n nominally dry to water-saturated peralkaline (aluminium saturation i ndex, A.S.I. 0.64) to peraluminous (A.S.I. 1.22) granitic melts at 800 to 1035 degrees C and 800 to 5000 bars. The attainment of equilibrium is demonstrated by the concurrence of the solubility products from di ssolution, crystallization, Mn-doped and Nb- or Ta-doped experiments a t the same pressure and temperature. The solubility products of MnNb2O 6 (K-sp (Nb)) and MnTa2O6 (K-sp (Ta)) at 800 degrees C and 2 kbar both increase dramatically with alkali contents in water--saturated peralk aline melts. They range from 1.2 x 10(-4) and 2.6 x 10(-4) mol(2)/kg(3 ), respectively, in subaluminous melt (A.S.I. 1.02) to 202 x 10(-4) an d 255 x 10(-4) mol(2)/kg(2), respectively, in peralkaline melt (A.S.I. 0.64). This increase from the subaluminous composition can be explain ed by five non-bridging oxygens being required for each excess atom of Nb5+ or Ta5+ that is dissolved into the melt. The K-sp (Nb) and K-sp (Ta) also increase weakly with Al content in peraluminous melts, rangi ng up to 1.7 x 10(-4) and 4.6 x 10(-4) mol(2)/kg(2), respectively, in the A.S.I. 1.22 composition. Columbite-tantalite solubilities in subal uminous and peraluminous melts (A.S.I. 1.02 and 1.22) are strongly tem perature dependent, increasing by a factor of 10 to 20 from 800 to 103 5 degrees C. By contrast columbite-tantalite solubility in the peralka line composition (A.S.I. 0.64) is only weakly temperature dependent, i ncreasing by a factor of less than 3 over the same temperature range. Similarly, K-sp (Nb) and K-sp (Ta) increase by more than two orders of magnitude with the first 3 wt% H2O added to the A.S.I. 1.02 and 1.22 compositions: whereas there is no detectable change in solubility for the A.S.I. 0.64 composition over the same range of water contents. Sol ubilities are only slightly dependent on pressure over the range 800 t o 5000 bars. The data for water-saturated sub- and peraluminous granit es have been extrapolated to 600 degrees C, conditions at which pegmat ites and highly evolved granites may crystallize. Using a melt concent ration of 0.05 wt% MnO, 70 to 100 ppm Nb or 500 to 1400 ppm Ta are req uired for manganocolumbite and manganotantalite saturation, respective ly. The solubility data are also used to model the fractionation of Nb and Ta between rutile and silicate melts. Predicted rutile/melt parti tion coefficients increase by about two orders of magnitude from peral kaline to peraluminous granitic compositions. It is demonstrated that the gamma Nb2O5/gamma Ta2O5 activity coefficient ratio in the melt pha se depends on melt composition. This ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compo sitions. Accordingly, all the relevant accessory phases in subaluminou s to peraluminous granites are predicted to incorporate Nb preferentia lly over Ta. This explains the enrichment of Ta over Nb observed in hi ghly fractionated granitic rocks, and in the continental crust in gene ral.