Partitioning of Nb and Ta between rutile and felsic melt and the fractionation of Nb/Ta during partial melting of hydrous metabasalt

Abstract

In order to fully assess the role of rutile in fractionation of Nb/Ta during partial melting of hydrous metabasalt, we have measured rutile – felsic melt partition coefficients ( D values) for Nb and Ta with tonalitic to trondhjemitic compositions at 1.5–3.5 GPa, 900–1350 °C and ∼5.0–20 wt% H 2O. D Nb, D Ta and D Nb/ D Ta range from 17 ± 1 to 246 ± 13, 34 ± 2 to 232 ± 25 and 0.51 ± 0.04 to 1.06 ± 0.13, respectively. For the compositions investigated, melt composition appears to have no observable effect on the partitioning; the effect of pressure is also slight; whereas temperature and H 2O have marked effects. D Nb, D Ta and D Nb/ D Ta increase with decreasing temperature and H 2O content, showing a reversal of D Nb/ D Ta from <1.0 to >1.0. Using the data that approached equilibrium and obeyed Henry’s law, expressions describing the dependences of D Nb, D Ta and D Nb/ D Ta on temperature, pressure and melt H 2O content were obtained: (1) ln ( D Nb ) rutile/melt = - 2.846 ( ± 0.453 ) + 9621 ( ± 470 ) / T + 0.207 ( ± 0.101 ) P - 0.042 ( ± 0.009 ) H 2 O (2) ln ( D Ta ) rutile/melt = - 0.775 ( ± 0.398 ) + 6954 ( ± 413 ) / T + 0.140 ( ± 0.089 ) P - 0.009 ( ± 0.007 ) H 2 O (3) ln ( D Nb / D Ta ) rutile/melt = - 2.075 ( ± 0.289 ) + 2657 ( ± 301 ) / T + 0.075 ( ± 0.065 ) P - 0.033 ( ± 0.006 ) H 2 O where T is in K, P is in GPa, and H 2O are in wt%. These expressions are applicable to calculate Nb and Ta partitioning coefficients between rutile and felsic (highly siliceous) melts and Nb/Ta fractionation during partial melting of metabasalt. The derived models suggest that fluid-present partial melting of metabasalt always leads to D Nb/ D Ta lower than 1.0, whereas dehydration melting may lead to D Nb/ D Ta higher than 1.0 at temperatures <1000 °C and melt H 2O contents <10 wt%. Low-degree melting cannot substantially change the Nb/Ta ratio of rutile-bearing eclogite residues relative to their basalt precursors, whereas high-degree melting leads to lower Nb/Ta ratios in the residues. These results thus confirm that partial melting of hydrous metabasalt cannot produce rutile-bearing eclogites with superchondritic Nb/Ta. Fractionation of Nb/Ta for the melts relative to their basalt precursors can occur during low-degree melting due to the high D Nb and D Ta and variable D Nb/ D Ta. The fractionation direction depends on melting degree (temperature) and protolith initial H 2O content (assuming that hydrous phases are consumed entirely during partial melting), whereas the fractionation extent depends on protolith initial H 2O and TiO 2 contents. Fluid-present melting at <20 wt% melt fractions will significantly elevate melt Nb/Ta ratio, whereas dehydration melting with low initial H 2O (<1.2 wt%) may reduce melt Nb/Ta ratio. In general, rutile in eclogite residues accounts for the highly variable Nb/Ta ratios in the Archean TTG magmas. In particular, the very high Nb/Ta ratios of some TTG magmas may only be explained by low-degree (<20 wt%) melting in the presence of both rutile and a hydrous fluid.