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

摘要

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 1.5-3.5GPa, 900-1350°C and ～5.0-20wt% 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_2O (2)ln(D_(Ta))rutile/melt=-0.775(±0.398)+6954(±413)/T+0.140(±0.089)P-0.009(±0.007)H_2O (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_2O 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 <10wt%. 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 <20wt% melt fractions will significantly elevate melt Nb/Ta ratio, whereas dehydration melting with low initial H_2O (<1.2wt%) 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 (<20wt%) melting in the presence of both rutile and a hydrous fluid.