Natural and experimental constraints on formation of the continental crust based on niobium-tantalum fractionation

摘要

Fractionation between Nb and Ta, elements generally regarded as geochemical 'identical twins', is a key to deciphering the formation of the continental crust (CC). Here we show that Nb/Ta of rutile grains in eclogitic rocks from the Chinese Continental Scientific Drilling (CCSD) project are remarkably heterogeneous but overall subchondritic at core depths of 100-700 m, and are less variable and mainly suprachondritic at core depths of 700-3025 m, indicating clear Nb/Ta fractionation across a subducted slab. To understand the potential mechanism of Nb/Ta fractionation within the subducted plate, we analysed by laser ablation ICPMS a thermal migration experiment in which a wet andesite was placed in a large thermal gradient (300 deg C/cm with ends ranging from 950-350 deg C) at 0.5Gpa. Results show that Nb, Ta and Ti, driven by the thermal gradient, preferentially migrate by diffusion through supercritical fluids into the cooler end of the experiment (at approx 650-350 deg C). Due to contrasting Nb and Ta thermal migration patterns, dramatic fractionation between Nb, Ta, and Ti took place in the cooler end. Experimental results are consistent with the measured Nb, Ta in rutile from CCSD drillhole samples. We consider that major fractionation between Nb, Ta must occur before rutile appears, most likely during the prograde blueschist to amphibole-eclogite transformation, when Ti is also mobile. Before rutile appears, partitioning between Ti-rich dominant minerals such as amphiboles and fluids in the hotter region where dehydration preferentially occurs, produces Nb-Ta-Ti-rich fluids with subchondritic Nb/Ta, and dehydration residues with suprachondritic Nb/Ta. Meanwhile, owing to evolution of the thermal gradient within the subducting slab, thermal migration of Nb, Ta, and Ti in aqueous fluids result in Nb, Ta, and Ti enrichment in the cooler region and depletion in the hotter region. As a result of high-pressure metamorphism, hydrous rutile-rich eclogites with overall subchondritic Nb/Ta form in the cooler region, whereas relatively anhydrous rutile-poor eclogites with suprachondritic Nb/Ta form in the hotter region. Subsequently, partial melting of hydrous rutile-rich eclogites with initial subchondritic Nb/Ta at deeper levels transfers overall subchondritic Nb/Ta coupled with Nb, Ta, and Ti depletion characteristics to the CC, leaving dry rutile-poor eclogites with suprachondritic Nb/Ta and rutile-rich residual eclogites with overall, heterogeneous subchondritic Nb/Ta as a complementary reservoir to the CC.