Nb–Ta fractionation by partial melting at the titanite–rutile transition


During the evolution of the Earth, distinct geochemical reservoirs with different Nb/Ta ratios have developed. Archean granitoids of the tonalite–trondhjemite–granodiorite (TTG) suite, which represent the Earth’s early continental crust, show larger Nb/Ta variations than any other Earth reservoir. This implies that significant Nb–Ta fractionation must have occurred during early crust formation, while the underlying mechanism behind is still unclear. Here, we present a new model on how Nb may be fractionated from Ta during partial melting of subducted oceanic crust. Our data show that Nb/Ta ratios in melts derived from rutile- and titanite-bearing eclogite are largely controlled by the modal relative abundances of rutile and titanite in the source. High modal ratios of titanite over rutile generate melts with very high Nb/Ta (>60), whereas low modal titanite/rutile produces melts with much lower Nb/Ta (≤30). Very low Nb/Ta (<16) occur when all Ti-phases are consumed at very high degrees of melting. As the modal ratio of titanite to rutile is a function of pressure, the Nb/Ta of melts is a function of melting depth. Our new model helps to explain the extreme variation of Nb/Ta observed in many TTGs and thus how Nb and Ta were fractionated during the early evolution of the Earth. Furthermore, the model also indicates that simple one-stage melting models for mafic crust are not sufficient to explain the formation of TTGs.

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This study was partly funded by the DFG (KL 692/17-2). H. Brätz is thanked for help with the LA-ICP-MS analyses. We would also like to acknowledge the discussions at the Institut für Mineralogie at the Universität Münster, in particular with K. Mezger, and thank C. Münker for his support. G. Franz and M. Barth are thanked for their helpful reviews, which clearly helped to improve our contribution, and our thanks also go to J. Hoefs for his swift editorial handling of the manuscript.

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Correspondence to Timm John.

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Communicated by J. Hoefs.

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John, T., Klemd, R., Klemme, S. et al. Nb–Ta fractionation by partial melting at the titanite–rutile transition. Contrib Mineral Petrol 161, 35–45 (2011) doi:10.1007/s00410-010-0520-4

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  • Crustal evolution
  • Nb–Ta
  • Partial melting
  • Rutile
  • Titanite
  • Subduction zone
  • Eclogite