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The effect of disequilibrium crystallization on Nb-Ta fractionation in pegmatites: Constraints from crystallization experiments of tantalite-tapiolite

  • Marieke Van Lichtervelde EMAIL logo , Francois Holtz and Frank Melcher
Published/Copyright: August 28, 2018
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American Mineralogist
From the journal American Mineralogist Volume 103 Issue 9

Abstract

Tapiolite [FeTa2O6] and columbite-group minerals [(Fe,Mn)(Ta,Nb)2O6] are common Nb-Ta-bearing accessory minerals in rare-element granites and pegmatites. Their compositional gap has inspired several experimental studies, but none of them have succeeded in reproducing the parameters that influence the compositional gap. In this study, tapiolite and columbite-group minerals (CGM) were crystallized from water-saturated, flux-rich granitic melts at various conditions of pressure, temperature, oxygen fugacity, and Ti contents. Crystals with a size as small as 500 nm were analyzed with a field emission gun (FEG) electron microprobe. The results show that temperature, pressure, and Ti content only slightly affect the compositional gaps between tapiolite and CGM, whereas high fO2 leads to complete solid solution between a rutile-structured component Fe3+TaO4 and (Fe,Mn)Ta2O6. The experimental CGM-tapiolite compositional gaps are compared with natural CGM-tapiolite pairs from rare-element granites and pegmatites worldwide. This study reveals that the crystallographic structure of tapiolite and CGM could be the dominant parameter that influences the position of the compositional gap. Order-disorder in CGM and tapiolite is tightly linked to disequilibrium crystallization triggered by supersaturation. Significant isothermal Nb-Ta fractionation is observed inside CGM crystals that grow at high degrees of supersaturation. The effect of supersaturation prevails over the solubility effect that is known to increase the Ta/(Ta+Nb) ratio in CGM and coexisting melts. Thus, even if global equilibrium in terms of the solubility of Nb-Ta-bearing minerals is attained, the Ta/(Nb+Ta) ratio in the crystals may differ significantly from equilibrium. It implies that Nb-Ta fractionation in Nb-Ta oxides is controlled by crystallization kinetics rather than equilibrium chemical fractionation (or any other processes such as F-complexing of Ta or fluid exsolution) in dynamic systems that can rapidly reach supersaturated conditions. These results have important implications for the understanding of crystallization processes in highly evolved and pegmatite-forming magmas.

Keywords: Crystallization experiments; disequilibrium crystallization; Nb/Ta fractionation; pegmatites; tapiolite; From Magmas to Ore Deposits

Acknowledgments

This work benefited from the contribution of D. Guillaume, A. Wegorzewski, and A. Stechern in running the experiments, and S. Gouy and P. de Parseval in developing FEG-EPMA procedures. We are also grateful to S. Goldmann for helping with the BGR database, J. Lodziak for EPMA analyses of natural CGM, and C. Josse, T. Hungria, and L. Datas for FIB and TEM techniques. Constructive comments by D. London, J.M. Hanchar, M. Wise, and A. Stepanov greatly enhanced the quality of the manuscript. This work was supported by the German Research Foundation (DFG project Ho 1337/20) and the French National Research Agency (ANR project VARPEG).

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Appendix

Apendix Figure 1 Theoretical Gibbs free energy changes (ΔfG°) for the crystallization reactions of CGM end-members, calculated from the ΔfG° of pure oxide constituents (for example, ΔfG° of FeTa2O6 is the sum of the ΔfG° of FeO and Ta2O5). Thermodynamic data from Hong and Kim (2001), Jacob et al. (2010), and Robie and Hemingway (1995).
Apendix Figure 1

Theoretical Gibbs free energy changes (ΔfG°) for the crystallization reactions of CGM end-members, calculated from the ΔfG° of pure oxide constituents (for example, ΔfG° of FeTa2O6 is the sum of the ΔfG° of FeO and Ta2O5). Thermodynamic data from Hong and Kim (2001), Jacob et al. (2010), and Robie and Hemingway (1995).

Received: 2017-12-19
Accepted: 2018-05-04
Published Online: 2018-08-28
Published in Print: 2018-09-25

© 2018 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.2138/am-2018-6441
Keywords for this article
Crystallization experiments; disequilibrium crystallization; Nb/Ta fractionation; pegmatites; tapiolite; From Magmas to Ore Deposits

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. The tales of disequilibrium and equilibrium crystallization of rare metal minerals: Data from new experiments
  3. Pressure, temperature, water content, and oxygen fugacity dependence of the Mg grain-boundary diffusion coefficient in forsterite
  4. Questioning the biogenicity of Neoproterozoic superheavy pyrite by SIMS
  5. The effect of disequilibrium crystallization on Nb-Ta fractionation in pegmatites: Constraints from crystallization experiments of tantalite-tapiolite
  6. Titanite major and trace element compositions as petrogenetic and metallogenic indicators of Mo ore deposits: Examples from four granite plutons in the southern Yidun arc, SW China
  7. Kuliginite, a new hydroxychloride mineral from the Udachnaya kimberlite pipe, Yakutia: Implications for low-temperature hydrothermal alteration of the kimberlites
  8. Electron microprobe technique for the determination of iron oxidation state in silicate glasses
  9. Experimental investigation of F and Cl partitioning between apatite and Fe-rich basaltic melt at 0 GPa and 950–1050 °C: Evidence for steric controls on apatite-melt exchange equilibria in OH-poor apatite
  10. Carbonic acid monohydrate
  11. High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe
  12. Disturbance of the Sm-Nd isotopic system by metasomatic alteration: A case study of fluorapatite from the Sin Quyen Cu-LREE-Au deposit, Vietnam
  13. Segerstromite, Ca3(As5+O4)2[As3+(OH)3]2, the first mineral containing As3+(OH)3, the arsenite molecule, from the Cobriza mine in the Atacama Region, Chile
  14. Vestaite, (Ti4+Fe2+) Ti34+ O9, a new mineral in the shocked eucrite Northwest Africa 8003
  15. Decomposition boundary from high-pressure clinoenstatite to wadsleyite + stishovite in MgSiO3
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  17. Making tissintite: Mimicking meteorites in the multi-anvil
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