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Drivers of zirconium isotope fractionation in Zr-bearing phases and melts: the roles of vibrational, nuclear field shift and diffusive effects

Méheut, Merlin and Ibañez-Mejia, Mauricio and Tissot, François L. H. (2021) Drivers of zirconium isotope fractionation in Zr-bearing phases and melts: the roles of vibrational, nuclear field shift and diffusive effects. Geochimica et Cosmochimica Acta, 292 . pp. 217-234. ISSN 0016-7037.

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Conflicting results exist regarding the mechanisms, direction, and magnitude of Zr isotope fractionation in igneous systems. To better understand the origin of the fractionations observed in magmatic Zr-bearing minerals and bulk rocks, we theoretically investigated the main potential driving processes: thermodynamic equilibrium effects driven by either (i) vibrational energy or (ii) nuclear volume, and (iii) diffusion-driven kinetic effects. Vibrational equilibrium fractionation properties were estimated for zircon (^(VIII)ZrSiO₄), baddeleyite (^(VII)ZrO₂), gittinsite (^(VI)ZrCaSi₂O₇), sabinaite (Na₄^(VIII)Zr₂TiC₄O₁₆), and vlasovite (Na₂^(VI)ZrSi₄O₁₁). These properties show dependency on Zr coordination, as well as the presence of strong covalent bonds (C O, Si O by order of decreasing effect) in the material. More importantly, despite the large variety of structures investigated, the predicted mass-dependent equilibrium fractionations (Δ⁹⁴/⁹⁰Zr ∼±0.05‰ relative to zircon at 800 °C) are systematically one order of magnitude smaller than required to explain the natural variability observed to date in natural settings (δ⁹⁴/⁹⁰Zr from ∼+1 to −5‰). Likewise, careful evaluation of expected nuclear field shift (NFS) effects predict a magnitude of fractionation of ∼0.08‰ (at 800 °C), further supporting the conclusion that equilibrium effects cannot be invoked to explain extreme δ⁹⁴/⁹⁰Zr zircon values. Furthermore, the mass-dependency of all Zr isotope ratios reported in zircon crystals precludes a contribution of NFS effects larger than ∼0.01‰ on δ⁹⁴/⁹⁰Zr. On the other hand, we show that diffusion, and in particular the development of Zr diffusive boundary layers in silicate magmas during fractional crystallization, provides a viable and most likely mechanism to produce permil-level, mass-dependent isotope fractionations similar to those observed in natural systems. We propose testable scenarii to explain the large and contrasting Zr isotopes signatures in different magmatic zircons, which underline the importance of magmatic composition, Zr diffusivity, and crystallization timescales.

Item Type:Article
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URLURL TypeDescription
Ibañez-Mejia, Mauricio0000-0002-7839-2425
Tissot, François L. H.0000-0001-6622-2907
Additional Information:© 2020 Elsevier Ltd. Received 5 June 2020, Accepted 22 September 2020, Available online 1 October 2020. This research was supported by NSF-EAR grants 1823748 (to MIM) and 1824002 (to FT) and start-up funds to MIM provided by University of Rochester and to FT provided by Caltech. This work was performed using HPC resources from CALMIP (Calcul en Midi-Pyrénées; Grant 2019-P1037). We are grateful for critical evaluations from Editor Mark Rehkämper, Jörn-Frederik Wotzlaw, and two anonymous reviewers, which helped to improve the clarity of this manuscript. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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University of RochesterUNSPECIFIED
Calcul en Midi-Pyrénées2019-P1037
Record Number:CaltechAUTHORS:20201002-090438118
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Official Citation:Merlin Méheut, Mauricio Ibañez-Mejia, François L.H. Tissot, Drivers of zirconium isotope fractionation in Zr-bearing phases and melts: The roles of vibrational, nuclear field shift and diffusive effects, Geochimica et Cosmochimica Acta, Volume 292, 2021, Pages 217-234, ISSN 0016-7037,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105748
Deposited By: Tony Diaz
Deposited On:02 Oct 2020 16:19
Last Modified:05 Nov 2020 19:29

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