Drivers of Zirconium Isotope Fractionation in Zr-Bearing Phases and Melts: the Roles of Vibrational, Nuclear Field Shift and Diffusive Effects
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Journal Pre-proofs Drivers of zirconium isotope fractionation in Zr-bearing phases and melts: the roles of vibrational, nuclear field shift and diffusive effects Merlin Méheut, Mauricio Ibañez-Mejia, François L.H. Tissot PII: S0016-7037(20)30592-5 DOI: https://doi.org/10.1016/j.gca.2020.09.028 Reference: GCA 11934 To appear in: Geochimica et Cosmochimica Acta Received Date: 5 June 2020 Accepted Date: 22 September 2020 Please cite this article as: Méheut, M., Ibañez-Mejia, M., Tissot, F.L.H., 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 (2020), doi: https://doi.org/10.1016/j.gca.2020.09.028 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved. Drivers of zirconium isotope fractionation in Zr-bearing phases and melts: the roles of vibrational, nuclear field shift and diffusive effects. Merlin Méheut1, Mauricio Ibañez-Mejia2, François L.H. Tissot3 1Géosciences Environnement Toulouse, CNRS-UPS-OMP, 14 av. Édouard Belin, 31400 Toulouse, France 2Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA. 3The Isotoparium, Division of the Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Abstract (285 words) 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 VIII equilibrium fractionation properties were estimated for zircon ( ZrSiO4), baddeleyite VII VI VIII ( ZrO2), gittinsite ( ZrCaSi2O7), sabinaite (Na4 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 (Δ 94/90Zr ~±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 1 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. 1. Introduction Zirconium isotopes have long been studied in extraterrestrial materials as they hold clues to the chronology of, and stellar contributions to, the early Solar System (e.g., Yin et al., 2000; Akram and Schönbächler, 2016). Recently, investigations of mass-dependent isotopic 94/90 94 /90 effects in igneous samples have revealed significant δ Zr (i.e., [( Zr/ ZrSample/ 94 /90 3 Zr/ ZrStandard)-1]*10 ) variability at both the mineral (Ibañez-Mejia and Tissot, 2019; Zhang et al., 2019; Guo et al, 2020) and bulk-rock scales (Inglis et al., 2019; Feng et al., 2020; Tian et al., 2020), suggesting Zr isotopes may be useful tracers of magmatic differentiation. Yet, the mechanism(s) driving the observed variability and their petrogenetic significance remain poorly understood. In particular, the direction and magnitude of equilibrium Zr stable isotope fractionation between Zr-bearing phases and silicate melts is unknown. At thermodynamic equilibrium, heavy isotopes typically populate shorter and stiffer bonds with the lowest coordination number (Schauble, 2004; Young et al., 2015; Blanchard et al., 2017). Since Zr coordination in the melt (i.e., coordination number VI) is expected to be 2 lower than in baddeleyite (VII) or zircon (VIII) (Farges et al., 1991; Louvel et al., 2013), the first-order expectation is that these common Zr-rich phases would be isotopically light relative to the melt from which they crystallize. Such a scenario has recently been invoked by Inglis et 94/90 al. (2019) to explain a positive correlation between δ Zr and SiO2 wt. % in bulk volcanic rock samples from the Hekla Volcano, Iceland. In stark contrast, a detailed investigation (Ibañez-Mejia and Tissot, 2019) of single zircon and baddeleyite crystals from a well- characterized gabbroic igneous cumulate (FC-1 anorthositic gabbro from the Duluth Complex, USA) revealed that these Zr-rich phases are isotopically heavy (by up to ~1 ‰) relative to the starting melt composition (as represented by the bulk rock value). Adding to this conundrum, microbeam (Kirkpatrick et al., 2019; Zhang et al., 2019) and solution (Tompkins et al., 2020) δ94/90Zr data in zircon from several reference localities (e.g., Mud Tank, 91500) display homogeneous compositions, which is at odds with isotope fractionation as being driven by zircon crystallization. Recent LA-ICPMS data by Guo et al. (2020) further complicate the picture, reporting lighter composition in the core (compared to the rims) of zircon from the Gangdese Arc in Tibet, which the authors interpreted as indicative of negative and variable equilibrium fractionation factors between zircon and melt (from -0.12 to -0.45 ‰). Here, we approach these conflicting observations from a theoretical standpoint: (i) We performed first-principles, ab initio calculations to quantify the direction and magnitude of Zr isotope fractionation between silicate melts and Zr-rich phases at thermodynamic equilibrium. (ii) We explored the potential of Nuclear Field Shift effects for driving significant δ 94/90Zr variations, and established a maximum allowable contribution of this effect to the published data. (iii) We evaluated the potential for diffusion-driven (i.e., kinetic) effects as drivers of Zr isotope fractionation. Altogether, these investigations place important constraints on the drivers of Zr isotope variability in Zr-bearing phases and liquids at magmatic temperatures, and 3 highlight that diffusion in silicate melts may be the major mechanism of Zr isotopic fractionation. 2. Zirconium bonding environments in silicate melts and Zr-rich phases Differences in the local bonding environment of Zr in silicate liquids and co-existing minerals are the main drivers of vibrational equilibrium isotope fractionation. The speciation of Zr in fluids at high-P-T and in variably hydrated silicate melts of different compositions is fairly well known from studies using molecular dynamics, X-ray absorption spectroscopy near- edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and ab initio calculations (Farges et al., 1991; Galoisy et al., 1999; Montorsi et al., 2002; Wilke et al., 2012; Louvel et al., 2013; Wang et al., 2020). These studies have found Zr in silicate melts to exhibit short-range order and to be mainly in octahedral (6-fold) coordination, with six O nearest neighbors and mean Zr-O bond distances (RZr-O) from 2.05 to 2.13 Å for the first coordination shell. A second shell contribution arises from interactions with tetrahedral-network Si(Na/Al) atoms located at around 3.7 Å (Farges et al., 1991; Farges, 1996, Louvel et al., 2013). Although Zr4+ solubility in silicate melts is a strong function of temperature, pressure, and melt composition (Watson, 1979; Watson and Harrison, 1983; Farges et al., 1991; Boehnke et al., 2013; Louvel et al., 2013), experiments at conditions relevant to magmatic environments have found Zr speciation to be independent of temperature and pressure (Louvel et al., 2013), as well as water or halogen (F- and Cl-) contents (Farges et al., 1991; Farges, 1996; Louvel et al., 2013; Wang et al., 2020). Only at high–P-T in fluids with very low Si-Na contents (i.e., <35 wt.%) has Zr4+ been observed to occur in 8-fold coordination in a liquid phase (Wilke et al., 2012; Louvel et al., 2013). On the other hand, Zr4+ coordination in common Zr-bearing minerals, varies. It is 8-fold in the zircon structure (tetragonal ZrSiO4; Robinson et al., 1971), 7-fold in baddeleyite (monoclinic ZrO2; Smith and Newkirk, 1965) and can also replace 6-coordinated Ti in Ti-oxide minerals such as ilmenite (FeTiO3) and rutile (TiO2) (Abrahams and Bernstein, 4 4+ 1971; Farges et al., 1996). Mean RZr-O in the first coordination shell of Zr are 2.14-2.28 Å for zircon and 2.04-2.18 Å for baddeleyite (Louvel et al., 2013). In this work, two mineral structures are considered for Zr-bearing phases: zircon (tetragonal ZrSiO4), in which Zr is found in 8-fold coordination, and baddeleyite (monoclinic ZrO2), where Zr is found in 7-fold coordination (Fig. 1a,b; Robinson et al., 1971; Smith and Newkirk, 1971). To model Zr in the silicate melt, several mineral structures were computed: gittinsite (Fig.