Drivers of zirconium isotope fractionation in Zr-bearing phases and melts: The roles of vibrational, nuclear field shift and diffusive effects
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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DOI：
10.1016/j.gca.2020.09.028
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发表时间：
2021-01
影响因子：
5
通讯作者：
M. Méheut;M. Ibáñez-Mejia;F. Tissot
中科院分区：
文献类型：
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作者：
M. Méheut;M. Ibáñez-Mejia;F. Tissot
关于岩浆系统中Zr同位素分馏的机制、方向和大小，存在着一些令人满意的结果。为了更好地理解在岩浆含锆矿物和块状岩石中观察到的分馏的起源，我们从理论上研究了主要的潜在驱动过程：由（i）振动能或（ii）核体积驱动的热力学平衡效应，以及（iii）扩散驱动的动力学效应。振动平衡分馏性能的锆石（VIIIZrSiO 4），斜锆石（VIIIZrO 2），gittinsite（VIZrCaSi 2 O 7），sabinaite（Na 4VIIIZr 2 TiC 4 O 16），和vlasovite（Na 2 VIZrSi 4 O 11）进行了估计。这些性质显示依赖于Zr配位，以及在材料中存在强共价键（Csingle bondO，Sisingle bondO，按效果递减的顺序）。更重要的是，尽管研究的结构种类繁多，但预测的质量相关平衡分馏（相对于800 °C的锆石，Δ94/90 Zr为±0.05‰）系统地比解释迄今在自然环境中观察到的自然变化（δ94/90 Zr从± 1 ‰到-5 ‰）所需的小一个数量级。同样，对预期的核场位移（NFS）效应的仔细评估预测分馏的幅度为10.08 ‰（在800 °C），进一步支持了平衡效应不能用来解释极端的δ94/90 Zr锆石值的结论。此外，锆石晶体中所有Zr同位素比值的质量依赖性排除了NFS效应对δ94/90 Zr的贡献大于0.01‰。另一方面，我们表明，扩散，特别是锆扩散边界层在硅酸盐岩浆在分离结晶过程中的发展，提供了一个可行的和最有可能的机制，产生渗透水平，质量依赖的同位素分馏类似于在自然系统中观察到的。我们提出了可测试的锆来解释大的和对比鲜明的锆同位素签名在不同的岩浆锆石，这强调了岩浆成分，锆扩散率和结晶时间尺度的重要性。
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 (VIIIZrSiO4), baddeleyite (VIIZrO2), gittinsite (VIZrCaSi2O7), sabinaite (Na4VIIIZr2TiC4O16), and vlasovite (Na2VIZrSi4O11). These properties show dependency on Zr coordination, as well as the presence of strong covalent bonds (Csingle bondO, Sisingle bondO 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.