Extreme zircon Zr isotope fractionation caused by diffusion-driven zircon growth during magma evolution

Previous studies of natural samples indicated significant zircon Zr isotope variations. However, conflictive results and explanations were acquired regarding the mechanism, direction, and magnitude of Zr isotope fractionation. In order to resolve this issue, in situ Zr isotope analyses of magmatic zircons are carried out on a suite of mafic plutonic rocks from southwestern Tianshan, west China, and the results reveal that diffusion-driven Zr isotope fractionation during zircon growth may be the most likely mechanism to produce permil-level, mass-dependent isotope fractionation. This study shows zircon Zr isotope fractionations ranging from -4.44‰ to 1.53‰, with extreme variation of 5.97‰. Some zircons for isotope profile analyses exhibit internal zoning with light Zr isotope compositions in the core and heavy ones toward the rim with intragrain maximum variations of about 2.82‰. In addition, these zircons also display significant intragrain variations of Zr/Hf ratios and Ti contents, most of which are negatively correlated with their Zr isotope compositions. Although the variation of Zr/Hf ratios and Ti contents of zircons profile could reflect the effect of fractional crystallization, the extreme Zr isotope variations cannot be explained by mass-dependent stable isotope equilibrium fractionations, which only cause a finite fractionation of 0.08‰ at 800℃. Along with the decrease of the temperature and the increase of Zr supersaturation of the magma, the Zr isotope fractionations become more and more significant, with the high and low δ94Zr values gradually spreading out to each side but not increasing or decreasing in a single direction, and forming a ‘bell’ shape. However, the average δ94Zr values of each sample are similar to each other, without significant Zr isotope fractionation during magma evolution. Together with the results of model calculation, the extreme zircon Zr isotope fractionation can be explained by the diffusion-limited crystallization of zircon (DLC model), which is principally controlled by zircon crystallization temperature and Zr supersaturation in the melts. With the decrease of the magma temperature, the diffusion-driven zircon Zr isotope fractionation become larger during magmatic differentiation. Therefore, we suggest the diffusion mechanism may be a reasonable mechanism to cause the remarkable Zr isotope fractionation in zircon scale.

既往对自然样本的研究揭示了锆石中锆 (Zr) 同位素的显著变化。然而，关于锆同位素分馏的机制、方向和幅度，研究结果和解释存在分歧。为解决这一问题，本研究对来自中国西天山西南部的基性岩浆岩中的锆石进行了原位锆同位素分析，结果显示，在锆石生长过程中，由扩散驱动的锆同位素分馏可能是产生千分位级、质量依赖性同位素分馏的最可能机制。本研究发现，锆石的锆同位素分馏范围从 -4.44‰ 至 1.53‰，极端变化达 5.97‰。部分锆石的同位素剖面分析显示出内部分区现象，核心部分锆同位素组成较轻，而边缘部分则较重，晶粒内部的最大变化约为 2.82‰。此外，这些锆石还表现出显著的锆/铪 (Zr/Hf) 比例和钛含量晶粒内变异，其中大部分与锆同位素组成呈负相关。尽管锆石剖面中锆/铪比例和钛含量的变化可能反映分馏结晶的影响，但极端的锆同位素变化无法用质量依赖性的稳定同位素平衡分馏来解释，后者仅在 800℃ 时造成有限的 0.08‰ 分馏。随着温度的降低和岩浆中锆超饱和度的增加，锆同位素分馏逐渐变得更加显著，高δ94Zr和低δ94Zr值逐渐向两侧扩散，但并不单一方向增加或减少，形成“钟形”分布。然而，每个样品的平均δ94Zr值彼此相似，在岩浆演化过程中未观察到显著的锆同位素分馏。结合模型计算结果，极端的锆石锆同位素分馏可以通过锆石扩散限制结晶（DLC模型）来解释，该模型主要受锆石结晶温度和熔体中锆超饱和度控制。随着岩浆温度的降低，在岩浆分异过程中，由扩散驱动的锆石锆同位素分馏变得更加显著。因此，我们认为扩散机制可能是导致锆石尺度上显著锆同位素分馏的合理机制。