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.