Diffusion-driven zircon Zr isotopic fractionation during ultramafic-mafic magmatic differentiation

Previous studies of natural samples have documented significant zircon Zr stable isotope variations. However, the petrologic mechanisms responsible for Zr stable isotope fractionation during magmatic differentiation remain unclear. To resolve this issue, in situ Zr stable isotope analyses of magmatic zircons are carried out on a suite of mafic plutonic rocks from southwestern Tianshan, west China. The results suggest that diffusion-driven Zr stable isotope fractionation during zircon growth is the most likely mechanism to produce permil-level, mass-dependent isotope fractionation. Zircon Zr stable isotope in the samples ranges from -4.50 ‰ to 1.48 ‰, with an extreme variation of 5.98 ‰. Some zircon grains analyzed for isotope profile show internal zoning, with lighter Zr stable isotope compositions in the core and heavier ones toward the rim, exhibiting intragrain variations up to 2.82 ‰. In addition, these zircons display significant intragrain variations in Zr/Hf ratio and Ti content, most of which show a decreasing trend with increasing δ94/90Zr values. Although the Zr/Hf and Ti variations in zircon profiles likely reflect fractional crystallization effect, the extreme Zr stable isotope variations cannot be explained by mass-dependent stable isotope equilibrium fractionations, which would only cause a fractionation of 0.08 ‰ at 800℃. As the temperature decreases and Zr supersaturation in the magma increases, the Zr stable isotope fractionations become more pronounced. The δ94/90Zr values exhibit a ‘bell shape’ distribution, with high and low values spreading outward, rather than trending in a single direction. Together with previous reported less variable Zr stable isotope of ultramafic-mafic igneous rocks and results of model calculation, the extreme zircon Zr stable 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 decreasing temperature during magmatic differentiation, the diffusion-driven zircon Zr isotope fractionations become more significant. Therefore, the diffusion occurring the accumulation processes may be an effective mechanism to cause the remarkable Zr stable isotope variation in zircon scale.