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

Previous studies of natural samples have documented marked variations in zircon Zr stable isotopes. However, the petrological mechanisms responsible for Zr stable isotopic fractionation during magmatic differentiation remain unclear. To address this, in situ Zr stable isotopic analyses of magmatic zircons were carried out on a suite of mafic plutonic rocks from the southwestern Tianshan, west China. Our results suggest that diffusion-driven Zr stable isotopic fractionation during zircon growth produces per mil-level, mass-dependent isotopic fractionation. Zircon Zr stable isotopic compositions range from –4.50 ‰ to +1.48 ‰, spanning a variation of 5.98 ‰. Some zircon grains exhibit internal Zr stable isotopic zoning, with lighter Zr isotopes in the core and heavier Zr isotope toward the rim, with intra-grain variations of up to 2.82 ‰. In addition, these zircons exhibit significant intra-grain variations in Zr/Hf ratios and Ti contents, which generally decrease with increasing δ94/90Zr values. While the Zr/Hf and Ti variations likely record fractional crystallization processes, the Zr stable isotopic variations cannot be attributed to mass-dependent, stable isotopic equilibrium fractionation, which would produce only ∼0.08 ‰ fractionation at 800 °C. As the temperature decreases and Zr supersaturation in the magma increases, the Zr stable isotopic fractionation becomes more pronounced. The δ94/90Zr values exhibit increasing variability toward both higher and lower values, rather than trending in a single direction. These variable Zr stable isotopic compositions can be explained by the diffusion-limited crystallization (DLC), which is primarily controlled by the zircon crystallization temperature and Zr supersaturation in the melt. As temperature decreases during magmatic differentiation, the diffusion-driven zircon Zr isotopic fractionation is enhanced. Therefore, diffusion during crystal accumulation provides a plausible explanation for the large Zr stable isotopic variability observed in zircons.