Transition from kinetic to equilibrium Zr isotope fractionations during magma crystallization

Zirconium (Zr) isotopes are an emerging tool to study igneous processes. However, Zr isotope fractionation mechanisms are debated, especially regarding the relative roles of kinetic and equilibrium effects. Here, we report high-precision in-situ Zr isotope (δ⁹⁴Zr) measurements in zircons from granitoids with ages of 3.4 to 2.5 Ga. The δ⁹⁴Zr values in all magmatic zircons range from −0.63 to + 0.41 ‰. These zircons show negative correlations between δ⁹⁴Zr values and Zr/Hf ratios, indicating that the crystallization of zircon plays an important role in Zr isotope variations. However, our calculations suggest that neither fractional crystallization of zircon under equilibrium conditions nor that with a fixed kinetic effect cannot explain the δ⁹⁴Zr values observed in our zircons. Combined with theoretical models of crystal growth, we propose that the fractionation mechanisms evolve from diffusion-dominated situations in the early stage to equilibrium-controlled conditions in the later stage of a zircon population’s crystallization history. Our modelling results suggest that a gradual shift from diffusive-kinetic (growth-related) fractionation to near-equilibrium fractionation best explains our results. Zr isotopes, therefore, have the potential to probe the crystallization kinetics of high-temperature processes that are otherwise inaccessible to direct observation.