EXPERIMENTAL CONSTRAINTS ON ZIRCONIUM STABLE ISOTOPE FRACTIONATION DURING MAGMATIC ZIRCON CRYSTALLIZATION

Zirconium (Zr) belongs to a group of transition metals known as the High Field Strength Elements (HFSE), which, due to their distinctive geochemical properties, are widely used to trace magmatic differentiation as well as the co-evolution of Earth's mantle and crust [e.g., 1, 2]. Although mass-independent isotopic variations of Zr have been extensively studied [e.g., 3, 4], only recently have mass-dependent variations begun to be explored [e.g., 5, 6, 7, 8]. These investigations have debated whether magmatic zircon crystallization can drive equilibrium stable isotope fractionation given that Zr⁴⁺ undergoes a shift in coordination state (from 6- to 8-fold) as zircon precipitates from a silicic melt. Although Zr measurements from natural systems have confirmed significant 𝛿^94/90Zr variability, ab initio calculations predict negligible equilibrium fractionation between zircon and melt at magmatic temperatures, and therefore the exact mechanism(s) fractionating Zr stable isotopes remain unclear [9, 10]. To resolve this debate, we determined isotopic fractionation coefficients between zircon and silicic melt as a function of temperature and melt composition using controlled zircon-growth experiments. Our preliminary data show that the observed range of natural variability is not well explained by equilibrium isotope fractionation during zircon crystallization. This supports the hypothesis of a kinetic origin of isotopic fractionations during magma crystallization [9, 10].

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