Experimental Constraints on Mass-Dependent Zirconium Isotope Fractionation During Magmatic Zircon Crystallization

Abstract

Recent studies of zirconium isotopes in igneous systems have revealed significant mass-dependent variability, the origin of which remains intensely debated. Indeed, while magmatic zircon crystallization could potentially drive equilibrium isotope fractionation given that Zr4+ undergoes a shift in coordination as zircon precipitates from a silicic melt, ab initio calculations predict only negligible equilibrium fractionation between zircon and melt at magmatic temperatures. To resolve this debate, we determined isotopic fractionation factors between zircon and silicic melt (Δ94/90Zrzircon-melt) using controlled zircon-growth experiments. We show that experimental values are in excellent agreement with ab initio predictions. Our results imply that, for most natural systems studied to date, the observed variability is not due to equilibrium isotope fractionation during zircon crystallization.