The rises and falls of zirconium isotopes during zircon crystallisation

Abstract

Zircon trace element and isotopic analysis for an increasing number of isotopic systems provide valuable insights into magma crystallisation, and evolution of the continental crust. Zirconium stable isotopes have been measured recently and shown to vary by several per mille on Earth. Both positive and negative shifts of δ94/90Zr have been found in zircons and melts, and equilibrium isotopic fractionations are found to be small. We here employ and further develop a zircon crystallisation programme with two moving boundaries targeting zircon crystallisation in the presence of other minerals. We reproduce positive, negative, and inverse δ94/90Zr, Zr/Hf, and other compatible trace element variations in zircon and surrounding melt that can be entirely explained by the boundary layer processes during kinetics of zircon crystallisation in a realistic set of geologic conditions. Progressively lower δ94/90Zr values in zircon develop during rapid crystallisation, lower melt water contents, crystallisation in the lower temperature range, and whilst co-crystallising with the Zr-poor phase. Given large (multi-per mille) kinetic Zr isotope fractionations discovered, zircon may serve as a monitor of magmatic crystallisation conditions, but is unlikely to serve as a diagnostic tool for the bigger set of petrological problems (e.g., evolution of the continental crust) without proper context.