Kinetic Zr isotope fractionation on Mars recorded in ancient Zr-rich minerals

Zircon crystallisation induces stable Zr isotope fractionation by the preferential incorporation of the lighter Zr isotopes. The Zr isotope compositions recorded in terrestrial zircons show that, in most cases, the magnitude of isotope fractionation exceeds that expected from thermodynamic equilibrium. In other words, kinetic Zr isotope fractionation between zircon and silicate melt is the rule rather than the exception. Here, we investigate the stable Zr isotope compositions of ∼4.45 Gyr-old zircon and baddeleyite grains from the meteorite Northwest Africa (NWA) 7533, a regolith breccia sample from Mars. These Zr-rich minerals crystallised during impact reworking of the primordial Martian crust and, thus, probe a magmatic system fundamentally different from the plutons and batholiths represented in the existing zircon Zr isotope data. The 26 analysed grains reveal a total 94Zr/90Zr variation of ∼0.75 ‰, indicative of kinetic isotope fractionation. Further, the grains record a correlation between 94Zr/90Zr and Zr/Hf, similar to that observed in terrestrial zircon samples, suggesting that the fractionation behaviour in Pre-Noachian impact melt systems on Mars is similar to that occurring in younger magmatic intrusions on Earth.