Linking Significant Zr Isotopic Fractionation in Magmatic Zircons With Petrographic Textures

The origin of crystal-scale Zr isotopic variation remains enigmatic. Here, we report high-precision Zr isotopic compositions of thin-sectioned and hand-picked zircon crystals from monzodiorites and K-feldspar granites, which represent the mafic and felsic products of a differentiating magma. Our results reveal that zircons from both rock associations are dominated by a rim-ward increase in the Zr-94/Zr-90 ratio and accompanied by similar to 1 parts per thousand intracrystalline and intercrystalline variations, confirming the preferential incorporation of light Zr isotopes and thus kinetic isotopic fractionation during zircon growth. Notably, zircons enclosed by K-feldspar in monzodiorites generally yield lower Zr-94/Zr-90 and higher Zr/Hf ratios than that of separated crystals, while in K-feldspar granites, tiny zircons intergrown with apatite obtain higher Zr-94/Zr-90 but lower Zr/Hf ratios relative to that of sizable separates. The diversity in petrographic textures and sizes of zircon crystals, corresponding to their crystallization sequence, potentially accounts for some of the heterogeneity of Zr isotopes at the mineral scale.