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 94Zr/90Zr ratio and accompanied by ∼1‰ 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 94Zr/90Zr and higher Zr/Hf ratios than that of separated crystals, while in K‐feldspar granites, tiny zircons intergrown with apatite obtain higher 94Zr/90Zr 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. Plain Language Summary: Stable Zr isotopes of zircon have been proposed as a new and promising tracer for magmatic differentiation. However, how significant heterogeneity of Zr isotopes developed at the mineral scale in an evolving magma is highly debatable. Ab initio simulation calculations, experiments, and natural sampling have suggested that kinetic rather than equilibrium effect plays a dominant role in fractionating Zr isotopes during zircon crystallization, whereas how kinetic effect links with the textural contexts of zircons is not directly constrained. Here, petrographic and geochemical constraints on zircon crystals from the mafic and felsic products of a differentiating magma are provided, which reveal that earlier crystallized domains or crystals are isotopically lighter in Zr than the later ones. Besides, the δ94Zr values of zircon crystals, displaying intra‐grain and inter‐grain variations beyond ∼1‰, correlate well with Zr/Hf ratios that reflect diverse stages of magma differentiation, irrespective of the compositions of host rocks. These findings indicate that crystal‐scale elemental and stable isotopic variations (e.g., Zr isotopes in zircon) could be ascribed to the sequential crystallization of targeting minerals. Key Points: Early‐formed zircon crystals or domains are isotopically lighter in Zr than those crystallized late in both mafic and felsic rocksDiversity in textural occurrences and sizes of zircons potentially explain the Zr isotopic heterogeneity at the mineral scaleGeochemical information extracted from separated minerals does not always reflect the nature of primitive magma [ABSTRACT FROM AUTHOR]