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Recent studies of zirconium isotopes in igneous systems have revealed significant mass dependent variability, the origin of which remains intensely debated. While magmatic zircon crystallisation 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 limited equilibrium fractionation between zircon and melt at magmatic temperatures. To resolve this debate, we determined the isotopic fractionation between co-existing zircon and silicic melt using controlled zircon growth experiments. Our experimental results indicate that zircon has a lower δ94/90Zr relative to co-existing melt by ∼0.045 ‰ at magmatic conditions, which is in excellent agreement with ab initio predictions. Our results imply that, for most natural systems studied to date, the observed variability is predominantly a result of non-equilibrium rather than equilibrium isotope fractionation during zircon crystallisation.
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This content will become publicly available on April 10, 2026
Equilibrium Silicon Isotope Fractionation in Eclogites and Granites Constrained by Single Crystal X‐Ray Diffraction and the Force Constants Approach
Abstract Using our recently developed X‐ray diffraction basedforce constantsapproach, we have determined the equilibrium Si isotope fractionation between omphacite/garnet, quartz/kyanite, and quartz/zircon at temperatures relevant to the petrogenesis. We find that Na strongly affects the Si isotope fractionation between omphacite and garnet. Our results have suggested that the omphacite and garnet in eclogite collected in the Dabie Mountain, as well as the kyanite and its host quartz veins, are isotopically in equilibrium, which further suggests that the Dabie Mountain eclogites and its host veins underwent the same high pressure‐temperature condition during their formation. The Si isotope fractionation determined by our methods, together with published mass spectroscopy measurements, DFT‐CIPW calculations and sigmoid fitting on various felsic granites, have suggested that the Si isotope fraction between zircon and whole rock “saturates” at ∼0.45‰ at 1000 K when the SiO2content in the granite is above ∼70 wt%.
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- PAR ID:
- 10585113
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 8
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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