Kinetic isotope fractionation of lithium in granite–pegmatite systems: The role of diffusion at crystal–melt interfaces

Abstract Lithium (Li) isotopic compositions in granite–pegmatite systems often exhibit an inverse correlation between δ7Li values and Li concentrations, i.e., heavier Li isotope composition with lower Li concentration, which is not readily reproduced by simple expectations from residual-melt evolution during magmatic differentiation. To address this paradox, we integrated petrological modeling, diffusion simulations, and a global dataset to evaluate the roles of equilibrium crystallization versus diffusion in controlling Li isotope systematics. Neither equilibrium nor fractional crystallization models readily reproduced, under the limited conditions explored here, the observed δ7Li–Li trends, indicating the limitations of traditional differentiation processes. Instead, diffusion-driven kinetic isotope fractionation at crystal–melt interfaces is interpreted as a plausible first-order control. Rapid crystal growth under favorable transient conditions can form local interfacial boundary layers where 6Li diffuses faster than 7Li, leading to preferential 7Li incorporation into crystals and transient isotopic enrichment in the interfacial region. The extent of fractionation depends on the crystal growth rate relative to the diffusion coefficient and temperature-dependent diffusivity. Our models are broadly consistent with empirical data, suggesting that diffusion may play an important role in generating the first-order Li isotopic variations observed in evolved magmatic systems, although additional processes may also contribute to the observed scatter and extremes. These findings enhance our understanding of Li isotope dynamics and offer insights into the formation of Li-rich mineral deposits.