K isotopes reveal fluid exsolution and Li enrichment in the Koktokay No. 3 pegmatite dike

Introduction: Lithium–cesium–tantalum pegmatites are critical sources of rare metals, yet the mechanisms of their formation, especially the role of fluids in elemental and isotopic fractionation, remain debated. Materials and methods: We investigated the Koktokay No. 3 pegmatite dike, the largest rare-element pegmatite in China, using K isotopes and chemical compositions of K-feldspar and muscovite across its concentric zones (I–VIII), combined with petrological modeling. Results: The results show a strong link between fluid exsolution, Li enrichment, and negative δ41K shift, corresponding to abrupt release of fluids and a rapid decline in solid phase abundance. Modeling indicates that low fluid–melt partition coefficients (Dfluid/melt = 0.1 for Li) best reproduce the observed Li enrichment, highlighting fluid exsolution as a key driver of rare-element accumulation. Conclusions: These results provide compelling evidence that late-stage fluid exsolution controls elemental enrichment and modulates K isotopic fractionation, offering new constraints on the petrogenesis of evolved pegmatite systems.