Nd-Hf isotope disequilibrium indicating Be-rich melts sourced from the anatexis of S-type granitic protolith

Beryllium (Be) is a critical metal whose enrichment mechanisms in anatectic pegmatites remain poorly constrained. A major challenge in understanding their genesis lies in the reliable identification of source rocks. This study examines Be-pegmatites in the Kelumute-Jideke field of the Chinese Altai in northwestern China to assess the utility of isotopic tracers and to constrain the origin of Be-rich melts. Systematic monazite U-Pb geochronology, whole-rock geochemistry, and Nd-Hf isotopic analyses of pegmatites, granites, and metasedimentary rocks are integrated together with partial melting simulations to determine petrogenetic conditions. Studies reveal a clear superiority of Nd isotopes over Hf isotopes in source discrimination. The Sm-Nd system is less susceptible than the Lu-Hf system to isotopic disequilibrium during melting, and magmatic monazite Nd isotopes in pegmatites preserve a robust record of initial melt composition. Nd isotopic data conclusively indicate that the Triassic Be-pegmatites and coeval granites were produced by partial melting of Early Devonian S-type biotite granites. This interpretation is supported by consistent Nd crustal evolution trends and consistent model ages (TDM2 = 1.3−1.2 Ga), ruling out significant contributions from direct metasedimentary anatexis or fractional crystallization. In contrast, the significant Hf isotope disequilibrium indicates the low-temperature anatexis of Devonian granite dominated the generation of the Be melt. A characteristic of anatectic pegmatites, in which Hf isotopic values decrease with increasing temperature, can be effectively distinguished from the uniform Hf isotopic values of highly fractionated pegmatites. Partial melting modeling demonstrates that muscovite-dehydration melting of the Devonian granite protolith under low-temperature, medium-pressure conditions (∼4 kbar, 640−700 °C) was essential for Be enrichment, efficiently concentrating Be released during mica breakdown.