Mg isotopes and TIMA tracing the mantle source and mineralization process at Bayan Obo Nb-REE-Fe deposit, China

• Ancient carbonate materials were introduced into mantle source via decarbonation. • Mantle source, magmatic differentiation and metasomatism are for δ 26 Mg variations. • Ancient carbonatites involve the addition of carbonate materials. • Acidic conditions promote the migration and enrichment of Nb and REE. The presence of carbonate materials in the mantle source of carbonatites has long been debated. TIMA and Mg isotope are conducted to reveal the genesis of carbonatites and Bayan Obo Nb-REE-Fe deposit. The δ 26 Mg values of dolomite carbonatite dykes are lighter than those of the normal mantle, ranging from −0.83 ‰ to −0.43 ‰. The δ 26 Mg values show marked variation in both coarse-grained dolomite carbonatites (ranging from −0.71 ‰ to −0.03 ‰) and Nb-REE-Fe ores, with the latter exhibiting a broader isotopic range from −0.72 ‰ to +0.31 ‰. Fenite has a δ 26 Mg value of −0.19 ‰, while limestones display the lowest δ 26 Mg values overall, with a narrow range of −1.88 ‰ to −1.86 ‰. This study indicates that Bayan Obo carbonatites originated from the low-temperature partial melting of carbonated peridotites. The ancient carbonate materials were introduced into the mantle source via decarbonation. Whole-rock δ 26 Mg values are controlled by multiple mineral phases rather than a single phase. The mantle source, magmatic differentiation processes and subsequent hydrothermal fluid metasomatism collectively influence the Mg isotope compositions in the Bayan Obo area. The δ 26 Mg values below the mantle value may result from minerals enriched in light Mg isotopes. Therefore, ancient carbonatites also involve the addition of carbonate materials. Element correlations suggest that acidic conditions favor the migration and enrichment of Nb and REE, whereas alkaline conditions promote the migration and enrichment of REE but are unfavourable for Nb migration.