Isotopic shift in carbonatitic magma source indicates sequential melting of precedent subducted oceanic slab

The carbonatite petrogenesis remains mysterious despite decades of research. A persistent enigma is the role of oceanic subduction and recycling in the formation of carbonatitic magmas. Our novel radiogenic isotope analyses of the Bayan Obo REE-bearing carbonatites (ca. 1.3 Ga) reveal a temporal shift in source, transitioning from PREMA-like signature as recorded in early pyrochlore (ε _Nd (t) = 4.4 to 6.5, I _Sr (t) = 0.702684 to 0.702867) to OIB-like signature as preserved in late apatite (ε _Nd (t) = -0.5 to 1.1, I _Sr (t) = 0.702688 to 0.702895), monazite, bästnasite and bulk rock of all different varieties of carbonatites. Stable isotope analyses of Li, S, C and O (δ ⁷ Li = -0.1‰ to 10.8‰; δ ³⁴ S _{= -4.5‰ to 14.7‰; δ ¹³ C = -5.6‰ to -0.4‰; δ ¹⁸ O = 8.8‰ to 14.4‰) explicitly prove the involvement of subducted sediments and carbonates in the magma source. Li-Mg isotope modeling indicates a hybrid source of about 30% lithospheric mantle, 20-60% marine carbonates, and 10-50% marine sediments for generating parental carbonate-rich alkaline magmas. Temporal isotopic shift occurred in the carbonatitic magma source from moderately depleted to slightly enriched signature. A plausible mechanism for such source shift is progressive melting of a subducted slab consisting of eclogitized oceanic basalts and overlying metasediments and carbonates. This slab melting model offers the first evidence of direct link between a preceding subduction and Precambrian carbonatites as opposed to Phanerozoic ones that often involve deeply-recycled crustal materials.}