Calcium-O-Sr-Nd isotope evidence for the origin of REE-bearing carbonatite from metasomatized lithospheric mantle without recycled carbonate

Abstract

Carbonatites, originating from mantle-derived magmas, provide unique insight into both mantle composition and deep carbon cycling. As primary host rocks for REE deposits, their petrogenesis carries significant economic implications. However, key controversies persist regarding to (1) their ultimate mantle sources (primitive vs. metasomatized mantle), and (2) whether subducted carbonates participate in the mantle source and consequently contribute to REE enrichment. Here we present comprehensive study of bulk-rock and in-situ mineral (olivine, apatite, calcite, and dolomite) major, trace elements, and Ca-O-Sr-Nd isotopes for the Qieganbulake carbonatite, which formed during Neoproterozoic breakup of the Rodinia supercontinent and hosts considerable REE contents but no economic mineralization. The Qieganbulake carbonatite shows enriched Sr-Nd isotopes [(⁸⁷Sr/⁸⁶Sr)_i = 0.7059–0.7067; εNd(t) = -9.1 to -4.3], consistent with derivation from a metasomatized subcontinental lithospheric mantle (SCLM). Mantle-derived olivine xenocrysts in the carbonatite yield δ¹⁸O values (4.75 ± 0.92‰ to + 5.76 ± 0.26‰) overlapping with that of the metasomatized SCLM without significant involvement of ¹⁸O-rich recycled carbonate. The bulk-rock δ^44/42Ca values (0.33 ± 0.01‰ to 0.38 ± 0.02‰) are comparable to those of primary carbonatites and basalts, further excluding significant recycled carbonate contributions. Coupled Ca and olivine O isotopes thus implicate that slab-released fluids, rather than sedimentary carbonates, served as the predominant metasomatic agent in the mantle source. This finding also highlights that source fertilization by subducted carbonates is not a prerequisite for REE enrichment in carbonatites. Furthermore, we identify that the limited REE mineralization potential primarily results from extensive early apatite crystallization, which effectively sequestered REEs and inhibited the formation of REE minerals.