Mg isotopes and in-situ Rb—Sr dating reveal carbonate-rich and carbonate-poor sediments in mantle sources of lamproites beneath the North China Craton

Lamproite is a distinctive ultrapotassic alkaline rock, considered to originate from a volatile-rich metasomatized lithospheric mantle. However, the roles of carbonate-rich and carbonate-poor sediments in their mantle source remain debatable. Here, bulk-rock Mg isotopes and in-situ mineral analyses of lamproites from the northern North China Craton were conducted to elucidate their petrogenesis and provide insight into the deep carbon cycle. In-situ phlogopite Rb—Sr and apatite U—Pb dating yielded a consistent emplacement age of ∼212 Ma, linking them to Early Mesozoic extensional tectonics. The studied lamproites are characterized by high MgO and K₂O, low Al₂O₃ and TiO₂ contents, geochemical signatures similar to those of orogenic lamproites, and phlogopite pyroxenites are identified as the mantle sources. Their high large ion lithophile element contents, negative high field strength element anomalies, and enriched Sr—Nd isotopic values indicate a metasomatized mantle source influenced by recycled crustal materials. The δ²⁶Mg values (−0.22 to −0.37 ‰) of lamproites vary, ranging from the normal mantle (−0.25 ± 0.04 ‰) to lower values. Lamproites with low δ²⁶Mg are characterized by high CaO/Al₂O₃ and Sr/Nd ratios, suggesting an enriched mantle source metasomatized by carbonate-rich sediments with light Mg isotopes. In contrast, lamproites with normal mantle-like δ²⁶Mg values exhibit low CaO/Al₂O₃ and Sr/Nd ratios, along with enriched Sr isotopes, implying mantle metasomatism by carbonate-poor sediments. The integration of Mg—Sr isotopes in these lamproites reveals that they were derived from a lithospheric mantle metasomatized by the incorporation of approximately 25 % calcium carbonate sediments and 38 % clay sediments via subducted slabs. The carbonate metasomatism is likely associated with the subduction of the Paleo-Asian Oceanic slab, consistent with other coeval alkaline magmas in adjacent areas, while the mantle modification by silicate sediments may be related to an earlier metasomatic event. Overall, this study demonstrates that lamproites act as the products of the deep carbon cycle, highlighting the role of subducted carbonates in modifying the lithospheric mantle and driving alkaline magmatism.