Mantle Source and Metasomatic Agent of Olivine Nephelinites: Insights from Olivine Geochemistry, Oxygen Isotopes and Whole-Rock Os Isotopes

Abstract Nephelinites are a distinctive group of silica-undersaturated, strongly alkaline ultrabasic volcanic rocks that are occasionally associated with carbonatites. Their genesis has been attributed to the melting of a mantle source enriched by either recycled crustal materials or other metasomatic agents. This study investigates olivine nephelinites and constrains their mantle source from the West Qinling orogen, Central China—a locality noted for its unique occurrence of rocks previously classified as kamafugites or melilitites in China. Olivines in these rocks are categorized into Group I mantle xenocrysts (Fo89–92) and Group II magmatic crystals (Fo81–88). In situ δ18O values of the Group II magmatic olivines (5.04 ± 0.25‰; 1σ, n = 67) are slightly lower than those of Group I mantle xenocrysts (5.26 ± 0.20‰,1σ, n = 199). Measured 187Os/188Os ratios for these nephelinites are moderately to highly radiogenic (0.138–0.682), with age-corrected γOs values varying from +6.57 to +409. Magmatic olivines with elevated 100 × Mn/Fe (mean = 1.65), 100 × Ni/Mg (mean = 0.70), and high 100 × Ca/Fe (mean = 2.33) ratios fingerprint a metasomatized mantle peridotite source. Binary mixing indicates that the observed radiogenic Os isotopes can be explained by the incorporation of <30% melts/fluids derived from a subducted oceanic slab. Our findings suggest that mantle metasomatism, driven by recycled oceanic slabs, played a major role in alkaline enrichment observed in the West Qinling magmas, and may provide new insights into the crust–mantle interaction.