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Mantle Source and Metasomatic Agent of Olivine Nephelinites: Insights into Olivine Geochemistry, Oxygen Isotopes and Whole-Rock Os Isotopes
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DOI：10.1093/petrology/egag031.png)
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Abstract
En 中文
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 (Fo(89-92)) and Group II magmatic crystals (Fo(81-88)). In situ delta O-18 values of the Group II magmatic olivines (5.04 +/- 0.25 parts per thousand; 1 sigma, n = 67) are slightly lower than those of Group I mantle xenocrysts (5.26 +/- 0.20 parts per thousand, 1 sigma, n = 199). Measured Os-187/Os-188 ratios for these nephelinites are moderately to highly radiogenic (0.138-0.682), with age-corrected gamma Os values varying from +6.57 to +409. Magmatic olivines with elevated 100 & times; Mn/Fe (mean = 1.65), 100 & times; Ni/Mg (mean = 0.70) and high 100 & times; 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.
Keywords:
olivine nephelinites
olivine chemistry and oxygen isotopes
Os isotopes
West Qinling orogen
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