Two types of grain-boundary glasses, one phosphorous rich and the other phosphorous poor, have been discovered during a detailed electron- and ion-microprobe study of spinel peridotite xenoliths exhumed from the mantle lithosphere of the Massif Central by Neogene alkali basalts. The high-P glasses are SiO2 poor and enriched in CaO, rare earth elements (REEs), Ba, U, and Rb and have large negative Ce anomalies on chondrite-normalized REE plots. The low-P glasses are SiO2 rich and have variable major and trace element compositions reflecting mixing between in situ partial melts of the host mantle and the high-P melt. The major and trace element compositions of the P-rich glasses strongly resemble those of phosphatic sediments and suggest their derivation by partial melting of a sedimentary protolith subducted under the Massif Central during the Hercynian orogeny. Selective assimilation of similar P-rich melt veins from the lithospheric mantle may account for the high-P contents of some Tertiary-Quaternary basalts from the Massif Central.

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