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Lying atop the relatively motionless Antarctic plate (velocity 646mm a-1), the Crozet archipelago, midway between Madagascar and Antarctica, is a region of unusually shallow (1543-1756 m) and thickened oceanic crust (10-165 km), high geoid height, and deep low-velocity zone, which may represent the surface expression of a mantle plume. Here, new major and trace element data are presented for Quaternary alkali basalts of the subaerial eruptive stage of East Island, the most easterly and oldest island (9 Ma) of the Crozet archipelago. Crystallization at uppermost mantle depths and phenocryst accumulation have strongly affected the parental magma compositions. Trace element patterns show a large negative K anomaly relative to Ta-La, moderate depletions in Rb and Ba with respect to Th-U, and heavy rare earth element depletions relative to light rare earth elements. These characteristics allow limits to be placed upon the composition and mineralogy of the mantle source of the magmas. The average trace element pattern of the East Island basalts can be matched by 17% melting of a garnet-phlogopite-bearing peridotite source. The stability field of phlogopite restricts melting depths to lithospheric levels. The modelled source composition requires a multistage evolution in which the mantle has been depleted by melt extraction before being metasomatized by alkali-rich plume-derived melts. The depleted mantle component is inferred to be sourced from residual mantle plume remnants that stagnated at the melting locus owing to a weak lateral flow velocity inside the melting region, whose accumulation progressively forms a depleted lithospheric root above the plume core. Low-degree, alkali-rich melts are probably derived from the plume source. Such a mantle source evolution may be general to both terrestrial and extraterrestrial environments in which the lateral component velocity of the mantle flow field is extremely slow.