Grande Comore Island is formed by two contemporaneously active volcanoes, Karthala and La Grille. Previous work has shown that La Grille lavas are alkalic low-degree melts of a metasomatized lithospheric mantle source containing amphibole. This study presents major, trace element and Sr-Nd-Pb isotope analyses of a detailed sample suite from Karthala. The lavas are alkali basalts displaying trace element and isotopic variations that indicate mixing between the deep mantle plume source and the lithospheric mantle source of La Grille. This study delineates how the process of plume-lithosphere interaction occurs in Grande Comore, characterizes the composition of the metasomatized lithospheric mantle, and identifies its origin. Lithosphere-derived La Grille lavas show similar trace element patterns to plume-related Karthala lavas, but lower trace element contents far comparable degrees of partial melting. Addition of a few percent of plume melt to anhydrous depleted lithospheric mantle generates a mantle composition that can serve as the source of La Grille lavas. This plume-related metasomatism occurs pervasively at the base of the lithospheric mantle, lowers its solidus temperature, and causes it to be a major contaminant of plume melts. Karthala lavas are shown to be mixtures of plume melts with high-degree melts of the lithospheric mantle. To the extent that this process is valid in other locations, it implies that isotopic variations of other oceanic islands may reflect contamination of nearly uniform plume melts with lithospheric melts.
Zq379Times Cited:26Cited References Count:62