¹MAX-PLANCK-INSTITUT FÜR CHEMIE, POSTFACH 3060, D-55020 MAINZ, GERMANY ²LAMONT-DOHERTY EARTH OBSERVATORY OF COLUMBIA UNIVERSITY, P.O. BOX 1000, PALISADES, NY10964, USA ³UNIVERSITÄT HEIDELBERG, IM NEUENHEIMER FELD 236, D-69120 HEIDELBERG, GERMANY ⁴UNIVERSITÉ DE LA RÉUNION, 15 AVÉNUE RENÉE CASSAN, BP 7151, F-97715 SAINT-DENIS MESSAY, CEDEX 09, RÉUNION

RECEIVED SEPTEMBER 10, 1997; REVISED TYPESCRIPT ACCEPTED JANUARY 9, 1998

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 for 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 soli