The Process of Plume-Lithosphere Interactions in the Ocean Basins--the Case of Grande Comore

Abstract

Grande Comore Island is formed by two contemporaneously active INTRODUCTION volcanoes, Karthala and La Grille. Previous work has shown that Most individual oceanic islands display significant isotopic La Grille lavas are alkalic low-degree melts of a metasomatized variations in erupted basalts, and the cause of these lithospheric mantle source containing amphibole. This study presents variations is still a matter of great debate. There is major, trace element and Sr–Nd–Pb isotope analyses of a detailed still no consensus as to whether they reflect intrinsic sample suite from Karthala. The lavas are alkali basalts displaying heterogeneity of the deep mantle source, mixtures of the trace element and isotopic variations that indicate mixing between plume source with entrained mantle, or contamination the deep mantle plume source and the lithospheric mantle source of of plume-derived melts by shallow lithospheric mantle La Grille. This study delineates how the process of plume–lithosphere or crust. Recently there has been increased recognition interaction occurs in Grande Comore, characterizes the composition of the significance of lithospheric contributions to the of the metasomatized lithospheric mantle, and identifies its origin. isotopic diversity found in oceanic islands, especially those Lithosphere-derived La Grille lavas show similar trace element located near continents and on oceanic plateaux (Gerlach patterns to plume-related Karthala lavas, but lower trace element et al., 1988; Hoernle et al., 1991; Barling et al., 1994; contents for comparable degrees of partial melting. Addition of a Class et al., 1996; Milner & le Roex, 1996). The possibility few percent of plume melt to anhydrous depleted lithospheric mantle of lithospheric contributions to Hawaiian tholeiitic lavas generates a mantle composition that can serve as the source of La has been widely discussed [e.g. Tatsumoto (1978), StauGrille lavas. This plume-related metasomatism occurs pervasively digel et al. (1984), Chen & Frey (1985), Hegner et al. at the base of the lithospheric mantle, lowers its solidus temperature, (1986), Stille et al. (1986), Kurz & Kammer (1991), Eggins and causes it to be a major contaminant of plume melts. Karthala (1992), Eiler et al. (1996), Hauri et al. (1996), and references lavas are shown to be mixtures of plume melts with high-degree therein], although these studies disagree on the identity melts of the lithospheric mantle. To the extent that this process is of the lithospheric component. Nevertheless, models of valid in other locations, it implies that isotopic variations of other lithospheric contributions to plume lavas have met reoceanic islands may reflect contamination of nearly uniform plume sistance on theoretical, geophysical, and geochemical melts with lithospheric melts. grounds. Significant melting of anhydrous lithosphere as