Ra–Th–Sr isotope systematics in Grande Comore Island: a case study of plume–lithosphere interaction

Here, we present new mass spectrometry measurements of U–Th–Ra disequilibria and Sr isotopes for historical volcanics from the Karthala and La Grille volcanoes in Grande Comore Island, Comores Archipelago. Alkali basalts from the Karthala are characterised by large 230Th (33–47%) and 226Ra excesses (21–53%), and radiogenic Sr compositions (0.7034–0.7041). In contrast, La Grille basanites have less radiogenic 87Sr/86Sr (∼0.7032) with less 226Ra excesses (21%) but similar 230Th–238U disequilibria (45%). The Karthala samples display much more scatter in the Ra–Th isochron diagram than in the Th–U isochron diagram, suggesting a decoupling of the two parent–daughter systems. Correlations between the Sr isotopes and U–Th–Ra disequilibria suggest a mixing relationship between La Grille and Karthala sources. La Grille basanites are shown to result from partial melting of the old metasomatised oceanic lithosphere beneath the archipelago whereas Karthala lavas are derived from the Comore plume. The presence of amphibole in the lithosphere is responsible for Ra–Ba, Ba–Th and Ra–Th fractionation during melting such that DBa > DTh > DRa. The partitioning of these elements clearly differs from partial melting of garnet–lherzolite where DTh > DRa > DBa. Part of the variations in the 226Ra excesses is attributed to radioactive decay between the source and the surface. Short residence times (<1000 year) preclude the presence of a large magma chamber and may be an argument for rapid segregation of the melts through cracks or veins during melt transport.