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 Th-230 (33-47%) and Ra-226 excesses (21-53%), and radiogenic Sr compositions (0.7034-0.7041). In contrast, La Grille basanites have less radiogenic Sr-87/Sr-86 (similar to 0.7032) with less 226Ra excesses (21%) but similar Th-230-U-238 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 D-Ba > D-Th > D-Ra The partitioning of these elements clearly differs from partial melting of garnet-lherzolite where D-Th > D-Ra > D-Ba. 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. (C) 1998 Elsevier Science B.V. All rights reserved.