U–Th–Pa–Ra systematics for the Grande Comore volcanics: melting processes in an upwelling plume

High precision TIMS Pa-U disequilibrium data have been obtained on a series of recent samples from the Karthala and La Grille volcanoes, Grande Comore. We use this data together with U–Th–Ra and Sr isotope, major and trace element data to constrain melting-induced fractionation of the U series nuclides. (231Pa/235U) activity ratios range between 1.7 and 1.9 for the historical Karthala volcanics and are around 2.0 for two recent La Grille samples. While the Karthala magmas could have been generated in the upwelling asthenosphere, the La Grille lavas are thought to involve some lithospheric melting. The Karthala data is best explained by a model of slowly upwelling mantle during 230Th and 231Pa grow in from their parent nuclides. Pb isotope data is consistent with a recent fractionation of 230Th/232Th induced by a slow melting process in the source of the Karthala. The upwelling velocity of the mantle plume is constrained to be a few cm per year. The La Grille data can be explained by interaction of the Karthala melts with the lithosphere. The degree of melting of the lithosphere still needs to be small to preserve the U-series fractionation. The higher 231Pa/235U in the La Grille lavas probably reflects a higher cpx/garnet mode than for the Karthala.