Geochemical and isotopic (Sr, Nd, Pb) evidence for plume-lithosphere interactions in the genesis of Grande Comore magmas (Indian Ocean)

Three groups are distinguished among the Grande Comore lavas: Old Karthala, Recent Karthala and La Grille. Major and trace element data allow a clear distinction between Karthala saturated to slightly undersaturated lavas (shield-stage) and La Grille strongly undersaturated primary melts (post-shield stage). All these lavas are strongly enriched in incompatible elements, with relative enrichments in Ba, Nb, La and relative depletions in Rb, K, Sr, Sm. Low HREE contents and strongly fractionated REE patterns indicate that garnet is residual in their sources. La Grille lavas exhibit the highest and most variable degrees of incompatible element enrichment, the strongest relative K depletion and a wide range of the most incompatible element ratios. These lavas are low-degree melts of an amphibole-bearing source. Ratios of the most incompatible elements are homogeneous among Karthala lavas. They were produced by slightly higher degrees of melting, with much smaller amounts of residual garnet and amphibole, probably at shallower depth, than La Grille lavas. La Grille lavas exhibit the most radiogenic Nd-143/Nd-144 (0.51283-0.51287) and the least radiogenic Sr-87/Sr-86 (0.70315-0.70336), Pb-206/Pb-204 (18.98-19.31) and Pb-208/Pb-204 (38.80-39.13). Old Karthala lavas display the most radiogenic Pb-206/Pb-204 (19.47-19.86) of the data set and intermediate Sr-87/Sr-86 and Nd-143/Nd-144. Recent Karthala lavas exhibit the most radiogenic Sr-87/Sr-86 (0.70362-0.70396), the least radiogenic Nd-143/Nd-144 (0.51265-0.51272) and intermediate Pb-206/Pb-204. Karthala lavas having 'LoNd compositions' require the mixing of closely associated EM I and HIMU components. Recent Karthala lavas reflect a stronger contribution of the EM I component whereas Old Karthala lavas are the most enriched in the HIMU component and contain a additional DM component. La Grille lavas reflect the significant involvement of a DM component, in addition to HIMU and EM I components. A strong coupling between isotopes and incompatible element ratios is observed among La Grille lavas with the most isotopically depleted samples exhibiting the most enriched tract element signature. The geochemical and isotope variability recorded in Grande Comore lavas is a consequence of small and variable degrees of melting of the heterogeneous underlying mantle. We propose that the EM I acid HIMU components are closely associated within the mantle plume and that the DM component is located in the oceanic lithosphere. Karthala lavas, corresponding to shallower, higher-degree melts, give the best estimate of the plume composition and must have been produced close to the plume axis. La Grille low-degree melts require lower temperatures for their genesis and must have been generated at the periphery of the plume. The tao volcanoes belong to two distinct magmatic systems. The atypical 'LoNd' signature of the Grande Comore lavas, contrasting with the Indian Ocean Dupal signature, may be explained by the specific location of the Comores Archipelago, at the junction of two distinct oceanic crusts, in the transition zone between two distinct mantle domains. (C) 1998 Elsevier Science B.V.