Osmium isotopes in Grande Comore lavas: A new extreme among a spectrum of EM-type mantle endmembers

Osmium isotopes for volcanic rocks from Grande Comore, the youngest island of the Comores Archipelago, located between East Africa and Madagascar, show systematic differences between the two volcanoes of the island. Os-187/Os-188 ratios (0.15-0.16) in shield-stage alkali basalts from Karthala are among the highest yet measured in ocean island basalts with >40 ppt Os. They are uniform over the whole range of Os abundances of 20-100 ppt and form nearly linear correlations with Sr-87/Sr-86, Nd-143/Nd-144 and He-3/He-4 ratios. In contrast. (OS)-O-187/Os-188 ratios in alkaline lavas from La Grille volcano are more variable (0.129-0.159) and negatively correlate with Os abundances. but not with Sr-Nd-He isotope ratios. Os isotope variability of La Grille samples is consistent with shallow contamination of mantle xenolith-bearing lavas with low Os-187/Os-188 lithospheric mantle and contamination of low Os magmas with high Os-187/Os-188 crust. Karthala magmas are free of such shallow contamination despite the lower Os concentration in some of its lavas. Os-187/Os-188 ratios (excluding contaminated samples) indicate that Grande Comore lavas represent mixtures between melts from the Comores plume and the oceanic lithospheric mantle, with La Grille mostly lithosphere-derived and Karthala plume-derived. The inferred isotopic composition of the metasomatized hydrous lithosphere is typical of the upper mantle. The high Os-187/Os-188 ratio (0.159) of the Comores plume endmember requires dominance of pyroxenite (estimated to be similar to 60%) over periclotite in the melt source. The Comores plume forms yet another extreme among ocean island basalts in Sr-Nd-Pb-He-Os isotope space. In the global context, individual islands with enriched mantle affinities trend towards their own unique enriched mantle endmember. The global data thus do not support the existence of narrowly defined EMI and EMII components [Zindler and Hart (1986), Chemical Geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493-571] but rather indicate that enriched components in the mantle exhibit a spectrum of mantle endmembers. (C) 2009 Elsevier B.V. All rights reserved.