Contr. Mineral. and Petrol. 31,335-346 (1971) 9 by Springer-Verlag 1971 Rare Earth Element Distribution in Lavas and Uhramafic Xenoliths from the Comores Archipelago, Western Indian Ocean M. F. J. FLOWEI~ * Depax~ment of Geology, The University, Manchester M 13 9PL, England Received May 3, 1971 Abstra~A. Lavas and included xenoliths from the Comores Archipelago have been analysed for the rare earth elements (I~EE) L~--Lu. Among basaltic lava ~rpes fraetionation of REE rock/chondrite distribution patterns is more extreme with greater Si02 undersaturation and con- tents of incompatible elements. Enrichment and slight fractionation of REE in the rock series ba~nite-phonolit.e is considered compatible with a model of fractional erystallisation at low pressures involving mainly olivine and clinopyroxene, and to a much lesser extent, plagioclase. Apatite is probably effective in curtailing further enrichment of REE. High level fractional cryst~llisation and eclogite fraetiona~ion at depth appear unlikely causes for the relative enrichment of light REE (La--Eu) in the unders~turated basalts. This effect is more probably due to miueralogical control during partial melting in the upper mant~le. Lherzolite xenoliths are poor in REE, exhibiting a slight relative depletion in the light REE. These patterns are interpreted as those of possible mantle material subjected to small degrees of partial melting, although not necessarily related to those melts erupted as lava flows at the surface. Introduction Recent studies on crustal rock t323es reveal that rare earth elemenr contents have become both enriched and fraetionated relative to chomL~itie abundances during their evolution from the Earth's mantae; (Haskin and Gehl, 1962; Frey and ttaskin, 1964; Schmitt etal., 1963, 1964; Jensen and Brunfelt, 1965; Balashov, 1966; Ha.skin etal., 1966; Herrmann, 1968; Masuda, 1968; Frey, 1969, 1970; Graham and Nicholls, 1969; etc.). It has further been suggested that the rare earth element (REE) distributions of basaltic magmas can be interpreted as complementing those of postulated mantle "residua", such as lherzolite (Frey, 1969; Nagasawa et al., 1968). The crust-mantle REE mass balances ilLferred from such studies are consistent with ehondritie type REE distribution patterns for undifferentiated terrestrial materials. The view has been put forward that basaltic lavas have evolved chemically during their ascent to the surface, a consequence mainly of fractional crystalli. sation under both low and high pressure regimes, (0'Hara, 1968). Although enrichment of REE may well be attributed to such processes, their fractionation from the chondrite pattern is not easily explained in terms of s'unple crystal fraetionation (Schilling and Winchester, 1969). The observed RE ~raetionation patterns may therefore prove to be inherent characteristics of liquids formed in the upper mantle, retained by the erupted lavas in spite of subsequent chemical evolution. * Present address: Institut f~r Mineralogie, Ruhr-Universitat, BRD 4630 Bochum, Germany.