Rare earth element distribution in lavas and ultramafic xenoliths from the Comores Archipelago, Western Indian Ocean

Abstract

Lavas and included xenoliths from the Comores Archipelago have been analysed for the rare earth elements (REE) La-Lu. Among basaltic lava types fractionation of REE rock/chondrite distribution patterns is more extreme with greater SiO2 undersaturation and contents of incompatible elements. Enrichment and slight fractionation of REE in the rock series basanite-phonolite is considered compatible with a model of fractional crystallisation 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 crystallisation and eclogite fractionation at depth appear unlikely causes for the relative enrichment of light REE (La-Eu) in the undersaturated basalts. This effect is more probably due to mineralogical control during partial melting in the upper mantle. 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.