Summary: The incompatible trace element abundances of Rungwe mafic lavas require melting of an enriched and metasomatized peridotite source. Lavas of varying degree of silica saturation and erupted from a single volcano also appear to have experienced different degrees of interaction with the crust. At the same time, the helium isotopes measured in olivine and clinopyroxene phenocrysts suggest an asthenospheric contribution to magmatism in the Western Rift. Heterogeneities in trace element and isotope composition and, by inference, in minor phase mineralogy of the source regions, occur at scales of 10 km or less, similar to that beneath the Katwe-Kikorongo field in southwest Uganda (Davies and Lloyd, 1989) and beneath the Virunga province (Rogers et al., 1992; Williams and Gill, 1992).