9614508 Graham The long term goal of this study is to provide the geochemical framework to help detail the timing and the nature of the interactions between the mantle plume, the convecting upper mantle and the subcontinental lithospheric mantle during the evolution of the East African Rift. The preliminary data has shown marked geochemical differences between various provinces on the western branch of the Rift. These differences can be related to the timing of ancient metasomatic events and to the development of isotopically-discrete domains within the subcontinental lithosphere. Whether these isotopic systematics are a general feature of the western branch of the rift remains to be demonstrated. Continuation of this line of study will be done by analysing He and radiogenic isotope compositions for classic suites of the Toro-Ankole and Virunga provinces and by dating lavas from the South Kivu province where the largest variations in radiogenic signatures have been observed. High 3He/4He ratios are known to be present in volcanic rocks of the Afar region and the northern parts of the main Ethiopian Rift and they are interpreted to reflect the influence of a mantle plume. A second goal of this study is to track the effects of this plume signature in pre-rift and post-rift magmas of the southern Ethiopian Rift.
