HR: 14:25h
AN: V33D-04    [Abstracts]
TI: Istopically Defined Source Reservoirs of Primitive Magmas in the East African Rift.
AU: * Rooney, T O
EM: tor102@psu.edu
AF: Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802 United States
AU: Furman, T
EM: furman@geosc.psu.edu
AF: Dept. of Geosciences, The Pennsylvania State University, University Park, PA 16802 United States
AU: Hanan, B
EM: bhanan@mail.sdsu.edu
AF: Department of Geological Sciences, San Diego State University, San Diego, CA 92182 United States
AB: Extension within the East African Rift is a function of the interaction between plume-driven uplift and far-field stresses associated with plate tectonic processes. Geochemical and isotopic investigation of primitive basalts from the Main Ethiopian Rift (MER) reveals systematic spatial variations in the contributions from distinct and identifiable source reservoirs that, in turn help identify the mechanisms by which along-axis rifting has progressed. The Sr-Nd-Pb isotopic characteristics of MER basalts can be described by a three-component mixing model involving the long-lived Afar plume, a depleted mantle component similar to the source region for Gulf of Aden MORB from east of 48° E and a reservoir that is likely lithospheric (sub-continental mantle lithosphere, magmatic underplate or lower crust). Quaternary basalts in the central MER exhibit a systematic decrease in plume influence southward from 9.5° N to 8° N, i.e., away from the modern surface expression of the Afar plume in Djibouti and Erta 'Ale. The composition of the Afar plume component is comparable to the "C" mantle reservoir. This southward decrease in plume influence is coupled with an increase in the influence of the lithospheric and depleted mantle components. Linear arrays observed within Pb-Pb isotopic space at each eruptive center require distinctive ratio of lithospheric + depleted mantle components mixing with variable amounts of the "C"-like plume component. This isotopic evidence suggests the depleted mantle and lithosphere mixed prior to the generation of the recent magmas. To the south, the Sr-Nd-Pb isotopic compositions of Turkana (Kenya) rift basalts record a mix of a similar "C"-like plume component and a fourth HIMU-like source component. Low ³He/⁴He values observed in the HIMU-dominated lavas from Turkana contrast with the higher ratios found in basalts associated with the "C"-like Afar plume. Further analysis of "C"-HIMU lavas at Turkana is required to fully constrain the He isotopic signatures. Thus, along-axis patterns in Quaternary EARS magmatism are compatible with two "C"-like plumes with contributions from the upper mantle and chemically distinct lithospheric components. Alternatively, a single "C"-like plume can account for these relationships. In the single plume scenario, the HIMU source component present in the 30 Ma Turkana lavas may represent melting of metasomatised lithosphere, derived from the accretion of island-arc-backarc basins during Pan-African events (e.g. Schilling et al., 1992). The recent plume-dominated activity in Turkana and Afar are separated by a region characterized by waning plume influence and a greater contribution from the depleted mantle. This intermediate zone, which is located in the south-central MER represents the modern site of contact between the northward propagating Kenya / Turkana Rift and the southward propagating Afar Rift zone.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005