HR: 11:10h
AN: V42B-04    [Abstracts]
TI: Two Plumes Beneath the East African Rift System: a Geochemical Investigation into Possible Interactions in Ethiopia
AU: * Nelson, W R
EM: wnelson@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802, United States
AU: Furman, T
EM: furman@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802, United States
AU: van Keken, P E
EM: keken@umich.edu
AF: University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109, United States
AU: Lin, S
EM: skylin0@ntu.edu.tw
AF: National Taiwan University, Department of Geosciences, Taipei, 106, Taiwan
AB: East African Rift System magmatism began over 40 my ago and has continued through the present. Numerical models have determined two plumes are necessary to create the spatial and temporal distribution of volcanism. Geochemical data support the presence of two chemically distinct plumes initially located beneath the Afar Depression (NE Ethiopia) and the Turkana Depression (SW Ethiopia/N Kenya). The timing and eruptive of the Afar and Kenya plumes are also distinct. While there is growing evidence to support the existence of two dynamically and chemically distinct plumes beneath the East African Rift System, the interactions between them remain unclear. Our study focuses on the geochemistry of mafic shield lavas from three locations on the eastern flank of the Ethiopian plateau. These lavas are spatially located between the surface manifestation of the Afar and Kenya plumes. The majority of the lava is alkaline and has experienced varying degrees of olivine and pyroxene fractionation. The northernmost lavas (9°10'N) are transitional and display the most fractionation. Primitive mantle melts were generated at depths near the fertile mantle garnet-spinel transition zone and deeper (80-100km) and are free of metasomatic influence. Minor HREE depletions also support derivation of melts from a garnet-bearing source. Lavas with lithospheric influence are generated from shallower depths and show minor amphibole influence. Overall, geochemical data show the lavas in this study closely resemble those from various episodes of Kenya plume magmatism with modifications attributed to lithospheric contamination. This interpretation is consistent with current numerical models suggesting episodic northward movement of Kenya plume magmas along the lithosphere-asthenosphere boundary. The data imply that the Kenya plume has a much larger spatial influence and therefore a larger geodynamic influence in the EARS than previously recognized.
DE: 1065 Major and trace element geochemistry
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3621 Mantle processes (1038)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting