HR: 14:35h
AN: S52G-04    [PDF]
TI: Magma Genesis during Continental Rifting: Isotopic Constraints from Quaternary lavas of the Main Ethiopian Rift
AU: * Bryce, J G
EM: julie.bryce@unh.edu
AF: Dept. Earth Sciences, University of New Hampshire, Durham, NH 03824 United States
AU: Furman, T
EM: furman@geosc.psu.edu
AF: Dept. Geological Sciences, Pennsylvania State University, University Park, PA 16802 United States
AU: Hanan, B B
EM: barry.hanan@sdsu.edu
AF: Dept. Geological Sciences, San Diego State University, San Diego, CA 92812 United States
AU: Ayalew, D
AF: Dept. Geology and Geophysics, University of Addis Ababa, Addis Ababa, Box1176 Ethiopia
AU: Yirgu, G
AF: Dept. Geology and Geophysics, University of Addis Ababa, Addis Ababa, Box1176 Ethiopia
AB: The East African Rift System (EARS) provides an unparalleled opportunity to study the links between tectonic activity and magma genesis in a plume-driven continental rift setting. As an example, the geochemistry of primitive EARS lavas offers detailed information about the interplay between lithospheric and sublithospheric magma sources in the continental rifting process. In order to test for geographical control on this process in the active Main Ethiopian Rift, we have studied magnesian basalts from five Quaternary volcanic centers in the Main Ethiopian Rift (MER: Fantale, Kone, Boseti, Gedemsa, and Tulu Moye), which lie immediately south of the highly extended Afar region. Incompatible element ratios (e.g., Zr/Nb, La/Nb) and isotopic data (Sr,Nd) of these MER lavas trend towards primitive earth values, signaling incorporation of both enriched and depleted mantle components. There are, however, lithospheric or crustal overprints in the more evolved basaltic samples (those with MgO $<6$ wt.~%), as have been found in the southern portion of the study area (1). Both isotopic and incompatible trace elemental signatures of the more primitive samples support the notion of increased lithospheric involvement in the generation of basaltic magmas with increasing distance from the Afar region. This phenomenon of lithospheric erosion during the onset of continental rifting has been observed in other temporally controlled suites in the EARS (e.g., (2),(3)) as well as other sites of continental rifting (e.g., (4)). High precision Pb isotopic analyses currently underway on the Quaternary MER samples will provide a more detailed picture of the number of source contributions and the nature of the mixing and transport processes significant in the genesis of MER lavas. (1) Trua et al., Chem Geol, 155, 201-231, 1999. (2) Furman and Graham, Lithos, 48, 237-262, 1999. (3) Furman et al., J Pet, in press (4) DePaolo and Daley, Chem. Geol. 169, 157-185, 2000.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Seismology [S]
MN: 2003 Fall Meeting