HR: 16:45h
AN: S52J-04    [PDF]
TI: 40 Million Years of Mafic Volcanism in Turkana, Kenya: Geochemical Insights
AU: Knight, K
EM: kelly.m.knight@exxonmobil.com
AF: ExxonMobil Development Company, 12450 Greenspoint Drive, Houston, TX 77060 United States
AU: * Furman, T
EM: furman@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Bryce, J
EM: julie.bryce@unh.edu
AF: University of New Hampshire, Department of Geosciences, Durham, NH 03824 United States
AB: The Turkana region in northern Kenya is the most highly extended portion of the East African Rift System. At least three superposed rifting episodes have affected this area since the Cretaceous, resulting in a highly attenuated crustal section. Mafic volcanism commenced in Turkana over 40 Ma and continues through the present. The geochemistry of Turkana mafic lavas appears dominated by contributions from an asthenospheric source region throughout this time period, with recognizable but volumetrically lower input from the continental lithosphere. Mantle plume inputs cannot be identified uniquely in this area but cannot be ruled out. Major element compositions of mafic lavas, back-corrected for olivine fractionation, indicate substantial shallowing of melt generation depth from Tertiary through the present. Concomitant changes in Ce/Y and Zr/Nb support this model, though all melts appear to be derived from clinopyroxene-rich spinel peridotite. Values of Ce/Pb, La/Nb, Nb/U differ only slightly between Tertiary and Quaternary lavas, and all are within the ranges expected for asthenosphere-derived basalt for most samples. Tertiary Turkana mafic lavas have Sr and Nd isotope values that overlap those of Quaternary Turkana basalts (0.7031-0.7035, 0.5128-0.5130). Pb isotope signatures of Turkana lavas have remained homogeneous over the past 3.5 Ma and are consistent with contributions from a deep mantle (e.g., FOZO or C) source. In contrast, late Eocene through Oligocene mafic lavas from Kajong and Porr volcanic centers have Pb isotope values that are more highly radiogenic than the Quaternary Turkana lavas, suggesting input from a lithospheric source region. The temporal geochemical changes observed at Turkana contrast with those seen in southern Ethiopia less than 100 km to the north. Most importantly, trace element and isotopic data require southern Ethiopian basalts of all ages to reflect primarily lithospheric melting. In addition, the radiogenic Pb signatures observed at Turkana 17-40 Ma are found in Ethiopian basalts 11-19 Ma but not 35-45 Ma, making it possible to determine a rough chemical stratigraphy of the regional lithosphere.
DE: 1040 Isotopic composition/chemistry
DE: 3670 Minor and trace element composition
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Seismology [S]
MN: 2003 Fall Meeting