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Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa |
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| Authors: | FURMAN TANYA; KALETA KELLY M; BRYCE JULIA G; HANAN BARRY B |
| Institution: | 1 DEPARTMENT OF GEOSCIENCES, PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PA 16802, USA 2 SHELL EXPLORATION AND PRODUCTION COMPANY, 200 N. DAIRY ASHFORD, HOUSTON, TX 77079, USA 3 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF NEW HAMPSHIRE, DURHAM, NH 03824, USA 4 DEPARTMENT OF GEOLOGICAL SCIENCES, SAN DIEGO STATE UNIVERSITY, SAN DIEGO, CA 92182, USA |
| Abstract: | The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (143Nd/144Nd = 0·51267–0·51283,87Sr/86Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb  30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar |
| Keywords: | : East African Rift System mantle plumes HIMU geochemistry Afar |
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