238 U– 230 Th disequilibrium in recent basalts and dynamic melting beneath the Kenya rift N.W. Rogers a, ⁎ , L.E. Thomas a , R. Macdonald b , C.J. Hawkesworth c , F. Mokadem a a Department of Earth Sciences, CEPSAR, The Open University, Milton Keynes, MK7 6AA, UK b Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK c Department of Earth Sciences, The University of Bristol, Wills Memorial Building, Queens Rd., Bristol, BS8 1RJ, UK Received 7 October 2005; received in revised form 27 April 2006; accepted 2 May 2006 Abstract Trace element and U-series isotope analyses are presented for a suite of recent (<10 ka) basalts from the axial portion of the Kenya rift. Samples from throughout the rift have LREE-enriched patterns with HREE > 10 × chondrite and the LREE between 60 and 200 × chondrite. REE fractionation is consistent with melting a garnet lherzolite source region with between 2% and 6% modal garnet. Other trace element ratios are distinct from OIB, notably Zr/Hf which ranges from 43 to 48, whilst at a given Zr content the Zr/Hf ratio is significantly greater than that found in OIB. ( 238 U/ 232 Th) range from 0.362 to 1.036, ( 230 Th/ 232 Th) from 0.503 to 1.109, with ( 230 Th/ 238 U) ranging from 0.783 to 2.966. All but two samples are in 230 Th excess or in secular equilibrium. Samples with elevated ( 238 U/ 232 Th), also have Rb/Cs > 120, but unexceptional 208 Pb ⁎ / 206 Pb ⁎ and hence κ Pb values. These samples have experienced U and Cs loss and are excluded from further consideration. Of the unaltered samples, all have ( 238 U/ 232 Th) generally lower than OIB, with maximum values of < 0.8, and some < 0.6. The maximum ( 230 Th/ 238 U) is 1.39, similar to OIB. Although none of the basalts has a primary composition, ( 230 Th/ 238 U) does not vary systematically with indices of fractionation, and comparison with evolved rocks from Kenya indicates that 238 U– 230 Th disequilibrium in the basalts is not the product of fractionation and crustal residence, but a product of melt generation. The maximum ( 230 Th/ 238 U) that can be generated by batch melting, assuming a source mineralogy consistent with the REE variation is 1.05 and so the variation in ( 230 Th/ 238 U) is attributed to more complex models of melt generation and/or transport. Both dynamic melting and equilibrium porous flow suggest mantle upwelling rates of ≤ 2 cm year - 1 . It is suggested that the Kenya basalts represent melts derived from lithospheric mantle that has been thermally reactivated by and incorporated into the underlying (East African) mantle plume. © 2006 Elsevier B.V. All rights reserved. Keywords: U-series; Trace elements; Basalts; Kenya rift; Melt generation 1. Introduction Continental basaltic magmatism reveals a composi- tional diversity that is somewhat greater than that observed in the oceanic realm. While in part this relates to the interaction of mafic magmas with the more easily fusible parts of the continental crust, much composi- tional diversity has been attributed to magma sources located in the continental mantle lithosphere—that part of the mantle attached to the base of the continents, forming part of the plate structure that does not take part in mantle convection. Petrological investigations Chemical Geology 234 (2006) 148 – 168 www.elsevier.com/locate/chemgeo ⁎ Corresponding author. Tel.: +44 1908 652013; fax: +44 1908 655151. E-mail address: n.w.rogers@open.ac.uk (N.W. Rogers). 0009-2541/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2006.05.002