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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. (238U/232Th) range from 0.362 to 1.036, (230Th/232Th) from 0.503 to 1.109, with (230Th/238U) ranging from 0.783 to 2.966. All but two samples are in 230Th excess or in secular equilibrium. Samples with elevated (238U/232Th), also have Rb/Cs>120, but unexceptional 208Pb⁎/206Pb⁎ 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 (238U/232Th) generally lower than OIB, with maximum values of <0.8, and some <0.6. The maximum (230Th/238U) is 1.39, similar to OIB. Although none of the basalts has a primary composition, (230Th/238U) does not vary systematically with indices of fractionation, and comparison with evolved rocks from Kenya indicates that 238U–230Th disequilibrium in the basalts is not the product of fractionation and crustal residence, but a product of melt generation. The maximum (230Th/238U) that can be generated by batch melting, assuming a source mineralogy consistent with the REE variation is 1.05 and so the variation in (230Th/238U) 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. [Copyright &y& Elsevier]

A suite of peralkaline trachytes from Longonot volcano, Kenya, which erupted during the last 6000 years, has been analysed for major and trace elements, Pb and Nd isotopes, and U–Th–Ra disequilibria. The lavas are divided into three stratigraphic groups of trachytes (Lt2a, Lt2b and Lt3), and hybrid lavas, designated LMx1 and LMx2, which, respectively, pre-date and post-date the Lt2 lavas. Major and trace elements are consistent, with up to 37% within-group fractional crystallization of predominantly alkali feldspar. The parental magma for the different trachyte groups had a more mafic composition—probably hawaiitic. Nd and Pb isotopes show minimal variation, both within and between magma groups, and indicate that up to 10% comendite magma from the neighbouring Olkaria volcanic field may have intermixed with the Longonot magma. (230Th/238U) disequilibria indicate that limited U/Th fractionation occurred during the past 10 kyr, whereas (226Ra/230Th) disequilibria reflect the effect of alkali feldspar fractionation >8 kyr ago in the Lt2a lavas, between 3 and 7 kyr ago in the Lt2b lavas and in the past 3 kyr for the Lt3 lavas. (226Ra/230Th) disequilibria in the Lt2b lavas are interpreted using a model that combines the equations of radioactive decay and in-growth with Rayleigh crystallization to give fractionation rates of about 0·2 × 10−4/year for the evolution of hawaiite to trachyte, but more rapid rates of up to 3 × 10−4/year for fractionation within the trachyte sequence. (226Ra/230Th) from two whole-rock–alkali feldspar pairs are interpreted to show the crystals formed at 5800 years BP (Lt2b) and 2800 years BP (Lt3), implying that phenocryst formation continued almost up to the time of eruption. The results strongly indicate that fractionated magmas can be stored for periods on the order of 1000–2500 years prior to eruption, whereas other magmas were erupted as fractionation was proceeding. [ABSTRACT FROM AUTHOR]