Osmium isotopes and Fe/Mn ratios in Ti-rich picritic basalts from the Ethiopian flood basalt province: No evidence for core contribution to the Afar plume Nick W. Rogers a, ⁎, Marc K. Davies a , Ian J. Parkinson a , Gezahegn Yirgu b a Department of Earth and Environmental Sciences, CEPSAR, The Open University, Milton Keynes, MK76AA, UK b Department of Geology and Geophysics, University of Addis Ababa, PO Box 1176, Addis Ababa, Ethiopia abstract article info Article history: Received 7 December 2009 Received in revised form 18 May 2010 Accepted 20 May 2010 Available online 25 June 2010 Editor: R.W. Carlson Keywords: Afar plume osmium isotopes iron–manganese ratios picrites Variations in the Fe/Mn ratio and Osmium isotopes in magnesium-rich mafic rocks from plume-related volcanic provinces have been exploited to imply the entrainment of core material in mantle plumes and the involvement of ancient recycled oceanic lithosphere. Here we present new major and trace element, 187 Os/ 188 Os ratios and precise Fe/Mn ratios on a suite of MgO-rich basalts, picritic basalts and ankaramites from the 30 Ma Ethiopian flood basalt province that shed new light on these arguments. The lavas show a range of compositions with MgO varying from 5 to 20 wt.% although the primary magma is inferred to have an MgO content of 15–16 wt.%. The lavas are also characterised by low Al 2 O 3 contents (7–9 wt.% at 15 wt.% MgO), implying an origin from ∼ 150 km depth at a temperature in excess of 1600 °C, consistent with an origin in the early phases of Afar mantle plume activity. Osmium isotopes in samples with N 10 wt.% MgO are unradiogenic with 187 Os/ 188 Os b 0.127 while those with lower MgO define a positive correlation with Re/Os and appear to have been contaminated by crustal material. Fe/Mn ratios determined by ICP-MS vary from 65.4 to 78.5 in rocks with N 10 wt.% MgO and show greater variation to both higher and lower values in less magnesian samples. These values are high compared with MORB and Icelandic basalts, comparable with the Fe/Mn ratios of Hawaiian and other selected ocean island basalts, and are a characteristic of the primary magma. There is no evidence to suggest that Fe/Mn is fractionated during peridotite melting at low pressures less than 5 GPa, implying in agreement with previous studies that high Fe/Mn ratios are a compositional feature of the magma source region. The lack of association of radiogenic 187 Os/ 188 Os with the high Fe/Mn ratios of the Ethiopian picritic basalts calls into question the link to possible entrainment of core material in the source of the Afar mantle plume. Similarly, the unradiogenic 187 Os/ 188 Os ratios preclude a significant contribution from ancient recycled oceanic lithosphere. An alternative model is suggested in which melts generated at high pressures (N 7 GPa) during the initial turbulent ascent of the Afar plume head form pyroxene rich veins with high Fe/Mn ratios and high incompatible element contents in a peridotite matrix. These highly fertile source regions contribute significantly to melt compositions during the early phases of plume emplacement. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The source of mantle plumes remains the subject of controversy, in particular whether or not they originate from the core–mantle boundary and, if so, whether or not they entrain components from the core and carry them towards the Earth's surface (Courtillot et al., 2003; Anderson, 2000; Meibom et al., 2003; Brandon et al., 1998; Humayun et al., 2004). Models of the convective flow in the mantle reveal the likelihood of whole-mantle convection (van Keken et al., 2002) while tracer models clearly demonstrate that much of the mantle must have been processed by melting in an environment close to the Earth's surface at least once during the history of the Earth (Huang and Davies, 2007). Mantle plumes, while not universally accepted as a real component of mantle convection (Foulger and Natland, 2003), originate at major thermal or compositional bound- aries within the Earth and the most significant occurs at the base of the mantle. Images from seismic tomography reveal velocity anomalies that may be related to temperature variations extending from regions affected by intra-plate volcanic activity, through the upper and lower mantle to the core–mantle boundary (Montelli et al., 2004; Lei and Zhao, 2006). Thus there is the possibility that material once close to the core-mantle boundary is convected into the upper mantle where it may undergo partial melting and contribute to surface volcanism. Compositional evidence for core material in surface volcanism is much more equivocal. While some have argued that the core may act as a source of unradiogenic He in ocean island and other plume- related basalts (Dale et al., 2009; Porcelli and Halliday, 2001; van Earth and Planetary Science Letters 296 (2010) 413–422 ⁎ Corresponding author. E-mail address: n.w.rogers@open.ac.uk (N.W. Rogers). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.05.027 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl