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*, Marc K. Davies, Ian J. Parkinson, Gezahegn Yirgu

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

24 Citations (Scopus)

Abstract

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 similar to 150 km depth at a temperature in excess of 1600 degrees C, consistent with an origin in the early phases of Afar mantle plume activity. Osmium isotopes in samples with >10 wt.% MgO are unradiogenic with (187)Os/(188)Os10 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 (>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. (C) 2010 Elsevier B.V. All rights reserved.

Original languageEnglish
Pages (from-to)413-422
Number of pages10
JournalEarth and Planetary Science Letters
Volume296
Issue number3-4
DOIs
Publication statusPublished - 1 Aug 2010

Keywords

  • RE-187-OS-187 SYSTEMATICS
  • MELTS
  • GEOCHEMICAL EVIDENCE
  • OCEANIC BASALTS
  • RE-OS ISOTOPES
  • osmium isotopes
  • ORIGIN
  • MANTLE PLUMES
  • Afar plume
  • ABYSSAL PERIDOTITES
  • LITHOSPHERE
  • EARTHS MANTLE
  • picrites
  • iron-manganese ratios

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