The oxygen isotope composition of Karoo and Etendeka picrites: High δ18O mantle or crustal contamination?

Abstract

Oxygen isotope compositions of Karoo and Etendeka large igneous province (LIP) picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation. Olivine and orthopyroxene phenocrysts from lavas and dykes (Mg# 64–80) from the Tuli and Mwenezi (Nuanetsi) regions of the ca 180 Ma Karoo LIP have δ18O values that range from 6.0 to 6.7 ‰. They appear to have crystallized from magmas having δ18O values about 1–1.5 ‰ higher than expected in an entirely mantle-derived magma. Olivines from picrite and picrite basalt dykes from the ca 135 Ma Etendeka LIP of Namibia and Karoo-age picrite dykes from Dronning Maud Land, Antarctica, do not have such elevated δ18O values. A range of δ18O values from 4.9 to 6.0 ‰, and good correlations between δ18O value and Sr, Nd and Pb isotope ratios for the Etendeka picrites are consistent with previously proposed models of crustal contamination. Explanations for the high δ18O values in Tuli/Mwenezi picrites are limited to (1) alteration, (2) crustal contamination, and (3) derivation from mantle with an abnormally high δ18O. Previously, a variety of models that range from crustal contamination to derivation from the ‘enriched’ mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as K, and average εNd and εSr values of −8 and +16 in Mwenezi (Nuanetsi) picrites. However, the primitive character of the magmas (Mg# 73), combined with the lack of correlation between δ18O values and radiogenic isotopic compositions, MgO content, or Mg# is inconsistent with crustal contamination. Thus, an 18O-enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated. High δ18O values are accompanied by negative Nb and Ta anomalies, consistent with the involvement of the mantle lithosphere, whereas the high δ18O themselves are consistent with an eclogitic source. Magma δ18O values about 1 ‰ higher than expected for mantle-derived magma are also a feature of the Bushveld mafic and ultramafic magmas, and the possibility exists that a long-lived 18O-enriched mantle source has existed beneath southern Africa. A mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during Archaean subduction.

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Acknowledgments

We are grateful to Falconbridge for supplying drill core samples for analysis and permission to publish, to Fayrooza Rawoot and John Lanham for technical assistance, to David Peate and Grant Cawthorn for discussion. The financial assistance of the National Research Foundation of South Africa (NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the authors and are not necessarily to be attributed to the NRF. UCT provided additional funding to CH for this research. We are grateful to the assistance of Malcolm Roberts in the operation of the EMP at CMCA-UWA. John Cliff is acknowledged for his technical support on the SIMS 1280 at CMCA-UWA. The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, the University of Western Australia, a facility funded by the University, State and Commonwealth Governments. Very constructive reviews from James Day, Joel Baker and an anonymous reviewer helped to improve the final version.

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Communicated by Jochen Hoefs.

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Harris, C., le Roux, P., Cochrane, R. et al. The oxygen isotope composition of Karoo and Etendeka picrites: High δ18O mantle or crustal contamination?. Contrib Mineral Petrol 170, 8 (2015). https://doi.org/10.1007/s00410-015-1164-1

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Keywords

  • Picrite
  • Olivine
  • Oxygen isotope
  • Karoo
  • Etendeka
  • Large igneous province