Letter | Published:

Unusual isotopic variations in Nyiragongo nephelinites

Nature volume 301, pages 141143 (13 January 1983) | Download Citation

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Abstract

Nd and Sr isotopic ratios in Nyiragongo nephelinites are fairly constant and close to ‘bulk Earth’1,2 estimates. By contrast, 206Pb/204Pb variations are the largest ever observed for young volcanic rocks. If the 500 Myr Pb–Pb age dates—as we believe—a mantle source event, then the inferred parent–daughter element fractionation patterns suggest this to be a metasomatic event. The data indicate that apparently primitive Nd and Sr isotopic compositions can be deceptive: the mantle under Nyiragongo does not seem to be primitive and undifferentiated but experienced, at different times, large-ioniclithophile element depletion and enrichment. We suggest here that mantle metasomatism, by causing erratic and sometimes severe fractionation of U from Pb, could be responsible for the more complex evolution of the U–Pb system relative to the Rb–Sr and Sm–Nd systems observed in oceanic basalts3,4.

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Affiliations

  1. Department of Earth Sciences, The University, Leeds LS2 9JT, UK

    • R. Vollmer
  2. Department of Geology, University of Leicester, Leicester LE1 7RH, UK

    • M. J. Norry

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DOI

https://doi.org/10.1038/301141a0

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