Nature 301, 141 - 143 (13 January 1983); doi:10.1038/301141a0

Unusual isotopic variations in Nyiragongo nephelinites

R. Vollmer^* & M. J. Norry^†

^*Department of Earth Sciences, The University, Leeds LS2 9JT, UK
^†Department of Geology, University of Leicester, Leicester LE1 7RH, UK

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 basalts^3,4.

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