Unusual isotopic variations in Nyiragongo nephelinites

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 basalts^3,4.