Petrology and geochemistry of a U and Th enriched nephelinite from Mt. Nyiragongo, Zaire: Its bearing on ancient mantle metasomatism

VOLLMER, R.; NIXON, P. H. and CONDLIFFE, E., 1985: Petrology and geochemistry of a U and Th enriched nephelinite from Mt. Nyiragongo, Zaire: Its bearing on ancient mantle metasomatism. Bull. Geol. Soc. Finland 57, Part 1—2, 37—46. Unusual large Pb isotope variations found by Vollmer and Norry (1983 a) in nephelinites from Nyiragongo and related volcanoes date a 500 Myr old U(Th)/Pb fractionation. U—Th—Pb systematics suggest that U,Th and radiogenic Pb are concentrated in an accessory xenocryst phase. We undertook a detailed mineralogical and chemical study of rock C9872, which is characterised by extreme radiogenic Pb isotope abundances, but did not find such xenolithic phase. Major element abundances of C9872 are close to average Nyiragongo nephelinite compositions. Among minor elements only Th and U are exceptionally enriched by an order of magnitude. Neither melilite nor perovskite were observed in C9872. Melilite may have broken down to a calcium bearing fayalite during late stage Fe metasomatism (Sahama 1961, 1978). Possible U and Th hosts include minerals normally associated with carbonatites such as perovskite or pyrochlore. However, either these phases remained undetected or were broken down during Fe metasomatism, although the undisturbed U—Th—Pb systematics imply that none of these elements were added or lost in this process. We propose that U and Th bearing minerals from a 500 Myr old carbonatite or carbonatite precursor (metasomatised mantle?) were incorporated into the nephelinite magma.