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Nd, Pb and Sr isotopic data from the Napak carbonatite-nephelinite centre, eastern Uganda: an example of open-system crystal fractionation

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Abstract

Nd, Pb and Sr isotopic data from nephelinite lavas from the Tertiary nephelinite-carbonatite complex of Napak, eastern Uganda, show large isotopic variations that can only be attributed to open-system behaviour. Possible explanations of the data include mixing between nephelinitic melts derived from an isotopically heterogeneous mantle, or interaction between a HIMU melt and mafic granulites. In both models crystal fractionation, involving olivine and clinopyroxene, played an important role. Major element chemistry, textural evidence and isotopic data from clinopyroxene phenocrysts from the olivine-bearing nephelinites, suggest that the pyroxenes did not crystallize from their host liquids. The isotopic data from the clinopyroxene phenocrysts support an interpretation of crystal fractionation in an open magma system that was undergoing continuous isotopic change. This study emphasises the importance of using combined isotopic data from both whole rock and mineral phases to interpret the evolutionary history of a single eruptive centre.

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  1. Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, K1S 5B6, Ottawa, Ontario, Canada

    Antonio Simonetti & Keith Bell

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  1. Antonio Simonetti
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Simonetti, A., Bell, K. Nd, Pb and Sr isotopic data from the Napak carbonatite-nephelinite centre, eastern Uganda: an example of open-system crystal fractionation. Contr. Mineral. and Petrol. 115, 356–366 (1994). https://doi.org/10.1007/BF00310774

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