Pyroxene phenocrysts from nephelinites associated with the Tertiary carbonatite-nephelinite Napak volcano are not in isotopic equilibrium with their host rocks and reveal a complicated evolutionary history that involved fractional crystallization of a melt undergoing continuous isotopic change. Intrasample pyroxene populations have different isotopic compositions and are interpreted as earlier formed pyroxenes that failed to isotopically equilibrate with the host liquids and that were later scavenged by ascending melts. The Napak data show that the pyroxenes can be effective in monitoring isotopic shifts that occur during magma evolution.

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