The Nd and Sr isotopic compositions of natrocarbonatite, nephelinite, phonolite and plutonic blocks from Oldoinyo Lengai form a distinct negative correlation, and closely follow the East African Carbonatite Line (EACL). The 143Nd/143Nd ratios of the lavas range from 0.51249 to 0.51269, those of plutonic blocks from 0.51174 to 0.51270. The 87Sr/86Sr of the lavas range from 0.70414 to 0.70512; plutonic blocks range from 0.70378 to 0.70861. The nephelinites fall into two distinct groups; those with the more depleted isotopic signature include the combeitebearing nephelinites. Published data from mantle xenoliths, both metasomatized and nonmetasomatized, fall along the same array and suggest the interaction of two components, one enriched and the other slightly depleted relative to bulk Earth and CHUR. Isotopic variations indicate that the rocks from Oldoinyo Lengai cannot be derived by magmatic differentiation alone; at least two spatially related components are involved in the origin of rocks from Oldoinyo Lengai.
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- Nd and Sr Isotope Systematics of the Active Carbonatite Volcano, Oldoinyo Lengai
J. B. Dawson
- Springer Berlin Heidelberg