Abstract
Carbonatite lavas are highly unusual in that they contain almost no SiO2 and are >50 per cent carbonate minerals. Although carbonatite magmatism has occurred throughout Earth’s history, Oldoinyo Lengai, in Tanzania, is the only currently active volcano producing these exotic rocks1. Here we show that volcanic gases captured during an eruptive episode at Oldoinyo Lengai are indistinguishable from those emitted along mid-ocean ridges, despite the fact that Oldoinyo Lengai carbonatites occur in a setting far removed from oceanic spreading centres. In contrast to lithophile trace elements, which are highly fractionated by the immiscible phase separation that produces these carbonatites, volatiles (CO2, He, N2 and Ar) are little affected by this process. Our results demonstrate that a globally homogenous reservoir exists in the upper mantle and supplies volatiles to both mid-ocean ridges and continental rifts. This argues against an unusually C-rich mantle being responsible for the genesis of Na-rich carbonatite and its nephelinite source magma at Oldoinyo Lengai. Rather, these carbonatites are formed in the shallow crust by immiscibility from silicate magmas (nephelinite), and are stable under eruption conditions as a result of their high Na contents.
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Acknowledgements
We would like to thank CNRS-INSU for financial support, the French Embassy in Dar Es Salaam and The University of New Mexico Research Allocation Committee for support of field work, the US National Science Foundation for analytical support at the Scripps Institution of Oceanography (EAR-0439122) and the University of New Mexico (EAR-0537618, EAR- 0827352). We thank the Tanzania Commission for Science and Technology for granting research permission (2005-217-NA-2005-74) for this study. We thank C. Ballentine for a thorough review that greatly helped to improve this paper.
Author Contributions T.P.F. collected gas samples for chemical and isotopic analyses and analysed gas chemistry and N isotopes; P.B. and B.M. designed the study, led the field expedition to Oldoinyo Lengai and obtained funding for the expedition and permits; D.R.H. led the analysis of He, Ar and C isotopes and CO2/3He ratios; T.P.F., P.B., B.M. and D.R.H. collectively wrote the paper; E.F. analysed noble gases and C isotopes at the Scripps Institution of Oceanography; F.P. helped with sample collection; Z.D.S. supported N isotope analyses at the University of New Mexico; and F.M. helped with logistics in Tanzania, organizing the field expedition and obtaining permits.
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Fischer, T., Burnard, P., Marty, B. et al. Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites. Nature 459, 77–80 (2009). https://doi.org/10.1038/nature07977
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DOI: https://doi.org/10.1038/nature07977
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