1. Department of Earth and Planetary Sciences, MSC03 2040, 1 University of New Mexico, New Mexico 87131-0001, USA
2. Centre de Recherches Pétrographiques et Géochimiques, Nancy-Université, CNRS-INSU, BP 20, 54501 Vandoeuvre Lès Nancy Cedex, France
3. Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, California 92093-0244, USA
4. Department of Mining and Mineral Processing Engineering, University of Dar Es Salaam, PO Box 35131, Dar Es Salaam, Tanzania

Correspondence to: T. P. Fischer¹ Correspondence and requests for materials should be addressed to T.P.F. (Email: fischer@unm.edu).

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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