Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites

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 rocks(1). 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, N-2 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.