An experimental determination of primary carbonatite magma composition

Abstract

Carbonatites are uncommon carbonate-rich rocks usually found in continental intra-plate regions and often associated with rifting. There has been much debate as to whether carbonatite magmas are primary melts derived from partial melting of mantle peridotite, or are formed by exsolution of an immiscible carbonate melt fraction from phonolitic or nephelinitic magmas. Our experiments on the phase relationships of carbonate and amphibole-bearing peridotite (containing 0.3% H₂O and 0.5–2.5% CO₂) show that sodic dolomitic carbonatite magma coexists with an amphibole lherzolite assemblage in a field ranging from 21 to 30 kbar and 930 to 1,080 °C, spanning a pressure and temperature interval between the solidus and the amphibole breakdown and melting curve. Thus primary carbonatite melts may occur under suitable geothermal conditions. The nature of the peridotite solidus and of the melting reactions differ considerably from published models^1–3. The carbonatite melt composition, determined by a series of 'sandwich' experiments, was found to be rich in Na, Mg, Ca and Fe, with a small dissolved silicate content. This melt quenches to an assemblage of dolomite and Na–Mg carbonate minerals, producing textures similar to those preserved in samples from Oldoinyo Lengai⁴, Homa mountains, Tanzania⁵ and Kaiserstuhl, Germany⁶.