IS THE WORLD'S ONLY CARBONATITE VOLCANO A DRY / ANHYDROUS SYSTEM?
For carbonatite systems in general, it is thought that the water is released upon crystallisation at depth, resulting in a fenitised aureole containing minerals with abundant H2O-rich fluid inclusions. However, recent investigations of the in-situ compositions of fluid inclusions trapped within fenitised rocks from Oldoinyo Lengai demonstrate that the metasomatism is characterised by CO2-rich and H2O-poor fluids, which is an entirely new discovery. The studied inclusions reveal the presence of alkali-rich and H2O-poor fluids that have precipitated secondary carbonatite minerals thereby resulting in a residual high-density CO2 fluid in the inclusions. The fluids and daughter minerals present are derived from a degassing, alkali-rich carbonatitic magma body with a high CO2/H2O value. Combined with trapping pressures equivalent to the lower crust at~30 km, which is at a much greater depth than the proposed magma-storage system for this volcano this would suggest a deeper-mantle origin for the carbonatitic melt.