Carbonate melt from the mantle in the volcanoes of south-east Zambia

Abstract

RECENTLY Wallace and Green1 reported the experimental formation of a dolomitic (Ca–Mg) carbonate melt in equilibrium with peridotite minerals in the range 21–30 kbar and 930–1,080 °C. These results confirm earlier deductions that, in the presence of CO2, initial melts from the mantle would be carbonatitic2,3, and extend the conditions of carbonate melt generation. The dolomitic melt composition also endorses earlier observations of quench dolomite after experimental melting of a natural garnet peridotite4. Dolomitic ashes, erupted from volcanoes near the confluence of the Rufunsa and Luangwa rivers in south-east Zambia5,6, offer the nearest analogue to the experimental melt yet reported. Quenched melt droplets in the volcanics now reveal new evidence indicating a mantle source for this natural dolomite liquid. Specifically, I present here results which show that the liquid contains magnesio-chromite crystals (52% Cr2O3) that match those in mantle peri-dotites, kimberlites and lamproites. In contrast with the experimental liquid, the natural dolomitic melt has a low iron content,and high manganese and strontium, with alkalis virtually absent. High potassium activity is recorded, however, in the intensely metasomatized rocks around the main volcanoes. These differences suggest that the mantle source region chemistry differs from the high-sodium source envisaged in the experiments. The Zambian carbonatites thus reveal new aspects of carbonate melt and fluid activity in the Earth's mantle.