LIQUID IMMISCIBILITY IN A NEPHELINITE CARBONATE SYSTEM AT 25-KBAR AND IMPLICATIONS FOR CARBONATITE ORIGIN

MANTLE-DERIVED carbonate-rich melts may have an important role in mantle metasomatism1,2, and may serve as parent liquids for crustal carbonatite magmas3,4. Experiments have shown that carbonatitic melts can be produced by partial melting of peridotite + CO2 + H2O above 22 kbar (ref. 3), and that silicate and carbonate liquids are immiscible between 2 and 15 kbar for a wide range of Ca/Na ratios5-7. We have determined the extent of silicate-carbonate liquid immiscibility at 25 kbar and 1,050-1,300°C using mixtures of magnesian nephelinite, dolomite and sodium carbonate with and without water. In contrast to the low-pressure data, the two-liquid field at 25 kbar is restricted to more sodium-rich compositions, far removed from natural mantle melts. Our experimental results suggest that neither partial melting of carbonated peridotite, nor extensive fractional crystallization of silicate magmas at depths corresponding to 25 kbar, are likely to generate carbonatitic magmas by liquid immiscibility. © 1990 Nature Publishing Group.