Relationship of melilite-bearing rocks to kimberlite: A preliminary report on the system akermanite-CO 2

Abstract

The maximum stability limit of akermanite in the presence of excess CO 2 was found to be about 6 kbar. Below that pressure, at relatively lower temperatures, akermanite reacts with CO 2 to form diopside + calcite. Akermanite is unstable below 1010°C, 2 kbar; 1180°, 5 kbar; and 1240°, 6.1 kbar. The melting temperature of akermanite is depressed by CO 2 owing to the solubility of CO 2 in the liquid: 1454°C, 1 kbar; 1405°, 2 kbar; 1365°, 5 kbar; and 1355°, 5.8 kbar. The complete melting of diopside + calcite (1:1 mole) above 6 kbar and the melting of akermanite in the presence of excess CO 2 below 6 kbar produce homogeneous liquids; therefore, carbonates are not necessarily immiscible in silicate melts of all ultrabasic compositions. The restriction of akermanite to relatively low pressures in the presence of CO 2 and H 2O, respectively, indicates that melilite-bearing rocks are not likely sources of kimberlite in the mantle. Melilite-bearing rocks recrystallize at high pressures to pyroxenites in the absence of CO 2 and H 2O. On the other hand, simplified reactions representing the conversion of ugandite or mafurite in the presence of CO 2 and H 2O to forsterite + phlogopite + calcite, the groundmass assemblage of kimberlite, indicates that a magma having melilite affinities could be transformed into kimberlite with the aid of suitable volatiles. The experimental results are also pertinent to Bowen's step 8 in the metamorphism of a siliceous dolomite. Because of the exceptionally high temperatures involved where CO pressure equals the total pressure, the reaction in nature appears to run at very low partial pressures of CO 2. Preliminary results on the melting of calcite cleavage rhombs indicate congruent melting at 1355°C, 2 kbar; 1395°, 5 kbar; 1430°, 7.5 kbar; and 1460°, 10 kbar.