Subsolidus and melting relations for the join CaCO_3-MgCO_3 at 10 kbar

Abstract

Mixtures of pure dry CaCO_3 and MgCO_3 were reacted at 10 kbar in a piston-cylinder apparatus. Solidus and liquidus boundaries were delineated by interpretation of quenched textures. X-ray determined compositions of quenched carbonates are not a reliable guide to the phase relations. The binary melting loop for CaCO_3-MgCO_3 extends from CaCO_3 at 1460°C through a liquidus minimum near 30 wt% MgCO_3 and 1075°C, and it is terminated at the incongruent melting reaction for dolomite solid solution at 1125° C (liquid with 32 wt% MgCO_3) Magnesite solid solution dissociates at 1090°C to produce dolomite + periclase + CO_2, truncating the dolomite-magnesite solvus. The 10 kb liquidus minimum at 1075°C and 30 wt% MgCO_3 occurs at lower temperature and higher Ca/Mg ratio than the 27 kbar liquidus minimum at 1290°C and 38 wt% MgCO_3. This relationship suggests that the first liquid produced by melting of a carbonate-bearing peridotite has increasing Mg/Ca ratio with increasing pressure. These phase relations provide part of the framework required to trace paths of crystallization of kimberlite and carbonatite magmas.