Subsolidus and melting relationships for calcite, magnesite and the join CaCO 3-MgCO 3 36 kb

Abstract

Subsolidus and melting relations for the CaCO 3-MgCO 3 join at 30 kb have been determined using piston-cylinder apparatus. Data are also presented for the melting curve of CaCO 3 to 30 kb, the decomposition and melting curves of MgCO 3 to 36 kb, and the calcite-aragonite transition at 800°C, 950°C and 1100°C. At 30kb, the melting loop for the CaCO 3-MgCO 3 join extends from 1610°C (CaCO 3) to 1585°C (MgCO 3) through a liquidus minimum at 1290°C (near 42 mole% MgCO 3). The dolomite-magnesite solvus barely intersects the 30 kb melting loop to produce a peritectic reaction at 1385°C. Integration of the new experimental data with other published data permits construction of a complete P- T projection and a sequence of isobars for the CaCO 3-MgCO 3 join for pressures between 5 and 30 kb. The phase relations for this join provide part of the essential framework of the model peridotite system CaO-MgO-SiO 8-CO 2-H 2O, which has particular application to the origin of carbonatitic and kimberlitic magmas. In light of the accumulating evidence for CO 2 in various forms within the upper mantle and of its effect on magmatic processes, analysis of the melting relations in this system is of considerable importance.