Subsolidus and melting relationships for calcite, magnesite and the join CaCO_3-MgCO_3 to 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.

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