Experimental study of the CaMgSi2O6–CO2 system at 3–8 GPa

The melting relationships in the system CaMgSi₂O₆ (Di)–CO₂ have been studied in the 3–8 GPa pressure range to determine if there is an abrupt decrease in the temperature of the solidus accompanying the stabilization of carbonate as a subsolidus phase. Such a decrease has been observed previously in peridotitic and some eclogitic systems. In contrast, the solidus in the Di–CO₂ system was found to decrease in a gradual fashion from 3 to 8 GPa. This decrease accompanies an evolution in the composition of the melt at the solidus from silicate-rich with minor CO₂ at 3 GPa to carbonatitic at 5.5 GPa, where the carbonation reaction Diopside + CO₂ = Dolomite (Dol) + Coesite (Cst) intersects the solidus. The near-solidus melt remains carbonatitic at higher pressure, consistent with carbonate being the dominant contributor to the melt. Based on previous studies in both eclogitic and peridotitic systems, this conclusion can be extended to more complicated systems: once carbonate is a stable subsolidus phase, it plays a major role in controlling both the temperature of melting and the composition of the melt produced.