The systems KCl-CaCO₃ and KCl-MgCO₃ at 6 GPa

Continuous variations in the composition of melt inclusions in diamonds from K-rich saline to carbonatitic indicate their possible genetic relationship. This causes interest in the study of chloride-carbonate systems at high pressure. Here we present experimental data on phase relations in the KCl-CaCO3 and KCl-MgCO3 systems at 6 GPa and 1000-1600 degrees C. The studied systems have the eutectic type of T-X diagrams. Subsolidus phases are represented by aragonite + KCl and magnesite + KCl. The KCl-CaCO3 eutectic is situated at 1200 degrees C and K2# 20, while the KCl-MgCO3 eutectic is located at 1400 degrees C and K2# 43, where K2# = 2KCl/(2KCl + CaCO3 + MgCO3)x100 mol%. The concentration of K and Cl in water-soluble chloride, and Ca, Mg, and CO2 in carbonate suggests that K-rich saline fluid entrapped by 'fibrous' diamonds could be a low-temperature derivative of a carbonatite melt containing KCl and H2O.