The system K2CO3-MgCO3 at 6 GPa and 900-1450 C

Abstract

Phase relations in the K 2 CO 3 -MgCO 3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 ± 0.5 GPa pressures and 900–1450 °C temperatures. Subsolidus assemblies comprise the fields K 2 CO 3 +K 2 Mg(CO 3 ) 2 and K 2 Mg(CO 3 ) 2 +MgCO 3 with the transition boundary near 50 mol% MgCO 3 in the system. The K 2 CO 3 -K 2 Mg(CO 3 ) 2 eutectic is established at 1200 °C and 25 mol% MgCO 3 . Melting of K 2 CO 3 occurs between 1400 and 1450 °C. We propose that K 2 Mg(CO 3 ) 2 disappears between 1200 and 1300 °C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 °C. At 6 GPa, melting of the K 2 Mg(CO 3 ) 2 +MgCO 3 assemblage can be initiated either by heating to 1300 °C under “dry” conditions or by adding a certain amount of water at 900–1000 °C. Thus, the K 2 Mg(CO 3 ) 2 could control the solidus temperature of the carbonated mantle under “dry” conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in “fibrous” diamonds. The K 2 Mg(CO 3 ) 2 compound was studied using in situ X-ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 °C, the structure of K 2 Mg(CO 3 ) 2 was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K 2 Mg(CO 3 ) 2 exhibited a trigonal R 3 m structure previously established at ambient conditions.