The system K2CO3-MgCO3 at 6 GPa and 900-1450 C
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Phase relations in the K2CO3-MgCO3 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 2CO3+K2Mg(CO3)2 and K2Mg(CO3)2+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The K2CO 3-K2Mg(CO3)2 eutectic is established at 1200 °C and 25 mol% MgCO3. Melting of K2CO 3 occurs between 1400 and 1450 °C. We propose that K 2Mg(CO3)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 2Mg(CO3)2+MgCO3 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 2Mg(CO3)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 K2Mg(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 K2Mg(CO3)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 K2Mg(CO3)2 exhibited a trigonal R3m structure previously established at ambient conditions.
author list (cited authors)
Shatskiy, A., Sharygin, I. S., Gavryushkin, P. N., Litasov, K. D., Borzdov, Y. M., Shcherbakova, A. V., ... Ohtani, E
complete list of authors
Shatskiy, A||Sharygin, IS||Gavryushkin, PN||Litasov, KD||Borzdov, YM||Shcherbakova, AV||Higo, Y||Funakoshi, K-I||Palyanov, YN||Ohtani, E