MELTING AND SUBSOLIDUS PHASE-RELATIONS IN THE MGSIO3-MGCO3 SYSTEM AT HIGH-PRESSURES - IMPLICATIONS TO EVOLUTION OF THE EARTHS ATMOSPHERE

Melting and subsolidus phase relations in the MgSiO3MgCO3 system were investigated at 8 and 15 GPa and temperatures up to 2100°C and at 26 GPa and 1600°C using a uniaxial split-sphere anvil apparatus. Mineral assemblages of enstatite + magnesite and perovskite + magnesite are stable at 8 and 15 GPa and at 26 GPa, respectively, under subsolidus conditions. The melting temperature of magnesite increases with increasing pressure. Melting of the system shows a simple eutectic relation; the eutectic point shifts to lower MgCO3 content and to higher temperature with increasing pressure. Carbon dioxide is no longer a volcatile component at a depth of 140-720 km in the mantle and is retained as magnesite. A considerable amount of CO2 has been stored in the Earth's mantle for a long period of the Earth's history and is successively released through volcanism. © 1990.