Post-spinel transitions in pyrolite and Mg_2SiO_4 and akimotoite-perovskite transition in MgSiO_3: Precise comparison by high-pressure high-temperature experiments with multi-sample cell technique

摘要

We precisely compared phase boundaries of post-spinel transition in pyrolite and Mg_2SiO_4 and of akimotoite-perovskite transition in MgSiO_3 at 21-28GPa and 1400-1800°C by detailed phase relation experiments using a multi-anvil apparatus. We used a multi-sample cell technique, in which pyrolite, Mg_2SiO_4 and MgSiO_3 were kept simultaneously at the same pressure-temperature conditions in each run. The experiments were performed in pressure and temperature intervals of 0.3GPa and 100°C, respectively. The post-spinel transition boundary in Mg_2SiO_4 is located at higher pressure by about 0.8GPa than the akimotoite-perovskite transition boundary in MgSiO_3. Both the transition boundaries have the same slope of -0.002GPa/°C. In pyrolite, the post-spinel transition occurs in a pressure interval within 0.4GPa at lower pressure by about 0.2-1.0GPa than that in Mg_2SiO_4 at 1400-1800°C. The Clapeyron slope of the post-spinel transition boundary in pyrolite is -0.001GPa/°C, which is half of -0.002GPa/°C of Mg_2SiO_4. When we assume that both the transition zone and the uppermost lower mantle have approximately pyrolitic composition, the above results imply that the Clapeyron slope of the transition boundary in pyrolite is more appropriate than that of Mg_2SiO_4 to evaluate effects of the post-spinel transition on mantle dynamics and the 660-km discontinuity topography. In pyrolite, the akimotoite-perovskite transition and the post-spinel transition occur at the same pressure at 1400°C. Above 1700°C, a part of ringwoodite in pyrolite transforms to garnet+magnesiowüstite at pressure below the post-spinel transition, and abundances of garnet and magnesiowüstite increase with increasing temperature.