Post-spinel transitions in pyrolite and Mg2SiO4 and akimotoite–perovskite transition in MgSiO3: Precise comparison by high-pressure high-temperature experiments with multi-sample cell technique

We precisely compared phase boundaries of post-spinel transition in pyrolite and Mg2SiO4 and of akimotoite–perovskite transition in MgSiO3 at 21–28 GPa and 1400–1800 °C by detailed phase relation experiments using a multi-anvil apparatus. We used a multi-sample cell technique, in which pyrolite, Mg2SiO4 and MgSiO3 were kept simultaneously at the same pressure–temperature conditions in each run. The experiments were performed in pressure and temperature intervals of 0.3 GPa and 100 °C, respectively. The post-spinel transition boundary in Mg2SiO4 is located at higher pressure by about 0.8 GPa than the akimotoite–perovskite transition boundary in MgSiO3. Both the transition boundaries have the same slope of − 0.002 GPa/°C. In pyrolite, the post-spinel transition occurs in a pressure interval within 0.4 GPa at lower pressure by about 0.2–1.0 GPa than that in Mg2SiO4 at 1400–1800 °C. The Clapeyron slope of the post-spinel transition boundary in pyrolite is − 0.001 GPa/°C, which is half of − 0.002 GPa/°C of Mg2SiO4. 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 Mg2SiO4 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.

► We compared formation pressures of perovskite in pyrolite, MgSiO3 and Mg2SiO4.
► The post-spinel transition in pyrolite occurs 0.2–1.0 GPa lower than that in Mg2SiO4.
► The Clapeyron slope in pyrolite is –0.001 GPa/°C which is half of that in Mg2SiO4.
► The formation of perovskite in MgSiO3 occurs 0.8 GPa lower than that in Mg2SiO4.
► Both the transition boundaries have the same slope of –0.002 GPa/°C.