Thermal Equations of State of Corundum and Rh2O3 (II)-Type Al2O3 up to 153 GPa and 3400 K

In this study, we present new experimental constraints on the thermal equations of state of corundum and Rh2O3(II)-type Al2O3 up to 153 GPa and 3400 K using synchrotron X-ray diffraction in laser-heated diamond anvil cells. Corundum was observed to transform into a mixture of corundum and Rh2O3(II)-type Al2O3 at 106 GPa and 2300 K, accompanied by a strong kinetic barrier. The Rh2O3(II)-type Al2O3 further transforms into the CaIrO3-phase at 156 GPa and 3700 K. The transition from corundum to Rh2O3(II)-type Al2O3 along a normal mantle geotherm could lead to a 1.9(5)% increase in density (rho) but a 0.6(3)% decrease in bulk sound velocity (V-phi). Al2O3 is one of the major components of the anorthositic crust once this old crust was subducted to the deep mantle. Using the obtained thermoelastic parameters, we further modeled rho and V-phi profiles of primordial anorthositic crust and found that the subducted fate of the primordial anorthositic crust depended strongly on its temperature. Our modeling reveals that primordial anorthositic crust along a normal mantle geotherm could descend to the transition zone but be trapped at the topmost lower mantle with a greater V-phi. Sinking of the anorthositic crust to the lower mantle can only occur along a 500-1000 K colder slab geotherm.