Increase of the oxygen vacancy component in bridgmanite with temperature

Abstract The oxygen vacancy (OV, MgAlO 2.5 ) contents in bridgmanite with periclase and/or a calcium ferrite-type MgAl 2 O 4 –Mg 2 SiO 4 phase (CF-phase) were investigated at temperatures of 1700, 2000, and 2400 K and a pressure of 27 GPa using an ultrahigh-pressure multi-anvil press. The OV content increases significantly with increasing temperature by consuming coexisting periclase and the charge-coupled (CC, AlAlO 3 ) component in bridgmanite. The increase in OV component at high temperature is promoted by its high entropy. The CC components increase largely with temperature compared with the OV content in bridgmanite coexisting with CF-phase and periclase because the partition coefficient of Mg and Al between bridgmanite and CF-phase approaches unity because of the mixing entropy. The partial molar volumes of OV and CC bridgmanite endmembers at ambient conditions were constrained to be 26.64 and 25.79 cm 3 /mol, respectively, with densities of 3.428 and 3.955 g/cm 3 . By assuming linear relationships between wave velocities and density, the OV component may therefore cause a velocity reduction by one order of magnitude larger than the CC component at constant Al content in bridgmanite. The concomitant increase and decrease in OV and CC contents with temperature enhance V P and V S reductions by 0.7% and 2.5% in addition to anharmonic effects. The OV component in bridgmanite in subducted slabs is zero because of low temperature, and the transport of water and argon trapped by the OV component into the lower mantle by bridgmanite is therefore not expected. The storage capacity of these volatiles in subducted slabs will thus be significantly lower than the warm ambient mantle. Since the OV component is expected to enhance diffusion creep, the present results may explain slab stagnation in the mid-mantle due to low OV contents at low temperature, and vertically straight plumes in the lower mantle due to high OV contents at high temperature.