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Mantle convection manifests in the subduction of cold slabs and the upwelling of hot plumes, driving both near-surface processes such as volcanism and seismicity and the chemical evolution of the Earth’s interior. Phase transitions of mantle minerals at high pressure are associated with changes in density and viscosity. Mantle convection is either enhanced or impeded depending on the sign of the slope of the phase transition boundary (the temperature dependence of transition pressures). Accurately determining phase boundary slopes is, therefore, essential for understanding mantle dynamics. Here we identified the phase boundary of the post-garnet phase transition—the breakdown of garnet to bridgmanite plus corundum—under mantle conditions using in situ X-ray diffraction multi-anvil techniques that can accurately determine phase stability. We find that the post-garnet phase boundary has a downward …