Buoyancy of slabs and plumes enhanced by curved post-garnet phase boundary

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-convex shape: the slope changes from negative to positive with increasing temperature. The negative slope at low temperatures would impart upward buoyancy on cold slabs that is significantly larger than that by thermal expansion. This could impede slab downwelling and may explain slab stagnation between 660 and 1,000 km depth. In contrast, the positive slope at high temperatures would impart upward buoyancy on hot plumes and enhance their upwelling, which may account for the invisibility of plumes in seismic observations above 1,000 km depth.