Buoyancy of slabs and plumes enhanced by curved post-garnet phase boundary
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We studied the phase transition of garnet to bridgmanite and corundum (called the post-garnet transition) at high pressures and temperatures similar to those in Earth's lower mantle.
We found that the boundary of this transition has an unusual curved shape - the slope changes from negative at lower temperatures to positive at higher temperatures.
This curved boundary could have important effects on mantle dynamics:
- In cold subducting slabs, the negative slope would create upward buoyancy, potentially causing slabs to stagnate between 660-1000 km depth.
- In hot upwelling plumes, the positive slope would create additional upward buoyancy, enhancing plume rise.
These effects may explain some observed features of Earth's mantle:
- Seismic observations of slabs stagnating below the 660 km discontinuity.
- The apparent disappearance of plumes in seismic images above 1000 km depth.
The study used advanced experimental techniques with in-situ X-ray diffraction to accurately determine the phase boundary under high pressure and temperature conditions.
This finding provides a new perspective on mantle dynamics and may help explain previously puzzling observations about slab and plume behavior in the lower mantle.
Phase relations of the post-garnet transition in Mg3Al2Si3O12. Blue and red data points show conditions where bridgmanite plus corundum increased or decreased relative to garnet, respectively. The phase boundary curves downward, with slopes of -1.5 MPa/K at 1,400-1,800 K and 2.5 MPa/K at 1,900-2,100 K. Pressures were determined using an MgO equation of state. Error bars show pressure uncertainties. Some error bars are smaller than the symbols.
Mantle dynamics in the upper part of the lower mantle controlled by phase transitions. The white line shows the post-garnet (Pgt) transition boundary determined in this study. Blue arrows indicate positive buoyancies in a slab caused by negative slopes of post-spinel and Pgt transitions. Pink arrows show positive buoyancies in a plume due to positive slopes of the Pgt transition. White arrows represent buoyancies from thermal expansion. Arrow sizes indicate buoyancy magnitude.
Ishii, T., Frost, D.J., Kim, E.J., Chanyshev, A., Nishida, K., Wang, B., Ban, R., Xu, J., Liu, J., Su, X., Higo, Y., Tange, Y., Mao, H.-k., Katsura, T. Buoyancy of slabs and plumes enhanced by curved post-garnet phase boundary. Nature Geosci. 16, 828-832, 2023. 10.1038/s41561-023-01244-w
