Abstract
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.
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Data availability
The X-ray diffraction data that support the findings of this study are available via Zenodo (https://doi.org/10.5281/zenodo.3902732). Source data are provided with this paper.
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Acknowledgements
This work was funded by a research project approved by the Federal Ministry of Education and Research (BMBF) (grant number 05K16WC2) and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (proposal number 787 527) to T.K. and a DFG grant (grant number IS350/1-1) to T.I. The National Natural Science Foundation of China (NSFC) also supported this work (grant numbers U1930401 to J.L., 42150104 to H.M. and 92158206 to R. Tao and T.I.). The synchrotron X-ray diffraction experiments were performed at the BL04B1 beamline at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal numbers 2019A1353 and 2019B1133). We thank E. Posner for the English editing.
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T.I. designed and conducted the experiments and the majority of the analyses and wrote the paper. T.K. directed the project. D.J.F. and H.M. advised on and discussed the interpretation of the results. E.J.K. analysed the chemical compositions of the recovered samples. A.C., K.N., R.B., X.S., Y.H. and Y.T. together with T.I. and T.K. conducted the synchrotron radiation experiments at the BL04B1 beamline at SPring-8. All authors commented on the paper.
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Nature Geoscience thanks Juliane Dannberg, Ian Jackson and Baohua Zhang for their contribution to the peer review of this work. Primary Handling Editor: Tamara Goldin and Louise Hawkins, in collaboration with the Nature Geoscience team.
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Extended data
Extended Data Fig. 1 Back-scattered electron image of a recovered sample (M2877).
This consists of bridgmanite (Brg), corundum (Cor), pyrope (Gt), and a small amount of stishovite (St). The grain sizes of the phases are 1 micron or less.
Extended Data Fig. 2 Back-scattered electron images of recovered samples synthesized using the starting pyrope glass.
(a) 15 GPa and 1500 K. (b) 27 GPa and 2000 K. Brg, bridgmanite; Cor, corundum; St, stishovite; Gt, pyrope garnet.
Extended Data Fig. 3 Heat capacities of Mg3Al2Si3O12 pyrope (Gt) and the post-garnet phase (MgSiO3 bridgmanite (Brg) + Al2O3 corundum (Cor)).
The blue and red plots are proposed heat capacities of Gt and Brg+Cor, respectively, explaining the present post-garnet phase boundary. The blue and red curves are equations for the heat capacities of Gt38 and Brg+ Cor55,56, respectively, experimentally determined at room pressure. The dashed gray line is 3nR the limiting value of the Debye heat capacity (3nR, n: number of atoms and R: gas constant).
Extended Data Fig. 4 A post-garnet phase boundary proposed from heat capacities of garnet and the post-garnet phase.
The orange line shows the post-garnet phase boundary (Extended Data Fig. 3). The phase boundary experimentally determined in the present study is shown by the gray curve. Bars on the circles indicate errors in pressure.
Extended Data Fig. 5 A simple model to calculate buoyancies in a subducting slab and upwelling plume.
The red, green, and violet curves are the phase boundaries of the post-garnet (this study), Mj-Brg22, and post-spinel9 transitions. The red, gray, and cyan lines, respectively, show the temperatures of 1900 K, 1600 K, and 2200 K, representing the average mantle, slab and hot plume geotherms. The dashed line of the post-garnet transition is an extrapolation of the phase boundary.
Extended Data Fig. 6 Buoyancies in a subducting slab and upwelling plume.
The buoyancies of a subducting slab (a) and upwelling plume (b) were calculated using the simple model shown in Extended Data Fig. 5 assuming that each mineral proportion at the phase transitions are 100%. The widths of buoyancies were estimated from the differences in the transition pressures at the ambient temperature and the slab or plume temperature. The buoyancy directed to a shallower depth is expressed to be positive.
Extended Data Fig. 7 Buoyancies considering mineral assemblages and proportions of subducting slab and upwelling plume.
(a) Density changes of average mantle (black), subducting slab (blue), and upwelling plume (red) based on the model in Extended Data Fig. 5 with a simple mineralogical model (see Supplementary Text 3). (b, c) Buoyancies in a subducting slab and upwelling plume based on (a). The buoyancy directed to a shallower depth is expressed to be positive.
Supplementary information
Supplementary Information
Supplementary Figs. 1 and 2, Tables 1–5 and Texts 1–4.
Source data
Source Data Fig. 1
XRD profiles of samples.
Source Data Fig. 2
P–T conditions of stable phases.
Source Data Extended Data Fig. 3
Heat capacities of pyrope, bridgmanite and corundum.
Source Data Extended Data Fig. 4
Calculated P–T conditions of the post-garnet transition.
Source Data Extended Data Fig. 6
Buoyancy force with depth.
Source Data Extended Data Fig. 7
Buoyancy force with depth considering mineral proportion.
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Ishii, T., Frost, D.J., Kim, E.J. et al. Buoyancy of slabs and plumes enhanced by curved post-garnet phase boundary. Nat. Geosci. 16, 828–832 (2023). https://doi.org/10.1038/s41561-023-01244-w
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DOI: https://doi.org/10.1038/s41561-023-01244-w
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