The strength of subducted slabs in the mantle transition zone influences the style of mantle convection. Intense deformation is observed particularly in relatively old subducted slabs in the deep mantle transition zone. Understanding the cause of this regional and depth variation in slab deformation requires constraint of the rheological properties of deep mantle materials. Here, we report results of in situ deformation experiments during the olivine to ringwoodite phase transformation, from which we infer the deformation process under the conditions of cold slabs deep in the mantle transition zone. We find that newly transformed fine-grained ringwoodite deforms by diffusion creep and that its strength is substantially smaller than that of coarser-grained minerals but increases with time. Scaling analysis, based on a model of transformation kinetics and grain-size evolution during a phase transformation, suggests that a cold slab will be made of a mixture of weak, fine-grained and strong, coarse-grained materials in the deep transition zone, whereas a warm slab remains strong because of its large grain size. We propose that this temperature dependence of grain size may explain extensive deformation of cold slabs in the deep transition zone but limited deformation of relatively warm slabs.
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We thank H. Chen from 6BM-B beamline at Argonne National Laboratory for her help with the experiments (proposal ID: 52991) that are partly supported by COMPRES. We also thank K. Kisslinger from the Center for Functional Nanomaterials at Brookhaven National Laboratory for his help with TEM sample preparation and imaging. We are also grateful to Z. Jiang for his help with SEM and to W. Samella and C. Fiederlein for helping with preparation of parts for rotational Drickamer apparatus cell assembly. This work was supported by National Science Foundation grant no. EAR-1445356.
The authors declare no competing interests.
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Mohiuddin, A., Karato, S. & Girard, J. Slab weakening during the olivine to ringwoodite transition in the mantle. Nat. Geosci. 13, 170–174 (2020). https://doi.org/10.1038/s41561-019-0523-3