Abstract
IT has been suggested1–7 that deep earthquakes in subduct ion zones may be caused by mineralogical transformations in the subducting slab. These transformations, from olivine to modified spinel to spinel, are also thought to affect the rheology of the descending slab2–4. Experiments in the system Mg2SiO4–Fe2SiO4 (ref. 8) have shown that at still higher pressure, coinciding with the 670-km seismic discontinuity between the upper and lower mantle, spinel dissociates to a fine-grained mixture of perovskite plus magnesiowiistite. Here we argue that the dissociation product in the mantle will also be very fine-grained, and that its eutectoid texture, combined with the low temperature in the slab, will prevent grain growth. We note (as have others9,3) that fine-grained materials at moderately high temperatures and low strain rates exhibit super-plastic behaviour; the resulting viscous behaviour of the slab below the spinel dissociation boundary may therefore account for the observed absence of seismicity below ∼700 km depth, even if slabs do penetrate into the lower mantle.
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Ito, E., Sato, H. Aseismicity in the lower mantle by superplasticity of the descending slab. Nature 351, 140–141 (1991). https://doi.org/10.1038/351140a0
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DOI: https://doi.org/10.1038/351140a0
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