Aseismicity in the lower mantle by superplasticity of the descending slab

Abstract

IT has been suggested^1–7 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 slab^2–4. 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 others^9,3) 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.