Mantle superplasticity and its self-made demise

Abstract

The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth's lower mantle. In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite + enstatite + diopside (7:2.5:0.5), which are good analogues for Earth's mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain-grain size-viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (≤1 μm) rocks to coarse-grained (1-10 mm) aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.

Non-Destructive In Situ Study of Plastic Deformations in Diamonds: X-ray Diffraction Topography and µFTIR Mapping of Two Super Deep Diamond Crystals from São Luiz (Juina, Brazil)

“…The lobate shape of subgrains found in BZ270 suggests a formation mechanism by solid state diffusion creep under HTHP conditions similar to sublithospheric mantle conditions. When a crystal deforms by diffusion creep to accommodate space problems from simultaneous movement of whole grains along grain boundaries, a process of superplastic-like flow occurs [44,45]. This process could also explain the nitrogen aggregation state found by FTIR.…”

Superplasticity in Ti–6Al–4V: Characterisation, modelling and applications

“…Examples exist in the aerospace, defence, biomedical, sports and automotive sectors [2]. Although usually found in metallic systems, for example those based upon aluminium [3], magnesium [4] and zinc [5], superplasticity has also been reported in some ceramics [6] and geological materials [7]. However, it is for titanium alloys of the type studied in this paper that the effect has the greatest technological relevance.…”

Stress evolution and associated microstructure during transient creep of olivine at 1000–1200 °C

“…At smaller grain sizes (sub µm), dislocations are increasingly captured by grain boundaries, resulting in only few organized sub grain boundaries. In such small grains, the rate-limiting factor is dislocation glide or ionic diffusion rates, as opposed to dislocation climb in larger grains and for higher finite strains (e.g., Hiraga et al, 2011 ;Miyazaki et al, 2013).…”