The olivine-ringwoodite transformation triggers deep slab seismicity and rheological weakening

The subducting cold oceanic plates (slabs) exhibit the paradoxical deformation behaviors of deep seismicity and rheological weakening at the mantle transition zone (MTZ, ~400–700 km depths). The high-pressure transformation in olivine, a major constituent silicate mineral, is kinetically inhibited forming the metastable olivine wedge (MOW) in the cold center of the subducting slabs. Although the transformation of metastable olivine to ringwoodite (spinel structure) has been proposed as the possible trigger for the two contrasting behaviors of cold slabs, there have been almost no direct experimental evidence so far. Here we report new experimental results on the transformation-deformation coupling processes at ~20 GPa corresponding to the MTZ pressures. Ringwoodite is produced as nano-polycrystalline lamellae (NPL) under uniaxial stress. The thin ringwoodite NPL trigger unstable slips with coseismic stress drops by superplastic flow coupled with thermal instability at ~760–860°C. The thickening of NPL at ~950–1,330°C stabilizes the localized deformation, at which the transformation is effectively enhanced utilizing their incoherent nature, leading to homogeneous superplastic flow. Thus, the superplasticity in newly-formed ringwoodite plays key roles in the transformation-deformation coupling that occurs differently with temperatures. The paradoxical deformation behaviors of MTZ slabs can be explained by the stress-induced formation of ringwoodite NPL triggering deep seismicity at low temperatures in the vicinity of MOW, and the subsequent bulk transformation leading to homogeneous weakening at higher temperatures outside MOW.