Geophysics and Tectonics Seminar fall-2021

At the top of the mantle transition zone, olivine (α) transforms to wadsleyite (β) at about 410 km depth under equilibrium conditions, i.e., a pressure around 14 GPa and a temperature of about 1350 °C. The subsequent wave speed increase upon the α-β phase transition led to the discovery of the “410-km discontinuity” as a global feature thanks to seismology. In contrast, lower temperatures (< 1000°C) within the slab inhibit diffusive processes, thus diffusionless transformations might occur, such as the transition from α-olivine to ω-olivine (ε*-phase), i.e. new high-pressure polymorphy recently discovered in heavily shocked wadsleyite and ringwoodite meteorites. Our seismic waveform inversion results of the triplicated P wave datasets (in the Kuril subduction zone) show drastic variations of P-wave velocity inside the slab: a zone of extremely low wave speed (wave speed reduction < -20%) is located between 383 and 415 km depth, close to the cold core of the slab. These observations indicate that a layer of destabilized olivine exists within the cold slab, which highlights the transient (meta)stability of the ε*-phase, under substantial shear stress. Nonetheless, any phase transformation at relatively low temperatures necessarily induces long-lived grain-size reduction, which is also known to reduce P wave velocities. Whichever the transformation, we propose that the extremely low wave speed zone corresponds to a layer of partially transformed material, possibly consisting of a mixture of α, ω, and β-olivines.