Olivine-Wadsleyite Transformation within the Subducting Pacific Slab in Kuril

At the top of the mantle transition zone, it is commonly accepted that olivine (α) transforms to wadsleyite (β) at about 410 km depth under equilibrium conditions, i.e., a pressure of ~ 14 GPa and a temperature of ~ 1350 °C. The subsequent wave speed increase upon the α-β phase transition leads to the discovery of the 410-km discontinuity as a global feature seismologically. However, the complex topography of the “410-km discontinuity” is unclear within cold subducted oceanic lithospheres sinking into the lower mantle, partly due to the sparsity of seismic waves sampling the pertaining complex 3-D structures.

This study uses triplicated P waves (~ 2 seconds), most sensitive to the 410-km discontinuity, to invert for the characteristic parameters of its depth and radial wave speed gradients near the discontinuity. Six distinct wave propagation directions are investigated for a carefully chosen earthquake. These directions are sub-parallel to the slab depth contours in the Kuril subduction zone to guarantee a simplified layered earth modeling. Our results show azimuthal variations of the discontinuity depth either above or within the slab.

For example, the 410-km discontinuity is uplifted by 5-10 km at a depth of about 100 km above the slab upper interface. The uplift increases up to 15-20 km when the 410-km discontinuity is closer to, i.e., only 50 km above, the cold slab. This observation is consistent with the expected phase transition in equilibrium with temperatures greater than 1000°C. In contrast, within the cold slab (< 1000°C), the α-β transition exhibits drastic variations of P-wave speed. Our non-gradient-based inversion results show optimal models that place the following unique seismic constraints: 1) a significant P-wave speed increase within the slab (+5.5 ±1.5 %) compared to the ambient mantle; 2) a zone of extremely low wave speed (LVZ) within the slab with a P-wave speed reduction of -14 ±4 %. The observed LVZ is located near a depth of 350 km with an apparent thickness of 15-30 km, which can be much thinner in the direction normal to the slab upper interface.

These observations indicate a layer of destabilized olivine (LVZ) exists inside the slab. The α-β transition involves atomic diffusion highly dependent on temperature. Once olivine becomes unstable within a cold wedge, it cannot directly transform into wadsleyite. The drastic P-wave speed reduction is likely caused by the sudden grain-size reduction induced by the phase transition, and possibly also by the transient (meta)stability of an intermediate phase, the ω-olivine, under substantial shear stress during the transformation within the cold wedge of the sinking slab.