Kinetics of the olivine→spinel transition: Implications to deep-focus earthquake genesis

The rate of the olivine→spinel transition at high overpressure increases with diminishing grain size, or increasing shear stress, temperature, and possibly pressure. The transition rate is higher in Fe-rich compositions than in Mg-rich compositions, and it can be greatly increased by adding water or other mineralizers. Of all variables controlling the kinetics of the olivine→spinel transition in the mantle, temperature is the most critical. The olivine→spinel transition can be suppressed below 500°C in Mg-rich compositions, even in geological period of time. Since the temperature within a downgoing slab varies greatly according to different models of calculation, it is not clear at this stage whether the temperature is low enough to suppress the olivine→spinel transition. If the olvine→spinel transition cannot be suppressed, it may not be responsible for the genesis of deep-focus earthquakes. However, the rise of the olivine-spinel boundary in the cold interior of downgoing slabs provides an additional driving force for the plunging of these slabs. The distortion of the olivine-spinel boundary may also control the stress distribution in downgoing slabs and may be responsible for the observed alignment of principal stress axes of deep-focus earthquakes.