410-km discontinuity

The discontinuity at the depth range around 410 km (the 410) is a dominant feature of most upper mantle models. The 410 characterizes the beginning of the mantle transition zone.
This seismic discontinuity is marked by a P-velocity increase of 5% - 6% (Melbourne and Helmberger, 1998) and an S-velocity and density increase of $\sim$4% and 5%, respectively (following earth model IASP91, Kennett and Engdahl, 1991).
The discontinuity is associated with the first order phase transition of the $\alpha$-olivin $\rightarrow$ $\beta$-spinel phase within the olivine component of the mantle (olivine $\rightarrow$ $\beta$-wadsleyite) at pressures of 13 GPa and temperatures of 1400$^{\circ}$C, appropriate for depths around 410 km (see figure 2.3).
High pressure and high-temperature experiments with mantle material indicate a sharpness of 5 - 25 km for this transition in agreement with studies of the equilibrium thermodynamic behaviour of an olivine mantle chemistry. Seismic data show a sharpness of the 410 from 2 - 4 km beneath oceans (Benz and Vidale, 1993) and up to 35 km beneath stable continents (Priestley et al., 1994). However, recent studies show consensus for a complex structure of the discontinuity (Melbourne and Helmberger, 1998; Xu et al., 1999) consisting of a relatively sharp discontinuity with a transition smaller than 5 km in the same depth interval as a broader gradient zone.
The $\alpha$ $\rightarrow$ $\beta$-spinel transition of olivine is an exothermic phase transition with a positive Clapeyron slope of approximately 3 $\frac{MPa}{K}$ (Bina and Helffrich, 1994). The positive slope of the transition within the p-T diagram results in a depression of the discontinuity for increased temperatures and a rise for decreased. This behaviour of the discontinuity is important for the study of subducting slabs and rising plumes within the Earth's mantle.
Several other conditions like the ratio of $\frac{Mg}{(Mg+Fe)}$, the amount of olivine, garnet and pyroxene within the mantle material, and the water content of the material can influence the depth and the sharpness of the discontinuity.
Several studies using different seismological methods focus on the depth of the 410. P $\rightarrow$  S conversions (Stammler et al., 1991; Vinnik, 1977; Bock, 1988), and underside reflections of P'P' (Davis et al., 1989; Benz and Vidale, 1993) show a sharp discontinuity (< 5 km) with moderate strong variations in depth and sharpness. Additionally, ScS reverberations (Revenaugh and Jordan, 1991 and 1991a), long period studies of PP-precursors (Shearer, 1991) as well as refraction and large-angle reflections have been used (Priestley et al., 1994; Walck, 1984; Mechi et al., 1993).