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Lehmann discontinuity

Several different seismological methods indicate a discontinuity near 220 km depth.
The first results were early seismic refraction studies in Europe and North-America (Lehmann, 1959; Lehmann 1961, Hales et al., 1980). The detection of this discontinuity was confirmed later by surface wave studies (Goncz and Cleary, 1976), underside reflections of depth phases from the discontinuity (Vidale and Benz, 1992), ScS reverberations (Revenaugh and Jordan, 1991b) and P $\rightarrow$ S conversions (e.g. beneath NORSAR, Sacks et al., 1979).
This discontinuity at a depth of $\sim$ 210 km is also called the Lehmann discontinuity (the L).
Despite the detection of the L, the existence of a global discontinuity at this depth is still under discussion. Most of the regions where the L has been detected belong to continental or island arc regions. The fact that a reflection from this depth is missing in the global long-period stacks by Shearer (1991) supports either the theory of the regional nature of the L, or indicates a strong depth variation of this reflector.
Nevertheless, the L, with an increase of P and S-velocity of $\sim$7 % at a sharp boundary, has been part of some global Earth models such as the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981) (compare Figure 2.3). Such a big velocity change is not supported by more recent studies, which report values of 2% - 4.5% (Anderson, 1989; Gaherty and Jordan, 1995).
A major solid-solid phase transition within mantle material has not been found at pressure conditions (7 GPa - 8 GPa) and upper mantle temperatures relevant at depths of 210 km. However, two minor phase transitions have been found. Akaogi and Akimoto (1977) found a transition from coesite $\rightarrow$ stishovite and the formation of garnet solutions in the (Mg,Fe)$_4$Si$_4$O$_{12}$ - (Mg,Fe)$_3$Al$_2$Si$_3$O$_{12}$ system at the appropriate conditions. But the correlated velocity jumps of these phase transitions have been found to be too small to explain the seismic data (Bina and Wood, 1984). Aside from these reactions, the L might be the result of a compositional change from harzburgite to garnet lherzolite, or the change from garnet lherzolite to eclogite. Additionally, the base of a pronounced low velocity zone (LVZ) is discussed (Leven et al., 1981).
An explanation of the velocity jump observed by seismology across the L with mineralogical and chemical transition models seems impossible. The L is unlikely to be the sharp lower boundary of the asthenospheric LVZ, because the L is often detected in areas where the LVZ is weak or absent.
A probable explanation for the L is a discontinuity resulting from lattice preferred orientation (LPO) of highly anisotropic olivine crystals in a deformation zone responsible for the mechanical decoupling of the lithosphere from the underlying mantle. This hypothesis is further discussed by Revenaugh and Jordan (1991b), and is generalized as a change in structure, from highly anisotropic lithosphere to an isotropic asthenosphere. This phenomenon of rapid decrease of anisotropy around a depth of 220 km is interpreted as the result of the pressure induced change of the deformation mechanism from dislocation to diffusion creep (Karato, 1992). A change of anisotropy would cause a velocity jump for vertically or almost vertically traveling waves (Figure 2.5).

Figure 2.5: Cartoon of the mechanism leading to a velocity change due to a change of anisotropy structure. Below 210 km the mantle material is isotropic (v$_v$ = v$_h$). Above the 210 the material is highly anisotropic (v$_v$ < v$_h$). If the vertical velocity above the discontinuity is smaller than below the transition a vertically or almost vertically travelling wave is reflected at a velocity discontinuity. The change of the anisotropy structure is explained by a change of the deformation mechanism (Karato, 1992).
\centerline {\psfig{figure=abb_2.5.eps,angle=0,width=14cm,height=9cm}}\hfill

The anisotropy change is not the result of a disappearance of the anisotropic structure of the olivine crystals, but only due to a change of the deformation mechanism of the mantle material.
In this model, the L would be shallower in wet and/or hot regions compared to cooler and/or drier regions, in good agreement to seismological detections of this discontinuity.

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