Anisotropy: mineral properties are different in different crystallographic directions! (as we saw in optical mineralogy)
This is true of olivine, the mineral we believe makes up most of the upper part of the mantle (above 400 km), and also of pyroxene and other mantle minerals.
Crystal system is orthorhombic, so a, b, and c are all distinctive.
How do we know olivine is the major mineral in the first few hundred km of the m antle?
Example of interest here: velocity at which compressional (and shear) waves ttravel through the crystals: velocities differ along a, b, and c.
Example: in olivine (e.g., travel of earthquake waves)
SEISMIC ANISOTROPY: in an olivine dominated region (mantle), seismic velocities vary with direction: suggests that random orientation of crystals is not found (i.e., crystal anisotropy would be averaged out if the crystals were randomly oriented).
Attribute seismic anisotropy to preferred orientation of olivine crystals. This is believed to be caused by plastic flow in the upper mantle.
Consider how olivine crystals deform: what slip systems operate at various pressures and temperatures
Note nomenclature: (hkl) indicates the plane of slip, [uvw] indicates the direction of slip - thus
(010)[100] indicates slip in the [100] direction occurring on (010) planes.
The slip systems in olivine at P and T in the mantle.
WHAT SLIP SYSTEMS ARE KNOWN TO OPERATE (experimental studies):
Slip along a preferred slip system (slip // to a) in olivine.
Crystals oriented with their optimal slip system highly inclined to the shear direction are 'unhappy', and rotate into the slip direction. Crystals oriented so their slip planes and directions are parallel to the shear direction become highly elongated. The mechanism is shown diagramatically The slip systems in olivine as a function of temperature and pressure.
FABRIC development: compare :
with one that has
