Geoscience Reference
In-Depth Information
a
Fig. 20.2
Possible olivine and
orthopyroxene orientations within
the upper mantle showing
compressional velocities for the
three crystallographic axes, and
compressional and shear velocities
in the olivine
a
--
c
plane and
orthopyroxene
b
--
c
plane (after
Christensen and Lundquist, 1982).
b
8.25 km/s
7.72 km/s
c
b
8.43 km/s
MOHOROVICIC
DISCONTINUITY
6.92 km/s
9.89 km/s
7.92 km/s
c
a
OLIVINE
ORTHOPYROXENE
10
10
8
6
6
4
2
4
2
b
a
10
c
10
b
10
8
6
4 2
2
2
4
6
8
10
8
6
4
2
2 4
6
8
4
6
4
6
8
10
10
c
a
ORTHOPYROXENE
OLIVINE
upper mantle. The anisotropy of
-spinel, a high-
pressure form of olivine that is expected to be
a major mantle component below 400 km, is
also high. The
β
is an ilmenite-type structure that is extremely
anisotropic. Thus, the deep part of slabs may
exhibit pronounced anisotropy, a property that
could be mistaken for deep slab penetration in
certain seismic experiments.
Petrofabric studies combined with field stud-
ies on ophiolite harzburgites give the following
relationships.
-spinel form of olivine, stable
below about 500 km, is much less anisotropic.
Recrystallization of olivine to spinel forms can
be expected to yield aggregates with preferred
orientation but with perhaps less pronounced
P-wave anisotropy.
γ
-spinel has a strong S-wave
anisotropy (24% variation with direction, 16%
maximum difference between polarizations). The
fast shear directions are parallel to the slow
P-wave directions, whereas in olivine the fast
S-directions correspond to intermediate P-wave
velocity directions. Orthopyroxene transforms to
a cubic garnet-like structure that is stable over
much of the transition region part of the upper
mantle. This mineral, majorite, is expected to
be relatively isotropic. Therefore, most of the
mantle between 400 and 650 km depth is
expected to have relatively low anisotropy, with
the anisotropy decreasing as olivine transforms
to the spinel structures. At low temperatures, as
in subduction zones, the stable form of pyroxene
β
(1) Olivine
c
axes and orthopyroxene
b
axes lie
approximately parallel to the inferred ridge
axis in a plane parallel to the Moho disconti-
nuity.
(2) The olivine
a
axes and the orthopyroxene
c
axes align subparallel to the inferred speading
direction.
(3) The olivine
b
axes and the orthopyroxene
a
axes are approximately perpendicular to the
Moho.
These results indicate that the compressional
velocity in the vertical direction increases with
the orthopyroxene content, whereas horizontal
velocities and anisotropy decrease with increas-
ing orthopyroxerie content.
