Geoscience Reference
In-Depth Information
but more recently it is often taken as the man-
tle below the 650 km seismic discontinuity.
This has caused immense confusion regarding
whether slabs sink into the lower mantle.
The
400 km discontinuity is mainly due
to the olivine-spinel phase change,
considered as an equilibrium phase
boundary in a homogenous mantle
. The
seismic velocity jump, however, is smaller
than predicted for this phase change. The
orthopyroxene--garnet reaction leading to a gar-
net solid solution is also complete near this
depth, possibly contributing to the rapid increase
of velocity and density at the top of the transi-
tion region. Some eclogites are less dense than
the beta-form of olivine and may be perched at
400 km depth. For these reasons the 400 km
discontinuity should not be referred to as the
olivine--spinel phase change. If the discontinuity
is as small as in recent seismic models, then the
olivine content of the mantle may be lower than
in the shallow mantle. This would make sense
since olivine is a buoyant product of mantle dif-
ferentiation and would tend to accumulate at the
top of the mantle.
The TR as described by Bullen is a dif-
fuse region of high seismic wave-speed gradient
extendingfrom410to1000km.InBu len s
nomenclature the lower mantle (Region D)
started at 1000 km. Birch suggested that the
Repetti discontinuity near 1000 km marked the
top of the lower mantle and that high seismic
wave-speed gradients are caused by polymorphic
phase changes. The early models of Jeffreys and
Gutenberg were smooth and had high wave-speed
gradients without abrupt discontinuities, but in
the 1960s it was discovered that there are abrupt
jumps in seismic velocity at depths of approx-
imately 400 and 650 km. During that decade,
various investigations, including detailed stud-
ies of the travel times of both first- and later-
arriving body waves, seismic array measurements
of apparent velocities, observations of reflected
waves such as precursors to the core phase
P
P
,
and analysis of the dispersion of fundamental
and higher-mode surface waves, all confirmed the
existence of the discontinuities, which define the
transition Z are, TZ.
Thermodynamic considerations have been
used to argue that the discontinuities are abrupt
phase changes of, mainly, olivine to the spinel
crystal structure, and then to a 'post-spinel'
phase, not chemical changes as in the standard
geochemical models, and that the deeper one has
a negative Clapyron slope. This means that cold
subducting material of the same composition
as the surrounding mantle would depress the
650 km discontinuity, inhibiting vertical motion,
and would change to the denser phase only after
warming up or being forced to greater depth.
Material of different composition and intrinsic
density can be permanently trapped at phase
boundaries. Geochemical, and many convection,
models assume that the 650 km phase change
separates the 'depleted convecting upper mantle'
from the 'primordial undegassed lower mantle.'
Geodynamic modelers assume that if the 650 km
discontinuity is not a chemical change then there
can be no deeper chemical change and the man-
tle is chemically homogenous. The TZ thus holds
the
key
to
whether
there
is
whole-mantle
or
layered-mantle convection.
In the transition zone the stable phases are
garnet solid-solution,
-spinel and, possi-
bly, jadeite. Garnet solid-solution is composed of
ordinary garnet and SiO
2
-rich garnet (majorite).
The extrapolated elastic properties of the spinel
forms of olivine are higher than those observed
(Figures 8.9 and 8.10). The high velocity gradients
throughout the transition zone imply a continu-
ous change in chemistry or phase, or in lithol-
ogy (eclogite vs. peridotite). Appreciable garnet
is implied in order to match the velocities. A
spread-out phase change involving clinopyroxene
(diopside(di) plus jadeite(jd)) transforming to Ca-
rich majorite(mj) can explain the high velocity
gradients.
Unusually low temperatures, as expected in
the vicinity of a downgoing slab, will warp the
410 km discontinuity up by about 8 km per 100 K,
and the 650 km discontinuity down by about
5 km per 100 K. The Clapyron slopes are uncer-
tain but most estimates for the total thickening/
thinning of the TZ lie in the range 12--17 km per
100 degrees. This assumes a purely olivine min-
eralogy, which is unrealistic.
β
- and
γ
