Geoscience Reference
In-Depth Information
overturn. Since chemical discontinuities can be
almost invisible to 1D seismology, compared with
phase-changes, and since even small chemical
density contrasts can stratify the mantle, the pos-
sibility must be kept in mind that there may be
multiple chemical layers in the mantle, some of
which may be subtle. The major seismic discon-
tinuities in the mantle are due to mineralogical
and phase changes, not chemical changes, but
this does not rule out a chemically heterogenous
mantle.
Many eclogites are seismically fast because
they have a large proportion of garnet and much
of the pyroxene is in the form of jadeite. Some
garnet--pyroxenite xenoliths, however, have large
amounts of clinopyroxene and very little jadeite,
and, technically, are not eclogites. Such garnet
pyroxenites, which can be called arclogites, are
slower than common eclogites. They have higher
V
p
/
V
s
ratios than peridotites. Velocity decreases
do not necessarily imply hot mantle.
In the upper mantle, residual dunites and
eclogites have shear velocities that vary mainly
from 4.60 to 4.95 km/s, while normal peridotites
and lherzolites typically vary from 4.5 to 4.9
km/s, all at standard temperature and pressure
(STP).
Xenoliths from the Sierran root
fall into two categories with inferred densities
of 3.45--3.48 g/cm
3
and 3.6--3.75 g/cm
3
respec-
tively. The associated STP shear velocities are
lower than 4.68 km/s and higher than 4.83 km/s
respectively. Low-density eclogites have densities
comparable to peridotites and Hawaiian lher-
zolites and should sink to depths no greater
than about 400 km, even when cold. The high-
density low-MgO arc eclogites should density-
equilibrate in the lower part of the transition
region, 500--650 km, where they will show up as
low-velocity anomalies. These could be confused
with hot plumes, even if they are cold dense
sinkers!
Eclogites and dunites in the surface TBL, or
delaminated, will have slightly higher seismic
velocities than the warmer peridotites and lher-
zolites that they displace. Eclogites are much
denser than dunites and are more likely to
delaminate, on their own, and sink into the
underlying mantle. Delaminated material will
displace asthenospheric material, causing adia-
batic decompression melting and a lowering of
the shear velocity. Between 60 and 200 km eclog-
ites can have seismic velocities that are lower
or higher (particularly when cold) than man-
tle peridotites of similar density; the denser (or
colder) ones may show up as high velocity cur-
tains. Below about 400 km eclogites are typically
much lower velocity than transition zone min-
erals and, in fact, they are also less dense than
much of the mantle below 500 km. If the mantle
is close to its normal (peridotitic) solidus, then
Chemical discontinuities?
Mantle peridotites and eclogites have a variety
of compositions, densities and seismic veloci-
ties. If the density differences are large enough,
the mantle can become chemically stratified.
Eclogites have higher seismic velocities and den-
sities than peridotites at the same pressure and
temperature, but they have much lower veloci-
ties than peridotites and peridotitic assemblages
of the same density. After delamination suffi-
ciently large eclogitic blobs will sink into olivine-
rich mantle that has higher seismic velocities,
and higher melting temperatures. Arc eclogites
are predicted to have lower densities and veloci-
ties than transition zone minerals and may be
trapped in and above the transition region.
The cold ultramafic portions of the lithosphere
have high seismic velocities and moderately high
density. When sufficiently cold, they can become
denser than the upper part of the subplate man-
tle. The delamination model predicts that regions
possibly undergoing delamination, such as the
Sierras, Yellowstone, and possibly most arc and
rifted areas will be underlain by low-melting-
point fertile material, and high seismic velocity
curtains. At most depths, even subsolidus eclog-
ite is predicted to have lower seismic velocities
than the surrounding mantle, even if it is dense
and sinking. If it is volatile-rich, or if it warms
uptothesolidus,itwillhaveevenlowerseis-
mic velocities. These seismic features may easily
be mistaken for plumes. The delaminated mafic
material eventually warms up and melts, cre-
ating fertility spots in the mantle. Such spots
become buoyant as garnet is removed by melting.
Melting anomalies may be due to these fertile
spots rather than hotspots.
