Geoscience Reference
In-Depth Information
regarding
the
fate
of
eclogite
in
the
mantle
Approx
Depth
Rock type
SHEAR VELOCITY
(
P
=
0
)
Vs (km/s)
involves this point.
The arrangement in Figure 9.2 approximates
the situation in an ideally chemically stratified
mantle. The densities of peridotites vary from
about 3.3 to 3.47 g/cm
3
while measured and
theoretical eclogite densities range from 3.45 to
3.75 g/cm
3
. The latter is comparable to the
inferred STP density near 650 km and about 10%
less dense than the lower mantle. The lower den-
sity eclogites (high-MgO, low-SiO
2
)havedensities
less than the mantle below 410 km and will there-
fore be trapped at that boundary, even when cold.
There are several things to note. Eclogites
come in a large variety of compositions, densi-
ties and seismic velocities. There is not a one-
to-one correlation of seismic velocity and den-
sity in mantle rocks, and shear velocity is not
a monotonic increasing function of density or
depth. Some chemically distinct layers have sim-
ilar seismic velocities. The velocities are quan-
tized at about the 4% level, a typical variation
observed in the shallow mantle globally, and
under hotspots in particular; such variations on
the slow side are usually attributed to partial
melting or high-temperature. The shear-velocity
quantum step is equivalent to a temperature vari-
ation of 1000
◦
C at constant pressure and about
the size of the correction to be made to STP
velocities to account for ambient mantle tem-
perature and pressure. Pressure and temperature
effects may change the ordering and the velocity
and density jumps at depth. Eclogite can settle
to various levels, depending on composition; the
eclogite bodies that can sink to greater depths
because of their density have low-velocity com-
pared to similar-density rocks. Some eclogites
have densities intermediate to the low- and high-
pressure asemblages at the various peridotite
phase boundaries (410 km, 500 km, 650 km);
they will be trapped at these boundaries, affect-
ing the seismic properties, and changing them
from the ideal phase-change conditions. Cold
eclogites with STP densities between 3.45 and
3.6 g
STP
density
(g/cc)
reflectors
(km)
3
4
5
6
Vs
(km/s)
pyroxenite
3.23
3.29
3.30
3.31
3.31
3.35
3.35
3.38
3.42
3.45
3.46
3.47
3.48
3.52
3.55
3.57
3.58
3.59
3.60
3.60
3.63
4.43
4.68
4.90
4.84
4.87
4.52
4.83
4.82
4.76
4.60
4.77
4.72
4.68
5.06
5.79
5.08
continental
moho
60
Avg.ultramafic rock
harzburgite
dunite
PHN1569
Vp = 8.4 km/s
80
90
sp.peridotite
Gt.Lhz.
pyrolite
UPPER
Vp = 8.3 km/s
130
PHN1611
MANTLE
arclogite(highMgO)
stable
Vp = 8.1 km/s
eclogite
Hawaii Lhz.
3
4
5
6
200
8.1 km/s
arclogite(highMgO)
majorite(mj)
gamma-spinel
280
330
9 km/s
garnet
Mgo
beta-spinel(.1FeO)
pyrolite(410km)
9 km/s
6.05
400
5.54
5.33
arclogite
4.93
4.84
8.3 km/s
arclogite
3
4
5
6
500
gamma-spinel(.1FeO)
3.68
3.67
3.70
3.74
3.75
3.75
3.92
4.07
4.11
4.13
4.22
4.23
4.26
5.59
5.40
pyrolite(500km)
arclogite
arclogite
arclogite(lowMgO)
MORB(mj
4.91
4.93
# # #
ultra
stable
(when cold)
+
st)
5.6
+
5.71
5.08
6.62
5.50
'ilmenite'(.1FeO)
650
mw(Mg.8)
11 km/s
pv
ca pv
pv(.1FeO)
800
900
6.44
6.6
MORB(pv
+
st)
+
mw(.2FeO)
5.08
Fig. 9.2
Same as Figure 9.1 with additional information and
fewer rock types.
of convection than the homogenous mantle usu-
ally treated by convection modelers or
geodynam-
icists
. Convection in the mantle is mainly driven
by the differences in density between basalt, melt
and eclogite. Note that sinking eclogite can be
trapped above the various mantle phase changes,
giving low-velocity zones. Although mantle strat-
ification is unlikely to be as extreme or ideal as
Figure 9.1 it is also unlikely to be as extremely
homogenous or well-mixed as often assumed.
Crustal type reflection seismology is required to
see this kind of structure.
Recycled oceanic crust, one kind of eclogite,
will have a particularly high density if it can
sink below about 720 km because the high silica
content of MORB gives a large stishovite con-
tent to MORB--eclogite. On the other hand, cumu-
late gabbros, the average composition of the
oceanic crust and delaminated continental crust
have much lower silica contents and this reduces
their
cm
3
may be trapped above the olivine-beta-
spinel phase boundary near 410 km depth, giving
a low-velocity zone (LVZ) at this depth. The obser-
vations of a
LVZ atop the 410 km discon-
tinuity
are usually interpreted in terms of par-
tial melting. A perched eclogite fragment will
/
high-pressure
densities.
The
controversy
