Geoscience Reference
In-Depth Information
components do not differ much in intrinsic den-
sity, of the order of 2 or 3%, depending on depth.
The Earth itself is stratified by composition and
density (atmosphere, hydrosphere, crust, mantle,
core) and the crust and upper mantle are strati-
fied as well. The region at the base of the mantle --
D
-- appears to be iron-rich and intrinsically
dense. There may be a buoyant refractory (melt
depleted) discontinuous layer at the top of the
mantle -- the
perisphere
. The perisphere may never
get cold enough to subduct and D
may never
get hot enough to rise. These are only the most
obvious candidates for chemical layers; internal
layers will likely be subtle and they need not be
continuous or flat.
Figure 9.1 shows the shear velocity in a vari-
ety of rocks and minerals, at STP (standard tem-
perature and pressure) arranged according to
increasing zero-pressure density. This represents
a stably stratified system. Many of the chemically
distinct layers differ little in seismic properties
and sometimes a denser layer has lower seismic
velocity than a less dense overlying layer. The
densities range from 2.6 to 4.2 g/cm
3
; the den-
sity scale is monotonic but nonlinear. Several esti-
mates of depth are given, calibrated according to
estimates of mantle uncompressed density. Given
enough time in a low enough viscosity mantle
this is the stable density stratification. Note that
shear velocity is not a monotonically increasing
function of density. A stable density stratification
has an irregular complex shear velocity struc-
ture. Even if the mantle achieves this stable strat-
ification it will not be permanent. The differ-
ent lithologies have different melting points and
thermal properties, phase changes and can rise
or sink as the temperature changes. Figure 9.2
is a similar plot, with some of the minerals and
rocks identified.
Eclogites occur at various depths because they
come in a variety of compositions; eclogite is
notauniformrocktype.Arclogitesaregarnet
clinopyroxenites that occur as xenoliths in arc
magmas. The deeper eclogite layers in the fig-
ures are low-velocity zones relative to similar
density rocks. Cold dense eclogite melts as it
warms up to ambient mantle temperature, and
becomes buoyant. The stable stratification of a
chemically zoned mantle is only temporary. This
kind of mantle convects but it is a different kind
SHEAR VELOCITY (P
=
0
)
VS
3
4
5
6
CRUST
D
E
N
S
I
T
Y
3
4
5
6
UPPER
MANTLE
400 km
500 km
650 km
Fig. 9.1
Chemical stratification of the mantle if mantle
rocks and minerals arrange themselves in the gravity field
according to intrinsic density (density increases downward
but is not tabulated). The velocities (horizontal axis) and
densities (vertical axis) are appropriate for Standard
Temperature and Pressure (STP) conditions.
