Geology Reference
In-Depth Information
low-velocity anomalies that have been observed
just above the 410-km discontinuity, with a shear-
wave velocity drop of
5%.
1.6
Transition Zone
The
transition zone
forms the lowermost layer of
the upper mantle. We have discussed in the previ-
ous section its important role in the global water
balance and mantle circulation. Here we shall fo-
cus on some key features regarding the chemistry
and thermodynamics of this region. There are
two principal models for the composition of the
transition zone. In the classic
pyrolite model
of
Ringwood (
1975
), the two major seismic discon-
tinuities in the Earth's mantle, which bound the
transition zone respectively at 410 and 670 km
depth, are isochemical phase transformations of a
hypothetical garnet peridotite composed mainly
by olivine and pyroxene (hence the term “pyro-
lite”) (Fig.
1.12
).
In this instance, the 410-km discontinuity is
associated with the pressure-induced transforma-
tion of the olivine phase in peridotite rocks to
wadsleyite:
Fig. 1.12
Chemical composition of the mantle (% vol)
in
'
.Mg; Fe/
2
SiO
4
!
“
.Mg; Fe/
2
SiO
4
the
pyrolite
model
of
Ringwood
(
1975
).
Maj
is
majorite garnet, Mg-Pv
Ca-
perovskite, the other symbols are explained in Table
1.1
D
Mg-perovskite, Ca-Pv
D
Similarly, a second-order discontinuity at
520 km depth (
18 GPa) determines new
collapse of this mineral to ringwoodite:
garnet structure starting from
300 km depth
(
10 GPa). Majorite garnet, in turn, dissolves
into the perovskite structure between 670 and
750 km depth (Fig.
1.12
).
In the
piclogite model
of Bass and Anderson
(
1984
) the composition of the mantle transition
zone is essentially based on the match of theoret-
ical seismic velocities with observed data and a
model of chemical differentiation of the primitive
Earth (Anderson and Bass
1986
). In this instance,
the transition region is assumed garnet-rich rather
than olivine-rich and it is viewed as the source
region of MORBs.
An
“
.Mg; Fe/
2
SiO
4
!
”
.Mg; Fe/
2
SiO
4
Finally, the 670-km discontinuity at the base
of the mantle transition zone (
23 GPa) is asso-
ciated with a complete breakdown of the mineral
phase to form perovskite and magnesiowüstite:
”
.Mg; Fe/
2
SiO
4
!
.Mg; Fe/ SiO
3
C
.Mg; Fe/ O
The other two phases of the upper mantle
mineral assemblage, pyroxenes and garnet, are
subject to more gradual transformations. For
example, pyroxene gradually dissolves into the
important
prediction
of
the
pyrolite
model
arises
from
the
Clapeyron
slopes
,
dp
/
dT
,
of
the
phase
transitions
at
410
and
