Geoscience Reference
In-Depth Information
transition are not the same as those found in
MgSiO
3
(Tsuchiya & Tsuchiya, 2007a; Usui &
Tsuchiya, 2010). For MgGeO
3
, all three velocities
of
V
P
,
V
S
,and
V
show negative jumps at
the static
P
T
of 50 GPa. For CaIrO
3
,
V
P
and
V
show negative jumps at 0 GPa, while
V
S
shows a positive jump (Tsuchiya & Tsuchiya,
2007b). Therefore, care must be taken when one
extrapolates the elastic properties of these analog
materials to MgSiO
3
PPv.
16
Pv (solid)
PPV (dashed)
V
P
V
12
Φ
V
S
8
7.3.2 Effects of Fe and Al
For realistic multi-component systems like the
peridotitic and basaltic compositions, not only Fe,
but also Al
2
O
3
is substantially accommodated in
MgPv (Irifune & Ringwood, 1993; Irifune, 1994).
Understanding the effects of these elements on
the stability and elasticity of MgPv and PPv is
therefore important. Since the solid solution of
these elements produces two phase coexisting
loops, narrow loop widths (typically less than
ρ
4
0
50
100
150
P (GPa)
Fig. 7.2
Elastic wave velocities and density calculated
for MgPv (solid lines) and PPv (dashed lines) as a
function of pressure at 300 K, 1000 K, and 2000 from
top to bottom. A vertical line represents the
post-perovskite transition pressure at the static
temperature.
80
km) are in general necessary to interpret the sharp
seismic discontinuity. Actually, recent
ab initio
simulations of finite temperature free energy in
the MgSiO
3
-Al
2
O
3
system clarified sufficiently
small
∼
which is observed generally in the deepest man-
tle (Masters
et al.
, 2000). The large shear wave
velocity of PPv is clearly caused by its large
G
,
despite the larger density of this phase. Nega-
tive change in
V
is because of density increase
and unchanged bulk modulus. The PPv transition
should be observed preeminently through S wave
discontinuities. The pressure dependence of the
isotropic
B
and
G
obtained from the
ab initio
lat-
tice dynamics study (Wentzcovitch
et al
., 2004)
are shown in Figure 7.2 along with those of the
Pv structure at some different temperatures. Cal-
culations reported that
PPv two-phase loop (Tsuchiya &
Tsuchiya, 2008b), where effect of the atomic
random distribution was captured through the
multiple configuration sampling technique. In
contrast, the phase relations in MgSiO
3
-FeSiO
3
(or Fe
2
O
3
) are still unclear. However, even though
the transition proceeds in a large two-phase loop,
iron partitioning between Pv (PPv) and ferroper-
iclase (Fp) affects to reduce the phase transition
width (Sinmyo
et al
., 2011). Some high-pressure
experiments recently reported considerably broad
coexisting regions (Nishio-Hamane
et al
., 2007;
Catalli
et al
., 2009). But two-phase widths and
even the transition pressures themselves are
quite different in different studies, strongly
suggesting fundamental difficulty in the accurate
determination of multi-component phase rela-
tions by using a laser-heated diamond anvil cell
(LH-DAC) technique.
Incorporation of iron in the structure raises
several complexities such as the oxidation state,
Pv
+
dV
P
dV
S
dT
,and
dV
dT
,
of Pv
dT
0.11 ms
−
1
K
−
1
are
−
0.32,
−
0.32, and
−
at 50 GPa
and 2000 K, respectively.
Investigations of some low-pressure analogs of
MgSiO
3
PPv such as MgGeO
3
and CaIrO
3
, which
are isostructural to the PPv phase of MgSiO
3
but (substantially) lower
P
T
, reported that they
shows the compression behavior roughly similar
to MgSiO
3
, but the velocity contrasts across the
