Geoscience Reference
In-Depth Information
PPv transition accompanies marginal,
+
1.5%,
An NaAlSiO
4
component is known to be
dissolved in MgAl
2
O
4
under mantle tempera-
tures (e.g., Ricolleau
et al
., 2010), but Figure 7.5
demonstrates the quite comparable wave speeds
and density of the end-member phases, strongly
suggesting that dissolving of NaAlSiO
4
in a small
amount marginally affects the wave speeds of
the aluminous phase (Kawai & Tsuchiya, 2010;
Tsuchiya, 2011). Stability and elasticity of the
calcium titanate (CT)-type phase of MgAl
2
O
4
were also reported in Tsuchiya (2011). Although
a high-pressure transition of the CF-type to
CT-type phase was experimentally suggested to
occur at
and
0.9% velocity jumps in
V
P
,
V
S
,and
V
respectively (Tsuchiya
et al
., 2004b), while the
CaCl
2
-
α
-PbO
2
−
transition accompanies
+
0.9%,
−
2.5% in
V
P
,
V
S
,and
V
respec-
tively. Both transitions can therefore produce the
so-called
V
S
/V
anticorrelation in different ways,
but this should be discussed considering the bulk
rock elasticity as in the later section.
1.3%, and
+
7.4.2 MgAl
2
O
4
An aluminous high-pressure phase with the
MgAl
2
O
4
-based composition was first identified
by Liu (1978) and later Irifune
et al
. (1991), which
assigned the calcium ferrite (CaFe
2
O
4
, CF)-type
structure to this phase. CF-type MgAl
2
O
4
has
the crystal structure with the space group
Pbnm
,
which has a framework composed of AlO
6
octahedral double chains with magnesium ions
located in tunnels formed by the octahedral
chains. Its elastic wave velocities and density are
shown in Figure 7.5 as a function of pressure.
40-50 GPa (Funamori
et al
., 1998;
Catti, 2001), no phase change was identified
at least up to 150 GPa irrespective of applying
LDA or GGA (Tsuchiya (2011), consistent with
the experimental observation by Irifune
et al
.
(2002). But the calculated enthalpies of both
phases were found to be quite comparable with
only a marginal difference of less than 5 kJ/mol
remaining, suggesting that the CT-type phase
could stabilize due to additional effects such as
temperature, contamination by some minor ele-
ments, etc. The compressional (
V
P
) and shear (
V
S
)
velocities of the CT-type phase were calculated
to be 12.7 and 7.1 km/s at 60 GPa, respectively
(Tsuchiya, 2011). These are almost comparable
to the wave speeds of the CF-type phase within
2%, strongly implying that the velocity discon-
tinuities associated with the CF-to-CT phase
change are likely smeared in the lower mantle
due to its minor amount and would be hard to
be detected seismologically. Recent experiments
have also reported some additional phases related
to MgAl
2
O
4
.An
ε
phase was found to stabilize
in a narrow pressure range around 40 GPa (Ono
et al
., 2006). This phase has, however, not been
identified in the natural rock composition so far.
Apart from this, an orthorhombic phase with
the Mg
2
Al
2
O
5
composition was yielded through
a dissociation reaction of MgAl
2
O
4
∼
16
MgAl
2
O
4
NaAlSiO
4
V
P
12
V
Φ
8
V
S
ρ
4
spinel at
0
50
100
150
200
∼
25 GPa (Enomoto
et al
., 2009). However, the
temperature where the reaction occurred is above
∼
P (GPa)
2300 K, much higher than the normal mantle
geotherm in this pressure range,
Fig. 7.5
Static elastic wave velocities and density
calculated for the CF-type MgAl
2
O
4
(solid lines) and
NaAlSiO
4
1900 K. Since
these additional phases are expected to only have
∼
(dashed lines) as a function of pressure.
