Geoscience Reference
In-Depth Information
the distinct compositional anomaly accompanied
with accumulated dense and hot large MORB
piles appears less likely to exist broadly in the
deep mantle. However, some chemical variations
would still be plausible in particular places. In
particular, large S velocity reductions reaching
∼−
km depth beneath Mariana (Niu
et al
., 2003)
of
ln
V
P
∼
2%
match with the present contrasts calculated be-
tween MORB and pyrolite (Figures 7.11 and 7.12),
suggesting that the observed mid-lower mantle
anomalies might be caused by the presence of
subducted oceanic crust.
0%,
ln
V
S
∼−
2%, and
ln
ρ
∼+
10% observed underneath the south Atlantic
and Indian Oceans (Wen, 2001; Wen
et al
., 2001)
might match the chemical anomaly due to some
unknown materials much denser than MORB or
partial melting.
Our calculations indicate that subducted
MORB should cause an S and P velocity per-
turbation ratio (
ln
V
S
ln
V
P
7.7
Concluding Remarks
In the present chapter, we reviewed the current
sttaus of computational studies of high-pressure
elasticity of major mantle phases and crustal
phases and discussed their aggregate properties.
Results clearly indicate that the pyrolitic com-
position can explain the properties of the lower
mantle better than more silicic compositions, and
that the accumulated basaltic piles seem unsuit-
able to explain the LLSVP. Note, however, that,
we assumed that the mineral phase proportion
and the element partitioning are unchanged with
pressure. Their pressure dependencies, in addi-
tion to including other factors such as anisotropy
(e.g., Panning & Romanowicz, 2004), would fur-
ther improve the present mineralogical model
of the lower mantle. The detailed optimizations
would also give some tight constraints on the
temperature structure of the lower mantle.
)of0
∼−
1. This ratio is
positive (
3) for most of the lower mantle
(Masters
et al
., 2000; Karato & Karki, 2001).
Therefore, we conclude that subducted MORB
is unlikely to be present in a large region
of the lower mantle. Figure 7.12 also shows
that subducted MORB has faster bulk sound
velocity
+
2
∼
than
pyrolite.
The
contrast
ln
V
reaches
1.5% to 2.5% throughout the lower
mantle region primarily caused by the high bulk
velocity of silica in MORB. As mentioned above,
MORB has slower shear velocities than pyrolite
(
ln
V
S
∼−
∼+
2%). These results lead to another
important implication for the interpretation of
the velocity heterogeneity. Global tomographic
models (Masters
et al
., 2000) and regional studies
(Niu
et al
., 2003) revealed the anticorrelation
between lateral heterogeneities in
V
and
V
S
in
the middle-to-lower part of the lower mantle
including regions above D
. Such an anticorre-
lation was interpreted to imply the presence of
chemical heterogeneity (e.g., Karato & Karki,
2001). However, our present study shows that
such an observation can be attributed to the role
of PPv that is present in the D
layer (because
PPV has unusually high Vs).
The present calculations also indicate that sub-
ducted MORB can generate a shear and bulk
perturbation ratio (
ln
V
ln
V
S
References
Akaogi, M., Hamada, Y., Suzuki, T., Kobayashi, M., &
Okada, M., 1999. High pressure transitions in the
system MgAl
2
O
4
-CaAl
2
O
4
: a new hexagonal alumi-
nous phase with implication for the lower mantle,
Phys Earth Planet Inter
,
115
, 67-77.
Akaogi, M., Tanaka A., Kobayashi, M., Fukushima N.
& Suzuki, T. 2002. High-pressure transformations in
NaAlSiO
4
and thermodynamic properties of jadeite,
nepheline, and calcium ferrite-type phase,
Phys Earth
Planet Inter
,
130
, 49-58.
Alf e, D., Alfredsson, M., Brodholt, J., Gillan, M. J.
Towler M.D., & Needs, R.J., 2005. Quantum Monte
Carlo calculations of the structural properties and
the B1-B2 phase transition of MgO,
Phys Rev B
,
72
,
014114.
1.0, which is
much larger than the ratio inferred from seismic
tomography (
)of
∼−
0.1), suggesting that even some
amount of MORB (
∼−
∼
10%) can explain the seismo-
logical observations. Interestingly, characteristics
of the velocity anomalies observed at
∼
1100
