Phase transition and density of subducted MORB crust in the lower ...

Density (g/cm 3 )
Density (g/cm 3 )
Density (g/cm 3 )
Density (g/cm 3 )
6.5
6.0
5.5
5.0
4.5
5.6
5.4
5.2
5.0
4.8
4.6
5.6
5.4
5.2
5.0
4.8
4.6
5.6
5.4
5.2
5.0
4.8
4.6
4.4
a
b
c
d
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 247
PREM
MgPv+MgPP
SiO 2 -phases
CaPv
CF
40 60 80 100 120 140
Pressure (GPa)
5.2. Density profile of MORB
On the basis of measured crystal chemistry and
volume data, the density of each constituent mineral
was calculated for both 300 K and high temperatures.
The chemical composition of each phase is assumed
to be constant in perovskite-dominant (below 100
GPa) and post-perovskite phase-dominant assemblies
(above 104 GPa), respectively. Phase transformation
between stishovite and CaCl 2-type structure is second-order,
and therefore both phases likely have similar
chemical compositions. The mineral densities
were plotted in Fig. 5, together with the PREM density
profile [42] in order to illustrate which phase
contributes to the buoyancy of the subducted
MORB crust in the lower mantle.
Mg-perovskite is the densest phase below 100 GPa
and is much denser than the mean lower mantle (Fig.
5a). The post-perovskite phase is denser than perovskite
by about 3% at the phase transition. SiO2 phases
are least compressible and are the lightest mineral in
MORB composition in a pressure range studied here
(Fig. 5b). Al-bearing stishovite and CaCl2-type SiO2
phase are less dense than the PREM density at high
temperatures. The density a-PbO 2-type SiO 2 phase
strongly depends on the substitution mechanism of
Al 2O 3. Two types of mechanism, (1) oxygen vacancytype
and (2) octahedral vacancy-occupied-type, are
considered. In both the cases, Al-bearing a-PbO2type
SiO2 phase is remarkably less dense than
CaCl2-type phase at equivalent pressure. The density
of SiO2 phase in MORB composition decreases at the
phase transition to a-PbO 2-type phase due to the
incorporation of much higher Al 2O 3 content. a-
PbO 2-type SiO 2 phase significantly contributes to
the buoyancy of subducted MORB crust in the lowermost
mantle. Ca-perovskite is marginally denser
than the mean lower mantle (Fig. 5c). The density
Fig. 5. Density profile of each constituent mineral in MORB
composition. Two types of Al substitution mechanisms in a-
PbO2-type SiO2 phase, oxygen vacancy-type (reversed triangles)
and octahedral vacancy-occupied-type (squares), are considered
here. Other symbols are same as those in Fig. 4. The PREM density
is shown for comparison by broken lines [42]. High-temperature
(2070–2410 K) data by Ono et al. [7] are also plotted by pluses. The
main difference is seen in the density profile of CaFe2O4-type Al
phase, which is primarily due to the difference in chemical composition,
especially in FeO content.