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

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K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251
disappeared and the sharp single (200) peak was
observed upon heating, indicating that Ca-perovskite
adopts cubic structure at high temperature. NALphase,
which is a K-bearing Al-phase, was not
found in X-ray diffraction patterns and TEM observations
in the present study [5,31]. It is possibly present
as a host of potassium, but the amount should be very
minor.
In runs #3 and #5, (101) peak of stishovite changed
to doublet both at 300 K and high temperatures,
showing a second-order structural phase transition
from tetragonal to orthorhombic. Representative Xray
diffraction spectrum is presented in Fig. 2a. A
mineral assemblage of Mg-perovskite +CaCl2-type
SiO2 phase + Ca-perovskite+CaFe2O4-type Al-phase
was obtained from 67 GPa and 1760 K to 100 GPa
and 2060 K (Fig. 1). Observed and calculated X-ray
diffraction pattern is presented in Table 3. Phase
transition boundary between Al-bearing stishovite to
CaCl2-type phase in MORB composition is located at
~62 GPa and 2000 K.
At higher pressures above 113 GPa at 2240 K
(runs #4, #6, and #7), X-ray diffraction pattern drastically
changed (Fig. 2b). These X-ray peaks can be
explained by the coexistence of MgSiO 3-rich postperovskite
phase +a-PbO 2-type SiO 2 +Ca-perovskite+CaFe
2O 4-type Al-phase. This mineral assemblage
was confirmed to 134 GPa and 2300 K,
corresponding to the condition at the base of the
mantle. Recently Ono et al. [7] reported CaTi2O4type
Al-phase with a unit-cell volume of 150.2 A˚ 3 in
MORB composition at 143 GPa. This volume is
much smaller than that of CaFe 2O 4-type phase observed
in this study at 132 GPa (173.8 A˚ 3 )(Table 1).
It contrasts with the fact that volume change is
minimal at the phase transition between CaFe2O4type
and CaTi2O4-type structures in Mg-end-member
MgAl2O4 [15].
Phase transition from (Al,Fe)-bearing MgSiO3 perovskite
to a post-perovskite phase occurred between
100 and 113 GPa at 2060–2240 K. This is consistent
with the result on a natural pyrolite composition [32],
in which the post-perovskite phase transition was
observed between 103 and 115 GPa at 2060–2550
K on the basis of Tsuchiya’s gold scale same as in this
study. Pressures of post-perovskite phase transition in
pyrolite and MORB compositions are apparently
lower by about 10 GPa than that in pure MgSiO3
composition determined by using platinum pressure
standard. It could be primarily due to the difference in
the pressure scale used in the experiments. In addition,
there should be compositional effects on post-perovskite
phase transition, but they are not yet well understood.
MORB contains high Al2O3, FeO, Fe2O3,
and Na2O. Presence of Al2O3 stabilizes perovskite
relative to post-perovskite phase [33]. Post-perovskite
phase includes much more Na 2O than perovskite
(Table 2), indicating that Na 2O expands the stability
P–T field of the post-perovskite phase. Partitioning of
iron is under hot debate [32,34]. It depends on the
valence state and spin state, both of which significantly
affect the ionic radius of iron. It is noted that
Mg 2+ site is smaller in post-perovskite phase than in
perovskite.
Phase transition from CaCl 2-type to a-PbO 2-type
SiO 2 phase occurred also between 100 and 113 GPa.
a-PbO 2-type phase includes large amount of Al 2O 3
(Table 2), which should remarkably expands its stability
P–T field. Murakami et al. [12] demonstrated
that phase transition between CaCl2-type and a-PbO2type
structures occurs above 121 GPa and 2400 K in
pure SiO2 based on platinum pressure scale.
4. Mineral chemistry
Chemical compositions of coexisting phases were
determined for both low-pressure (perovskite-dominant)
and high-pressure (post-perovskite phase-dominant)
assemblies, recovered respectively from 60 and
113 GPa (Table 2). A typical TEM image is presented
in Fig. 3. Analyses were made only for the part
coexisting with gold grains. The coexistence of gold
indicates that this portion was heated to high temperatures.
The TEM analyses confirmed the mineral
assemblages same as those observed in X-ray diffraction
patterns. Both MgSiO3-rich post-perovskite phase
and Ca-perovskite converted to amorphous on release
of pressure. Chemical analyses showed that the heated
area lost 10–15% iron, which is usually observed in
the LHDAC experiments due to relatively large thermal
gradient [32].
Post-perovskite phase is similar in composition to
Mg-perovskite, except that the former has remarkably
high Na2O content. Similar observation was made in a
natural pyrolite composition [32]. Here we consider