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

Phase transition and density of subducted MORB crust in
the lower mantle
Kei Hirose a,b, *, Naoto Takafuji a,c , Nagayoshi Sata b , Yasuo Ohishi d
a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
b Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa 237-0061, Japan
c Division of Earth and Planetary Sciences, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
d Japan Synchrotron Radiation Research Institute, Mikazuki-cho, Hyogo 679-5198, Japan
Abstract
Earth and Planetary Science Letters 237 (2005) 239–251
Received 7 March 2005; received in revised form 31 May 2005; accepted 16 June 2005
Editor: S. King
Phase relations, mineral chemistry, and density of a natural mid-oceanic ridge basalt (MORB) composition were investigated
up to 134 GPa and 2300 K by a combination of in-situ X-ray diffraction measurements and chemical analyses using transmission
electron microscope (TEM). Results demonstrate that the MORB composition consists of MgSiO3-rich perovskite, stishovite,
CaSiO 3 perovskite, and CaFe 2O 4-type Al-phase in the upper part of the lower mantle. The most abundant mineral of MgSiO 3rich
perovskite undergoes phase transition to a CaIrO3-type post-perovskite phase above 110 GPa and 2500 K. Post-perovskite
phase is similar in composition to perovskite except considerably high Na2O content. Stishovite transforms to CaCl2-type SiO2
phase above 62 GPa and 2000 K and further to a-PbO 2-type phase above 110 GPa. a-PbO 2-type SiO 2 phase includes large
amount of Al2O3, which significantly expands its stability relative to CaCl2-type phase. Phase transition of CaSiO3 perovskite
from tetragonal to cubic was also observed with increasing temperature. CaFe2O4-type Al-phase is stable to the bottom of the
mantle. The density of MORB crust was calculated using volume data, combining with measured chemical compositions and
calculated mineral proportions. The former MORB crust is denser than the average lower mantle at all depths greater than ~720
km, contrary to earlier predictions. The subducted basaltic crust may have accumulated at the base of the mantle.
D 2005 Elsevier B.V. All rights reserved.
Keywords: MORB; phase transition; density; lower mantle; post-perovskite; in-situ X-ray observations
* Corresponding author. Department of Earth and Planetary
Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguro, Tokyo 152-8551, Japan. Tel.: +81 3 5734 2618; fax: +81
3 5734 3538.
E-mail address: kei@geo.titech.ac.jp (K. Hirose).
0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.06.035
1. Introduction
www.elsevier.com/locate/epsl
It is known that MORB composition consists
of Mg-perovskite, stishovite, Ca-perovskite, and
CaFe2O4-type Al-phase below 720-km depth in the
upper part of the lower mantle [1–7]. Recent high-

240
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251
pressure studies on end-member compositions suggest
that these minerals may undergo structural phase
transitions at greater depths. Phase relation and crystal
chemistry in MORB bulk composition, however,
are not yet well understood under the deep lower
mantle conditions [1,7]. Both experimental and theoretical
studies showed that pure MgSiO3 perovskite
transforms to a CaIrO3-type post-perovskite phase
near the base of the mantle (e.g., [8–10]). MgSiO 3rich
perovskite includes large amounts of FeO,
Fe 2O 3, and Al 2O 3 in MORB bulk composition,
which could significantly affect the stability of postperovskite
phase. Pure SiO2 phase transforms from
stishovite to CaCl2-type structure [11] and further to
a-PbO2-type structure [12] in the lower mantle. Phase
transition from tetragonal or orthorhombic to cubic
structure in CaSiO 3 perovskite has been reported in
both pure [13] and Al-bearing compositions [14].
Funamori et al. [15] demonstrated that CaFe 2O 4type
MgAl2O4 transforms to CaTi2O4-type structure
above 40 GPa, suggesting that phase transition of
CaFe2O4-type Al-phase in MORB composition possibly
occurs in the lower mantle. Here we determined
the phase transition boundaries in MORB composition
up to the condition of core–mantle boundary
region, based on X-ray diffraction measurements insitu
at high-pressure and -temperature. Phase transitions
in subducted MORB crust may be the causes of
seismic anomalies observed locally but in a wide
range of depths in the lower mantle (e.g., [16–20]).
Subduction of basaltic crust gives rise to a strong
chemical heterogeneity in the mantle. The amount of
basaltic component that has ever subducted into the
Earth’s interior over the geological time possibly sums
up to several tens percent of the whole mantle [21].
The fate of basaltic crust may be controlled by the
density relationship with the surrounding mantle. Previous
studies suggested that basaltic crust is buoyant
in the transition zone at 660- to 720-km depth
[2,5,6,22] and possibly in the lower mantle below
1500–2000-km depth [4,23]. In this paper, we determined
the chemical composition of each constituent
mineral by using analytical TEM. The density of
MORB crust in the lower mantle was calculated
from measured volume and chemical composition
data. Results indicate that the subducted MORB
crust is denser than the surrounding below 720-km
depth to the bottom of the mantle.
2. Experimental and analytical procedures
Starting material was a glass (runs #1 to #4) or gel
(runs #5 to #7) (Table 1). Both are chemically identical
with a composition of normal MORB (Table 2).
The same glass starting material was used in our
previous studies [2,5]. High P–T conditions were
generated using laser-heated diamond anvil cell
(LHDAC) techniques. The sample was mixed with a
fine powder of gold (~10 wt.%) that served as an
internal pressure standard and a laser absorber. The
sample mixture (~20-Am thick) was loaded into a
rhenium gasket with an initial thickness of ~50 Am,
being sandwiched by pure basalt layers that was not
mixed with gold (~15-Am thick for both sides). About
50-Am area was heated with a focused multimode
continuous wave Nd:YAG laser using double-side
heating technique, which minimizes both radial and
axial temperature gradient [24]. The laser beam was
not scanned on the sample.
Angle-dispersive X-ray diffraction measurements
were conducted in-situ at high-pressure and -temperature
at BL10XU of SPring-8. A monochromatic
incident X-ray beam with a wavelength of 0.41328
or 0.41688 A˚ was collimated to 20-Am in diameter.
X-ray diffraction spectra were obtained on an imaging
plate with exposure time of 1 to 10 min. Twodimensional
X-ray diffraction image was integrated
as a function of two-theta in order to give a conventional
one-dimensional diffraction profile. The
diffraction patterns of the sample were repeatedly
collected during heating and after quenching to
room temperature.
The uncertainty in temperature within 20-Am area
from which X-ray diffraction was collected could be
F10% [14]. Pressure was determined by applying the
equation of state (EOS) of gold, using two to four
diffraction lines. Pressure determination strongly
depends on the choice of EOS (e.g., [7]). We primarily
used EOS recently proposed by Tsuchiya [25]. Tsuchiya’s
gold scale is practically useful because it is
consistent with platinum pressure scale at 300 K [26].
Pressures calculated by EOS of gold proposed by
Jamieson et al. [27] were also presented in Table 1,
which predicts larger thermal pressure than Tsuchiya’s
EOS. The pressure uncertainties shown in Table 1 are
derived mainly from large errors in temperature to
apply P–V–T EOS.

Table 1
Volumes of coexisting minerals
Run P (GPa) Au
by Tsuchiya
[25]
P (GPa) Au
by Jamieson
et al. [27]
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 241
T (K) Unit-cell volumes (A˚ 3 )
MgPv
[Z =4]
MgPP
[Z =4]
St (rutile-type
SiO2) [Z =2]
CaCl 2-type
SiO2 [Z =2]
a-PbO 2-type
SiO2 [Z =4]
CaPv
[Z =1]
CF [Z =4]
#1 55.4 (14) 56.4 (17) 1890 145.68 (21) 41.64 (4) 39.62 (1) 205.27 (66)
49.4 (13) 50.6 (16) 1750 147.94 (10) 42.09 (1) 40.12 (1) 208.02 (12)
39.7 (3) 38.8 (3) 300 147.96 (14) 42.21 (3) 40.25 (4) 207.86 (75)
#2 59.6 (16) 60.8 (19) 2140 144.86 (12) 41.27 (3) 39.22 (1) 202.65 (65)
56.6 (15) 57.9 (18) 2060 146.31 (15) 41.70 (2) 39.59 (2) 203.93 (116)
47.1 (3) 45.6 (2) 300 146.00 (9) 41.51 (6) 39.49 (5) 204.30 (36)
#3 70.6 (15) 70.9 (19) 2170 139.99 (9) 40.43 (16) 38.29 (1) 197.09 (35)
60.3 (3) 57.3 (3) 300 140.07 (7) 40.57 (5) 38.19 (6) 197.38 (15)
73.2 (12) 72.4 (15) 1770 138.75 (8) 39.97 (6) 37.93 (1) 195.43 (22)
68.6 (12) 68.3 (15) 1770 139.94 (12) 40.44 (12) 38.22 (0) 197.29 (5)
62.0 (3) 58.8 (3) 300 139.39 (13) 40.31 (6) 38.11 (5) 196.08 (21)
#4 132.0 (18) 124.4 (23) 2130 124.61 (8) 74.81 (13) 34.30 (2) 175.91 (33)
122.2 (14) 110.2 (12) 300 124.09 (14) 74.71 (15) 34.25 (4) 176.11 (49)
#5 85.0 (12) 83.3 (16) 1980 135.83 (21) 39.06 (11) 37.09 (1) 190.99 (28)
78.3 (8) 72.9 (7) 300 134.29 (8) 39.20 (8) 36.94 (5) 189.72 (18)
81.4 (9) 75.5 (8) 300 133.95 (12) 38.85 (6) 36.82 (5) 188.78 (19)
100.0 (24) 96.5 (27) 2060 131.81 (19) 38.04 (13) 36.17 (4) 186.24 (28)
86.4 (12) 79.8 (10) 300 132.88 (14) 38.50 (8) 36.35 (5) 186.83 (15)
#6 112.6 (25) 108.0 (29) 2240 128.93 (8) 77.38 (17) 35.46 181.82 (57)
104.3 (12) 95.0 (10) 300 128.52 (6) 76.89 (8) 35.41 (4) 181.47 (28)
104.3 (13) 95.0 (11) 300 128.43 (10) 76.64 (2) 35.25 (4) 181.44 (21)
106.9 (17) 97.2 (14) 300 128.62 (10) 76.51 (5) 35.16 (5) 180.34 (61)
#7 129.9 (17) 123.1 (22) 2290 125.30 (6) 75.19 (9) 34.22 (2) 177.42
127.0 (24) 120.5 (28) 2280 125.65 (15) 75.66 (13) 34.63 (3) 178.32 (62)
125.2 (13) 118.7 (18) 2170 126.18 (8) 75.92 (10) 34.73 (1) 178.62 (34)
116.4 (12) 105.2 (10) 300 125.50 (6) 75.48 (9) 34.64 (3) 177.62 (24)
118.7 (9) 107.2 (7) 300 125.40 (15) 75.24 (8) 34.53 (2) 177.72 (27)
132.3 (11) 118.9 (10) 300 122.76 (7) 73.79 (14) 33.91 (2) 173.75 (28)
Numbers in parenthesis indicate uncertainties in the last digits. MgPv, MgSiO3-rich perovskite; MgPP, MgSiO3-rich post-perovskite phase; St,
stishovite; CaPv, CaSiO3 perovskite; CF, CaFe2O4-type Al-phase.
Table 2
Chemical compositions of starting material and coexisting phases at 60 and 113 GPa
Sample SM 60 GPa (run #2) 113 GPa (run #6)
MgPv St CaPv CF MgPP a-PbO2-SiO2 CaPv CF
N a
7 6 3 9 7 6 3 4
SiO2 49.64 41.5 (24) 96.6 (11) 50.3 (21) 28.7 (26) 36.5 (22) 87.4 (18) 47.1 (24) 30.7 (13)
TiO2 1.64 2.4 (5) – 0.3 (2) 1.6 (7) 2.9 (8) – 0.7 (6) 0.5 (1)
Al2O3 14.88 13.7 (19) 3.4 (11) 2.9 (6) 35.5 (29) 12.1 (15) 12.6 (18) 4.2 (30) 37.1 (32)
FeO* 11.43 20.1 (12) – 0.8 (6) 10.5 (28) 23.2 (27) – 2.7 (18) 9.6 (8)
MgO 8.51 21.3 (32) – 1.6 (4) 9.0 (26) 19.5 (19) – 1.7 (7) 8.2 (12)
CaO 10.55 1.0 (6) – 42.9 (9) 2.2 (23) 0.9 (9) – 43.6 (16) 1.7 (13)
Na2O 2.90 – – 0.8 (4) 12.5 (12) 5.0 (14) – – 12.1 (11)
K2O 0.12 – – 0.4 (4) – – – – –
Proportion
(wt.%)
35 (5) 17 (2) 23 (3) 25 (4) 38 (3) 23 (2) 23 (2) 16 (3)
Numbers in parenthesis indicate one standard deviation in the last digits. SM, starting material [2]; MgPv, MgSiO3-rich perovskite; St,
stishovite; CaPv, CaSiO3 perovskite; CF, CaFe2O4-type Al-phase; MgPP, MgSiO3-rich post-perovskite phase.
a
Number of analyses.

242
We conducted seven separate experiments at pressures
from 40 to 134 GPa (Fig. 1). In each run, the
sample was heated at a single P–T condition of interest,
although both pressure and temperature slightly
changed during heating (Table 1). Only in run #5,
second heating cycle was made after the sample was
once quenched and further compressed at room temperature.
The heating duration was 49 to 154 min at
1750–2300 K.
After the sample was recovered from DAC, it was
Ar ion-thinned for TEM analysis. Chemical composition
of each constituent mineral was determined
by NORAN Instruments/Voyager energy-dispersive
(EDS) analytical system attached with JEOL-2010
TEM operating at 200 kV. We calculated the chemical
Temperature (K)
2500
2000
1500
1000
500
0
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251
compositions from measured X-ray intensities using
K-factors. These K-factors were determined experimentally
using natural alkali–basalt glass as standard
[28–30]. All the chemical analyses were made only
for extremely thin parts where electron transparent
rates were higher than 94%.
3. Phase transition in MORB
In the first two sets of experiments (runs #1 and
#2), four-phase assemblage of Mg-perovskite +stishovite
+Ca-perovskite +CaFe2O4-type Al-phase was observed
in the X-ray diffraction patterns, up to 60 GPa
and 2140 K (Fig. 1). Funamori et al. [3] reported
Depth (km)
800 1200 1600 2000 2400 2800
MgPv
+St
+CaPv+CF
MgPv
+CaCl2-SiO2
+CaPv+CF
20 40 60 80 100 120 140
Pressure (GPa)
MgPP
+α-PbO2-SiO2
+CaPv+CF
Fig. 1. Phase relations of MORB composition in the lower mantle. Squares and triangles indicate the phase assemblage of MgSiO 3-rich
perovskite (MgPv)+stishovite (St) or CaCl2-type SiO2 phase+CaSiO3 perovskite (CaPv)+CaFe2O4-type Al-phase (CF). Circles represent the
assemblage of MgSiO3-rich post-perovskite phase (MgPP)+a-PbO2-type SiO2 phase+CaPv+CF. A broken line shows the tetragonal to cubic
transition boundary in CaSiO 3 (+3 wt.% Al 2O 3), according to Kurashina et al. [14].

CaMgAl-rich perovskite with intermediate composition
and structure between Mg-rich and Ca-rich perovskites
at similar P–T conditions, but such CaMgAlperovskite
was not observed in this study. A splitting
Intensity
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 243
(a) T-quenched from
69GPa, 1770K
MgPv+CaCl2-SiO2 +CaPv+CF
CF200
PP020
CF200
SC011
SC101
11.00 11.08
MP002+110
CF220
MP111
SC110
(b) T-quenched from
113GPa, 2240K
MgPP
+α-PbO2-SiO2 +CaPv+CF
SA110
CP110+011
MP020
MP112
CF320
MP200
PP022
SA111
PP110
CP110+011
CF320
PP111
Au111
SA021
PP040
PP041
CP111, CF131
Au200
PP023, SA102
SA121
CF420
PP131, CF311
SA112
PP042
CF141+250
CP200
CF040
Au111
SC101
MP120
MP210
MP121
MP103
SC011
SC020
of (200) peak of cubic Ca-perovskite was recognized
after quenching to room temperature in all the pressure
range studied here (Fig. 2). This suggests a
distortion to tetragonal structure [13,14]. The splitting
CP111, CF131, MP211
Au200
CF420
MP202
CF311
MP113
SC120
SC210 MP122
MP212
CF141+250
CP200
CP002
MP004
MP220
MP023, CF241
MP221
CF411
14.0 14.2
6 8 10 12
2θ
14 16 18
CP002
PP132
PP113
PP004
CF331+241
CF401
CF411
CP200
CP002
MP213+130
MP114
MP131+222, SC211
SC121
SC220
CP211+121
Au220
MP204
Fig. 2. X-ray diffraction pattern at (a) 62 GPa and 300 K after heating at 67–73 GPa and 1770–2170 K and (b) at 104 GPa and 300 K after
heating at 113–114 GPa and 2050–2240 K. MP, MgSiO3-rich perovskite; SC, CaCl2-type SiO2 phase; CP, CaSiO3 perovskite; CF, CaFe2O4-type
Al-phase; PP, MgSiO 3-rich post-perovskite phase; SA, a-PbO 2-type SiO 2 phase; Au, gold. The calculated peak positions are shown by vertical
bars. Enlarged patterns around 118 and 148 of two-theta angle show the peak splitting for CaCl 2-type SiO 2 phase and CaSiO 3 perovskite,
respectively.
SA202
SA221
PP133+150
MP
SC
CP
CF
Au
PP
SA
CP
CF
Au

244
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

K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 245
Fig. 3. TEM image of the sample recovered from 113 GPa and 2240
K. Abbreviations are similar to those in Fig. 2.
Na2SiO3 end-member. Post-perovskite phase has a
chemical formula on the join [(Mg,Fe 2+ ,Ca)1 X
(Al,Fe 3+ ) 2X(Si,Ti) 1 X]O 3–Na 2SiO 3 (Table 2). It suggests
a high abundance of Fe 3+ in the post-perovskite
phase (Fe 3+ /total Fe =0.81).
Stishovite includes 3.4 wt.% Al2O3 at 60 GPa,
consistently with the results of previous studies on
MORB composition [1–6]. On the other hand, a-
PbO2-type SiO2 phase includes significantly high
Al2O3 content (12.6 wt.%) at 113 GPa. It has been
reported that a-PbO 2-type TiO 2 phase contains large
amount of Fe 2O 3 [35]. A trivalent cation of Al 3+ may
substitute Si 4+ with oxygen vacancy. Alternatively, Al
may be incorporated without oxygen vacancy under
such ultra-high pressure conditions. Half of the octahedral
sites of oxygen hexagonal close-packed structure
are vacant in a-PbO2-type structure. Such vacant
site may be partially occupied by Al ions when three
Si 4+ ions are substituted by four Al 3+ ions.
Both Ca-perovskite and CaFe 2O 4-type Al-phase
exhibit similar chemical compositions in perovskitedominant
and post-perovskite phase-dominant assemblies.
CaFe2O4-type Al-phase has complex chemical
formula approximately on the join NaAlSiO4–
(Mg,Fe)Al2O4 [36]. Relatively wide chemical variations
when synthesized in MORB bulk composition
were reported in the literature [1–6,22].
Mineral proportions were estimated by mass-balance
calculations using all oxides analyzed except
K2O. Results show 35% Mg-perovskite, 25%
CaFe2O4-type Al-phase, 23% Ca-perovskite, and
17% stishovite in weight in the low-pressure assembly.
The high-pressure assembly consists of 38%
MgSiO 3-rich post-perovskite phase, 23% Ca-perovskite,
23% a-PbO2-type SiO2, and 16% CaFe2O4type
Al-phase. A remarkable decrease of CaFe2O4type
Al-phase is due to the partitioning of high Al2O3
into a-PbO2-type SiO2 phase and high Na2O into
post-perovskite phase (Table 2).
5. Density of MORB crust in the lower mantle
5.1. Volume of each constituent mineral
The unit-cell volumes of coexisting phases were
determined from the X-ray diffraction patterns both at
high temperature (1750 to 2290 K) and at 300 K
(Table 1). These room temperature P–V data were
fitted to the Birch–Murnaghan equation of state in
order to obtain the bapparentQ compressibility of
each constituent mineral in MORB composition. Previous
experimental studies suggest that chemical composition
of each phase changes little with increasing
pressure to 100 GPa [1,3,4].
The isothermal bulk modulus (K 0) was obtained to
be 217(F2) GPa for Mg-perovskite, assuming that
pressure derivative of the isothermal bulk modulus,
KV, is 4 and the volume at ambient condition (V0) is
169.5 A˚ 3 [2,4,6] (Fig. 4a). This is much lower than the
values of ~261 GPa previously determined for Mgend-member
composition (e.g., [37]). Mg-perovskite
in MORB composition includes large amount of
Al 2O 3 and Fe 2O 3. These results suggest that solution
of Al 2O 3 and Fe 2O 3 significantly reduces the bulk
modulus, consistently with theoretical predictions on
the effect of Al2O3 [38,39]. It is, however, noted that
chemical composition of Mg-perovskite synthesized
in MORB bulk composition may be different at each
pressure, which could cause apparent increase in the
compressibility.
The volumes of post-perovskite phase are plotted
in Fig. 4a, together with those of Mg-perovskite.
The unit-cell parameters of post-perovskite phase
are a =2.472(1) A˚ , b =8.096(2) A˚ , and c =6.134(1)
A˚ at 132 GPa and 300 K. Each unit-cell length is
larger by about 6% than that of pure MgSiO3 postperovskite
phase measured at 121 GPa and 300 K
[8]. Previous experiments on pure MgSiO3 demon-

246
strated that the volume decreased by about 1% at
the post-perovskite phase transition [8]. These P–V
data in MORB composition, however, do not indicate
such a large volume decrease. It should be due
to a change in the chemical composition from perovskite
to post-perovskite phase in MORB bulk
composition.
The P–V data of stishovite and CaCl2-type SiO2
were fitted together to the Birch–Murnaghan equation
of state (Fig. 4b). Fitting result shows
K0=279(F13) GPa and V0=47.3(F0.3) A˚ 3 when
KV=4, which are quite consistent with the previous
results on Al-bearing stishovite [40]. a-PbO2-type
phase in MORB composition contains 12.6 wt.%
Al2O3 that significantly expands volumes. The unitcell
volume of a-PbO2-type phase synthesized in
MORB composition is 73.79 A˚ 3 at 132 GPa (Table
1), which is larger by about 4% than that of pure
SiO2 phase obtained at 136 GPa [12]. It is noted
that the unit-cell volume of a-PbO2-type phase
[Z =4] is remarkably larger than double unit-cell
volume of CaCl2-type phase [Z =2] at equivalent
pressure (Fig. 4b).
The volumes of Ca-perovskite were determined on
the basis of tetragonal structure (space group; P4/
mmm) proposed by Shim et al. [13] at 300 K and
cubic structure at high temperatures. The P–V data of
Ca-perovskite and CaFe2O4-type Al-phase from 40 to
132 GPa respectively lie on a single compression
curve (Fig. 4c and d). Fitting to the Birch–Murnaghan
equation of state show K0=245(F6) GPa and
V0=45.6(F0.2) A˚ 3
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251
for Al-bearing Ca-perovskite
and K 0=214(F8) GPa and V 0=239.7(F1.5) A˚ 3 for
CaFe 2O 4-type Al-phase, when KV is fixed at 4. Previous
measurements of the compressibility of CaFe 2O 4type
Al-phase showed relatively wide variation in the
value of K0. Present result is higher than that measured
by Guignot and Andrault [36] (~190 GPa) but is
lower than that by Ono et al. [41] (243 GPa).
Fig. 4. Change in the unit-cell volumes as a function pressure for (a)
MgSiO 3-rich perovskite (circles) and post-perovskite phase
(squares), (b) stishovite (circles), CaCl2-type SiO2 (triangles), and
a-PbO2-type SiO2 (squares), (c) CaSiO3 perovskite, and (d)
CaFe2O4-type Al-phase. Half unit-cell volumes are plotted for a-
PbO 2-type SiO 2 phase. Closed and open symbols indicate room
temperature and high temperature (1750–2290 K) data, respectively.
Solid lines show apparent compression curves at 300 K. See text for
details.
unit-cell volume (Å 3 unit-cell volume (Å )
3 ) unit-cell volume (Å 3 unit-cell volume (Å )
3 )
155
150
145
140
135
130
125
43
42
41
40
39
38
37
40
38
36
34
210
200
190
180
170
a
b
c
d
MgPv+MgPP
SiO 2 -phases
CaPv
CF
40 60 80 100 120 140
Pressure (GPa)

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.

248
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251
of CaFe 2O 4-type Al-phase is larger the PREM density
even at high temperatures (Fig. 5d). It is noted, however,
that chemical composition of CaFe 2O 4-type Alphase
is variable depending on the bulk MORB composition
as summarized by Guignot and Andrault
[36], which significantly affects the mineral density.
Ono et al. [7] calculated remarkably lower densities
for this mineral primarily due to the difference in
chemical composition.
The net density of MORB composition is calculated
from the density of each constituent mineral and
mineral proportions. The density profile of MORB
composition is presented in Fig. 6. Fitting of pressure–density
data for perovskite-dominant assembly
at 300 K to the Birch–Murnaghan equation of state
gives K0=222(F6) GPa and q0=4.22(F0.02) g/cm 3
when KV is fixed at 4 (Fig. 6a). The density of
MORB crust increases by about 1% at ~2400-km
depth, if Al substitutes Si by using octahedral vacancy
(without forming oxygen vacancy) in a-PbO2-type
SiO2 phase. In contrast, if oxygen vacancy-type substitution
is assumed, the calculation shows that density
of MORB crust marginally decreases at the
pressure-induced phase transition, which is physically
unreasonable. It indicates that the former Al substitution
mechanism is dominant in a-PbO 2-type SiO 2
phase.
The subducted MORB crust is denser than the
average lower mantle at all depths greater than 720
km where MORB crust becomes perovskite-dominant
lithology [2,5,6], even after thermal equilibrium is
attained. The same conclusions are obtained when
pressure is estimated by using different EOS of gold
proposed by Jamieson et al. [27] (Fig. 6b).
This conclusion contrasts with earlier predictions
[4,23]. The mineral volumes were not measured at
high P–T in these previous studies, and instead compression
and thermal expansion data of the end-member
composition were used for each constituent
mineral. This resulted in a serious underestimate of
the density of MORB crust in the lower mantle [4,23].
It is true that MORB crust has extensively wide
chemical variations. Present study demonstrated that
only the SiO2 phases, especially a-PbO2-type phase,
contribute to the buoyancy of subducted MORB crust
relative to the surrounding mantle, using the particular
MORB sample. Ono et al. [7] showed that CaFe2O4type
Al-phase is also less dense than the normal
Rock density (g/cm 3 ) Rock density (g/cm 3 )
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
a
40 60 80 100 120 140
b
Depth (km)
1200 1600 2000 2400 2800
Pressure (GPa)
Depth (km)
1200 1600 2000 2400 2800
40 60 80 100 120 140
Pressure (GPa)
PREM
Au_Tsuchiya
PREM
Au_Jamieson
Fig. 6. Net density profile of MORB composition. Pressure was
calculated based on EOS of gold proposed by (a) Tsuchiya [25] and
by (b) Jamieson et al. [27]. Circles, MgPv+St+CaPv+CF; triangles,
MgPv+CaCl 2-type SiO 2+CaPv+CF; squares, MgPP+a-
PbO2-type SiO2+CaPv+CF. Closed and open symbols indicate
300 K and high temperature (1750–2290 K) data, respectively.
Broken lines indicate the PREM density [42]. The error of density
is typically 0.02 g/cm 3 , derived from the uncertainties in volumes
of coexisting phases and in mineral proportion. (a) Solid line shows
a density profile at 300 K for perovskite-dominant assembly fitted
to the Birch–Murnaghan equation of state. (b) Data by Ono et al.
[7] using Jamieson’s gold scale were shown for comparison (pluses).
Slightly lower density reported by [7] is primarily due to the
lower density of CaFe 2O 4-type Al-phase with a different chemical
composition.
mantle (Fig. 5d) using the different MORB composition.
However, both reached the same conclusion on
the density relationship between MORB and the nor-

mal mantle. The density crossover is not likely to
occur below 720-km depth, even for the highly
evolved MORB with high SiO 2 contents.
6. Implications for seismic heterogeneities in the
mid–lower mantle
Phase transition of subducted MORB crust may
cause local but large seismic heterogeneities in the
lower mantle. A number of seismic heterogeneities
have been found in the upper to middle layers of the
lower mantle (900–1850-km depth) (e.g., [16–20]).
These are often observed beneath convergent margins,
but they are not global [43]. Subduction of
MORB crust gives rise to a chemical and mineralogical
heterogeneity in the mantle, which could be
the origin of such local seismic anomalies.
A couple of phase transitions occur in the subducted
MORB crust in the upper- to mid–lower mantle
(Fig. 7). One is second-order structural phase
transition in SiO2 phase from stishovite to CaCl2type
structure. It is well known that this is ferroelas-
Temperature (K)
2500
2000
1500
1000
500
600 800 1000 1200 1400 1600 1800 2000
cubic CaPv
tetragonal
K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251 249
Depth (km)
normal mantle
hot slab
cold slab
stishovite
20 30 40 50 60 70 80 90
Pressure (GPa)
CaCl 2 -type SiO 2
Fig. 7. Temperature profiles of normal mantle and subducting slabs
[48], superimposed on the phase transition boundaries of Al-bearing
SiO2 phase (solid line) and CaSiO3 (+3 wt.% Al2O3) perovskite
(broken line). Phase transition of SiO2 phase occurs at ~1500-km
depth in the former MORB crust. The depth of phase transition in
Al-bearing Ca-perovskite can be variable, depending on temperature
of slabs and Al2O3 content [14].
tic-type phase transition. Carpenter et al. [44] showed
that seismic wave velocities, especially S-wave speed,
are significantly reduced near the phase transition; Swave
velocity drops by ~25% at the phase transition
and by 12–15% 100 km above and below the phase
transition. Present results demonstrate that phase transition
boundary is located at ~62 GPa and 2000 K,
corresponding to 1500-km depth (Fig. 7). Kaneshima
and Helffrich [17,19] found seismic scatters (called
M-phase anomaly) with at least 4% slow S-wave
velocity at 1400- to 1600-km depth northeast of the
Mariana subduction zone. The M-phase anomaly is
likely attributed to this ferroelastic-type phase transition
of SiO2 phase. Such seismic scatters should be
present in the former MORB crust, in which SiO2
phase is included about 20%. It is noted that SiO2
phase is not stable in a peridotitic mantle composition
(e.g., [32]). This is consistent with the fact that the
heterogeneity is not observed globally.
The lower mantle seismic heterogeneities are not
restricted to 1400- to 1600-km depth. For example,
Kaneshima and Helffrich [19] reported similar scatters
called S-phase anomaly at 1100- to 1450-km depth
and D-phase anomaly at 1700- to 1850-km depth in
the same area, although these anomalies are less intense
than the M-phase. Niu et al. [20] also found a
reflector at a depth of 1115 km in nearby area within
high-velocity anomaly in global tomographic models.
These anomalies are difficult to be reconciled with the
phase transition of SiO2 phase. Alternatively, phase
transition in Al-bearing Ca-perovskite may be responsible
for these heterogeneities. Ca-perovskite adopts
tetragonal structure at low temperature and transforms
to cubic with increasing temperature [13,14,45]. The
solution of Al 2O 3 enhances the distortion of perovskite
structure, and remarkably increases the transition
temperature to cubic perovskite. Present experiments
that Ca-perovskite includes 3 to 4 wt.% Al2O3 in
MORB composition, consistently with earlier studies
[1–6]. According to the experimental work by Kurashina
et al. [14], the tetragonal to cubic transition
boundary in CaSiO 3 (+3 wt.% Al 2O 3) is located at
about 1200 K and 50 GPa (Fig. 7). The phase transition
boundary is temperature-sensitive with a slightly
positive Clapeyron slope. Phase transition in CaSiO3
(+3 wt.% Al2O3) perovskite can occur in basaltic
crust layer of cold subducting slabs. Phase transition
is not expected in peridotite layer because Ca-perov-

250
skite contains less than 1 wt.% Al 2O 3 [23,32] and
transition temperature in such Al-poor Ca-perovskite
is too low. The depth of phase transition can be quite
variable in the upper to mid–lower mantle, due to the
variations in temperature of slabs and Al2O3 content
in Ca-perovskite (Fig. 7).
The tetragonal to cubic transition in Ca-perovskite
may also be ferroelastic-type. It is already known that
many distorted perovskites exhibit a ferroelastic behavior.
Acoustic velocity in these perovskites remarkably
drops across the structural transition to high
symmetry phases [46,47]. The subducted MORB
crust includes ~23% Al-bearing Ca-perovskite, and
phase transition in Ca-perovskite possibly causes
large seismic anomalies. Part of the lower mantle
seismic scatters/reflectors may be reconciled with
the phase transition in Al-bearing Ca-perovskite included
in subducted MORB crust. The most intense
M-phase anomaly observed at 1400–1600-km depth
[17,19] may be caused by the combined effects of
phase transitions in SiO2 phase and Ca-perovskite.
Acknowledgments
We thank Y. Tatsumi and S. Kaneshima for helpful
comments. The in-situ X-ray experiments were carried
out at SPring-8 (proposal no. 2004A3013-LD2np
and 2004B4013-LD2-np). Comments by anonymous
reviewers improved the manuscript.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.epsl.2005.06.035.
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