IN SITU X-RAY DIFFRACTION STUDY OF
pv
POSTSPINEL TRANSFORMATION KINETICS
IN Mg2 SiO4 AT
pv
pv
pv
HIGH-PRESSURE AND HIGH-TEMPERATURE
pv
il
plateÕs descent into the Earth's deep interior [1]. In
situ X-ray diffraction using synchrotron radiation is
especially suitable for the study of transformation
kinetics at high pressure [2,3], because it is
possible to collect a large amount of kinetic data
efficiently. Simultaneously, precise measurements
of the overpressure from the equilibrium phase
boundary can be recorded, which should
significantly affect the transformation rate.
Dissociation of Mg 2 SiO 4 spinel into MgSiO3
perovskite and MgO periclase (the postspinel
transformation) is believed to be of utmost
10
Temperature (° C)
sp
50
60
70
Au Au
sp
sp
sp Au
80
90
100
110
Fig. 2. Changes of X-ray diffraction patterns of
the sample (2θ = 5.0° ) during the postspinel
transformation at 27.7 GPa and 1085 ° C (sp:
Mg 2 SiO 4 spinel, pv: MgSiO3 perovskite, pc:
periclase, il: MgSiO3 ilmenite, st: stishovite).
were performed using a SPEED1500 multi-anvil
high pressure apparatus installed at beamline
BL04B1 [4]. The starting material is a sintered
mixture of Mg 2SiO 4 spinel and gold. Pressure was
calculated from the lattice constants of gold [5].
The sample was compressed to the desired
pressure at room temperature, and then heated to
the desired temperature. The heating rate was
maintained at about 500 ° C/min. When the
temperature reached the desired value, it was kept
constant. Then, X-ray diffraction profiles were
taken every 45 − 600 seconds. In this way, we
were able to observe the postspinel transformation
at 22.7 − 28.1 GPa and 860 − 1200 °C (Fig. 1).
The equilibrium boundary of the postspinel
transformation determined by SPEED1500 [6] was
used in this study.
Figure 2 shows changes in diffraction patterns
from Mg 2SiO 4 spinel to MgSiO 3 perovskite and
periclase obtained at 27.7 GPa and 1085 °C.
900 ilmenite appears
with periclase
800
perovskite + periclase
28
Au
Energy (keV)
1000
24
26
Pressure (GPa)
sp
Au
Au
equilibrium
boundary
22
230 sec
Au
ilmenite and stishovite
do not appear
stishovite appears
with periclase
spinel
st
sp
upper and lower parts. Since the kinetics of this
transformation have never been examined, we
have addressed this issue in an in situ X-ray
diffraction study at high-pressure and hightemperature.
High-pressure in situ X-ray diffraction experiments
700
20
pvpv
40 sec
importance in the division of the EarthÕs mantle into
1100
pv
Intensity
dynamics of mantle convection and the oceanic
1200
pv
il
minerals are very important for understanding the
1300
pc
pvpv
High pressure transformation kinetics of mantle
1050 sec
pv
30
Fig. 1. Pressure - temperature diagram
showing the experimental conditions.
Conditions of metastable appearance of
MgSiO3 ilmenite and stishovite are also shown.
26
Under these conditions, the transformation was
Figure 3 shows the time dependence of the
completed in 1050 seconds. This figure indicates
transformed volume fraction estimated from the
that, in addition to perovskite and periclase,
MgSiO3 ilmenite and SiO2 stishovite were observed
integrated intensities of the spinel diffraction
concurrently. Ilmenite and stishovite are thought to
with previous results, we constructed
be metastable under these conditions, since these
phases disappeared upon completion of
transformation - temperature - time (t-t-t)
lines. Based upon these kinetic data combined
diagrams showing the 5% transformation line at
an overpressure of ∼1 GPa and ∼4 GPa (Fig. 4).
transformation. The metastable appearance of
0.8
0.6
0.4
0.2
0
0
Transformed fraction (vol.)
22.7 GPa
1200 °C
500
1000
Duration (s)
1
0.6
0.4
0.2
0
0
4000
8000
(∼10 6 years) in the cold interior (∼700 °C) of the
descending oceanic plates.
1
0.8
0.6
0.4
0.2
0
1500
23.7 GPa
1115 °C
0.8
temperatures could be useful in evaluating
whether or not the postspinel transformation
occurs within the time scale for subduction
Transformed fraction (vol.)
1
Extrapolation of these lines to lower
0
Transformed fraction (vol.)
Transformed fraction (vol.)
ilmenite and stishovite was often observed at other
pressure and temperature conditions (Fig. 1). The
in situ X-ray data combined with SEM and TEM
observations of the recovered sample indicate that
spinel transforms into fine lamellae of stishovite
and periclase, and ilmenite and periclase as
intermediates in the postspinel transformation [7].
500
1000
Duration (s)
1500
1
0.8
0.6
0.4
0.2
00
12000
Duration (s)
27.7 GPa
1085 °C
26.7 GPa
910 °C
2000 4000 6000 8000 10000
Duration (s)
Fig. 3. Plots of transformed volume fraction with time in the postspinel transformation.
27
Temperature (° C)
1200 1100 1000
900
800
700
106 year
25
104 year
20
ln (t (min))
102 year
15
1 year
∆P∼ 1 GPa
10
1 day
∆P∼ 4 GPa
5
0
1 min
-5
1 sec
0.65
0.7
0.75 0.8 0.85 0.9
1000/ T (K)
0.95 1.0
1.05
Fig. 4. Transformation - temperature - time diagrams for the
postspinel transformation showing the 5% transformation with
the overpressure of ~1 GPa (triangle) and ~4 GPa (circle).
Tomoaki Kubo and Eiji Ohtani
Tohoku University
E-mail: tkubo@mail.cc.tohoku.ac.jp
References
[1] A. E. Ringwood, Phys. Earth Planet. Inter. 86 (1994) 5.
[2] D. C. Rubie et al., J. Geophys. Res. 95 (1990) 15829.
[3] T. Kubo et al., Geophys. Res. Lett. 25 (1998) 695.
[4] W. Utsumi et al., Rev. High Pressure Sci. Technol. 7 (1998)1484.
[5] O. L. Anderson et al., J. Appl. Phys. 65 (1989)1534.
[6] T. Irifune et al., Science 279 (1998) 1698.
[7] Tomoaki Kubo, Eiji Ohtani, Takumi Kato, Satoru Urakawa, Akio Suzuki,
Yuichi Kanbe, Ken-ichi Funakoshi, Wataru Utsumi and Kiyoshi Fujino,
Geophys. Res. Lett. 27 (2000) 807.
28