Fig. 1. Pressure - temperature diagram s h o w i n g t h e e x p e r i m e n t a l c o n d i t i o n s . Conditions of metastable appearance of MgSiO 3 ilmenite and stishovite are also shown. 26 IN SITU X-RAY DIFFRACTION STUDY OF POSTSPINEL TRANSFORMATION KINETICS IN Mg 2 SiO 4 AT HIGH-PRESSURE AND HIGH-TEMPERATURE High pressure transformation kinetics of mantle minerals are very important for understanding the dynamics of mantle convection and the oceanic plate s 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 b o u n d a r y c a n b e r e c o r d e d , w h i c h s h o u l d significantly affect the transformation rate. Dissociation of Mg 2 SiO 4 spinel into MgSiO 3 perovskite and MgO periclase (the postspinel transformation) is believed to be of utmost importance in the division of the Earth s mantle into 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 high- temperature. High-pressure in situ X-ray diffraction experiments spinel perovskite + periclase Pressure (GPa) Temperature ( ° C) 20 22 24 26 28 30 1300 1200 1100 1000 900 800 700 ilmenite and stishovite do not appear stishovite appears with periclase ilmenite appears with periclase equilibrium boundary were performed using a SPEED1500 multi-anvil high pressure apparatus installed at beamline BL04B1 [4]. The starting material is a sintered mixture of Mg 2 SiO 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 2 SiO 4 spinel to MgSiO 3 perovskite and periclase obtained at 27.7 GPa and 1085 ° C. pv 1050 sec 40 sec Energy (keV) 10 50 60 70 80 90 100 110 230 sec pv pv pv pv pv pv pv pv pv pv pv pc sp Au sp Au sp sp sp Au Au Au Au sp Au il st il Intensity Fig. 2. Changes of X-ray diffraction patterns of the sample (2 θ = 5.0 ° ) during the postspinel transformation at 27.7 GPa and 1085 ° C (sp: Mg 2 SiO 4 spinel, pv: MgSiO 3 perovskite, pc: periclase, il: MgSiO 3 ilmenite, st: stishovite). 27 Under these conditions, the transformation was completed in 1050 seconds. This figure indicates that, in addition to perovskite and periclase, MgSiO 3 ilmenite and SiO 2 stishovite were observed concurrently. Ilmenite and stishovite are thought to be metastable under these conditions, since these p h a s e s d i s a p p e a r e d u p o n c o m p l e t i o n o f transformation. The metastable appearance of 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]. Fig. 3. Plots of transformed volume fraction with time in the postspinel transformation. Figure 3 shows the time dependence of the transformed volume fraction estimated from the integrated intensities of the spinel diffraction lines. Based upon these kinetic data combined w i t h p r e v i o u s r e s u l t s , w e c o n s t r u c t e d transformation - temperature - time (t-t-t) diagrams showing the 5% transformation line at an overpressure of ∼ 1 GPa and ∼ 4 GPa (Fig. 4). E x t r a p o l a t i o n o f t h e s e l i n e s t o l o w e r temperatures could be useful in evaluating whether or not the postspinel transformation occurs within the time scale for subduction ( ∼ 10 6 years) in the cold interior ( ∼ 700 ° C) of the descending oceanic plates. 23.7 GPa 1115 ° C 1 0.8 0.6 0.4 0.2 0 4000 8000 12000 Transformed fraction (vol.) Duration (s) 0 26.7 GPa 910 ° C 1 0.8 0.6 0.4 0.2 0 2000 0 4000 6000 8000 10000 Transformed fraction (vol.) Duration (s) 27.7 GPa 1085 ° C 1 0.8 0.6 0.4 0.2 0 500 1000 1500 0 Duration (s) Transformed fraction (vol.) 1 0.8 0.6 0.4 0.2 0 500 1000 1500 0 Duration (s) Transformed fraction (vol.) 22.7 GPa 1200 ° C 28 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. 0.65 0.7 0.75 0.85 0.8 0.9 1.0 0.95 1.05 -5 0 5 10 15 25 20 ln (t ( min ) ) Temperature ( ° C) 1 min 1 day 1 year 10 2 year 10 6 year 1 sec 10 4 year 800 1200 1100 1000 900 700 ∆ P ∼ 4 GPa 1000/ T (K) ∆ P ∼ 1 GPa
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