SHEAR WAVE VELOCITY JUMP AT THE OLIVINE- SPINEL TRANSFORMATION IN Fe2SiO4 BY ULTRASONIC MEASUREMENTS IN SITU. Akira FUKIZAWA* and Hajimu KINOSHITA**

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1 J. Phys. Earth, 30, , 1982 SHEAR WAVE VELOCITY JUMP AT THE OLIVINE- SPINEL TRANSFORMATION IN Fe2SiO4 BY ULTRASONIC MEASUREMENTS IN SITU Akira FUKIZAWA* and Hajimu KINOSHITA** * Institute for Solid State Physics, the University of Tokyo, Tokyo, Japan ** Department of Earth Sciences, Chiba University, Chiba, Japan (Received March 27, 1982; Revised June 12, 1982) Ultrasonic shear wave velocities of polycrystalline Fe2SiO4 olivine and spinel were measured at pressures of up to 5.2GPa and temperatures of up to X-ray diffraction analysis on the recovered sample indicates that the sudden increase in shear wave velocity is associated with the olivine-spinel transformation. About 60% completion of the transformation was observed in the particular run. No anomalous decrease in shear wave velocity was detected at the transformation. 1. Introduction Many reports have been made on the velocity measurements of candidate mantle minerals including high-pressure minerals quenched to the ambient state as metastable forms (e.g., MIZUTANI et al., 1970; LIEBERMANN, 1972). However, the majority of these measurements has been performed at 1 bar or at relatively low pressures, usually below 3.0GPa, and at room temperature. There has been a limited number of velocity measurements under high-pressure and high-temperature conditions comparable to those of the earth's mantle. In particular, no velocity measurements have been reported for the temperature range above at pressures higher than 2.0GPa. Recently, ITO et al. (1977) developed a technique for measuring ultrasonic technique for higher temperatures have been made in the present investigation. In this paper, we report the results of shear wave velocity measurements at up to of Fe2SiO4, with special reference to the measurement in situ of shear wave velocity jump at the olivine-spinel transformation. 245

2 246 A. FUKIZAWA and H. KINOSHITA A wedge-type cubic-anvil high-pressure apparatus equipped with 16-mmedge cemented tungsten carbide anvils was employed in the present experiment. used as a pressure-transmitting medium and a sample container. Details of the mechanism of pressure generation in the apparatus have been given by ICHINOSE et al. (1975) and WAKATSUKI and ICHINOSE (1982). Pressure values in the present experiment were calibrated against press load at room temperature by means of pressure fixed points such as low-bi (2.55GPa), T1 (3.67GPa), and low-ba (5.5 GPa). Polycrystalline powders of Fe2SiO4 olivine and spinel were hot-pressed and used as starting materials in the present ultrasonic measurements. Fe2SiO4 olivine for 1hr. Powder samples of Fe2SiO4 spinel, which had been synthesized in ad- for ultrasonic measurement by grinding both ends with polishing paper. The final dimensions of the samples were about 4mm in diameter and 2 to 3mm in s ervation of the polished surface of the samples. Bulk density measured by the Archimedes method was 96 to 98% of the X-ray density for each polymorph of Fe2SiO4. The sample assembly used in the present experiment is shown in Fig. 1. The pyrophyllite cube was divided into three blocks. Both sample and transducer were placed in a center hole of the central pyrophyllite block 10mm in thickness. Fe2SiO4 sample, which was connected to a LiNbO3 transducer through a metallic Fig. 1. Sample assembly used for high pressure and high temperature ultrasonic measurement.

3 Shear Wave Velocity Jump was attached to one end of the sample and used as a reflector. The buffer, 5mm in diameter and 5mm in length, which was made of tantalum, was placed in the well-fired pyrophyllite sleeve. This sleeve protected the buffer from deformation in the course of compression. The LiNbO3 transducer with shear vibration mode of 10MHz fundamental resonant frequency was bonded with silver paste to one end of the buffer. The other end of the buffer was attached to the sample without any bond. An NaCl disk placed on the transducer protected it from destruction during compression. The sample was heated by a tubular graphite furnace 6mm in length, 8mm in outer diameter and 7mm in inner diameter. Temperatures were measured with a chromel/alumel thermocouple, 0.2mm in diameter, at the surface of the sample. No correction was made for the pressure effect on the e.m.f. of the thermocouple. It was found in the course of the present experiment that the signals of the LiNbO3 transducer became very weak in the reducing condition at temperatures higher than As shown in Fig. 1, utlizing a furnace of short length, the end of which was separated by about 3mm from the transducer, and the use of buffer, proved to be effective in keeping the transducer at much lower temperatures. This made it possible to catch the signals from the The electrical circuit system used for the ultrasonic measurements was exactly the same as the one developed by KINOSHITA et al. (1979). Ultrasonic signals were fed to and received from the transducer via copper lead wire through the preformed gaskets and the anvil gaps. The time intervals of signals reflected from both ends of the sample were measured using a conventional pulse-echo comparison method. The precision of the time interval measurement was about The ultrasonic wave velocity of the sample was determined from the time interval and the length of the sample. In the determination of shear wave velocity (Vs) at high pressure and temperature, the change in length of the sample, L/L0, with pressure and temperature was corrected by the relations L0/L=1+(P/3K0) adopted the values reported by SUMINO (1979) and SUZUKI et al. (1981), respectively. For corresponding values of Fe2SiO4 spinel, those reported by LIEBERMANN (1975) and MAO et al. (1969) were used. In addition to the reversible deformation, less than 0.5% permanent contraction of the samples was observed after the conclusion of the experiments. We assumed the contracted length as the zero pressure length L0 and used the velocity calculation. The uncertainty in Vs determination was estimated to be 3%. In the present sample assembly, polycrystalline sample was in direct contact with buffer without any bonding, and therefore, considerable confining pressure was necessary to get the reflections from both ends of the sample. Actually, because of the bad contact of the sample with the buffer, no reflected signals from

4 248 A. FUKIZAWA and H. KINOSHITA other end of the sample could be detected at pressures below the ram load of 30 kg/cmcm2 (ca. 1.5GPa). 3. Results and Discussion The typical reflection pattern observed by the present technique is shown in cidental signal, R1 and R2 indicate the signals reflected from both the front and back faces of the sample. Figure 3 shows the change in shear wave velocity of Fe2SiO4 olivine and spinel with a pressure of up to 5.2GPa at room temperature. Apparently, the velocity of both olivine and spinel increases linearly with increasing pressure. These Vs vs. pressure data were well represented with a straight line by a least squares fit. As already mentioned, because of the limitation of the present technique, the velocity data below ca. 1.5GPa could not be obtained. The data obtained below 2.0 GPa were not used for the calculation, because the pressure values in the cubic press are less reliable in this pressure range owing to the formation of the gasket. up to 0.75GPa. This is far below the present value, as small as about one-sixth. The present value for Fe2SiO4 olivine is rather concordant with the corresponding and ANDERSON (1969). Based on the linear fitting, the shear wave velocity of Fe2SiO4 olivine and an incident signal and R1, R2 indicate the signals reflected from both ends of the sample.

5 Shear Wave Velocity Jump Fig. 3. Change in shear wave velocity of Fe2SiO4 olivine and spinel with pressure up to 5.2GPa. Different symbols denote the velocity data obtained from different experiments. Table 1. Shear wave velocity of Fe2SiO4 olivine and spinel at atmospheric pressure and at room temperature. spinel was extrapolated to zero-pressure. The zero-pressure velocities are compared in Table 1 with those appearing in previous publications (CHUNG, 1971; AKIMOTO, 1972; LIEBERMANN, 1975; SUMINO, 1979). In Table 1, the data by Sumino are the only ones for single crystal Fe2SiO4 olivine which was measured by the rectangular parallelepiped resonance method. The other data were obtained by the ultrasonic methods using hot-pressed polycrystalline aggregates. As we measured the velocity only along the cylindrical axis, there is a possibility that the preferred orientation of grains of polycrystalline sample produces anomalously low or high velocity values. To evaluate this possibility, we compared the present estimate of Vs for olivine with that of SUMINO (1979), calculated by using the Voigt-Ruess-Hill averaging schema. The present estimate is about 2% higher

6 250 A. FUKIZAWA and H. KINOSHITA than his. On the consideration of uncertainty for velocity determination, we assume that our polycrystalline samples possess a comparable degree of elastic isotropy. Recently, BASSETT et al. (1982) measured the elastic moduli of single crystal of forsterite at pressures up to 4GPa by the Brillouin scattering method. We hope that the pressure dependence of elastic moduli and velocities of fayalite single crystal will be measured soon to remove the undesirable effect of preferred orientation of grains in the present data. Figure 4 shows the change in Vs of Fe2SiO4 olivine and spinel with temperature, at constant pressure. It was observed that the velocity of Fe2SiO4 olivine at pres- Fig. 4. Change in shear wave velocity of Fe2SiO4 olivine and spinel with temperature, at Fig. 5. Change in shear wave velocity of Fe2SiO4 olivine with temperature at 5.2GPa.

7 Shear Wave Velocity Jump 251 ity jump finally reached 11% over 3.5hr. The recovered sample after the run indicates contraction of length by 2.1%. This permanent deformation is fairly large compared with any other ordinary run in which the contraction is less than 0.5%. With the X-ray powder diffraction method, the recovered sample was contraction of the sample length was produced by the volume change associated with the partial transformation. According to the phase diagram for the olivine-spinel transformation in is estimated to be 5.15GPa. This indicates that the present velocity measurement at 5.2GPa, shown in Fig. 5, was carried out in the spinel field throughout the varia- transformation. It is also expected that, because of the large volume change associated with the transformation, the cell pressure might be reduced a little while the ram load is held constant. This may explain the present result wherein the transformation ceased at an intermediate state, leaving about 40% untransformed olivine in the sample. If the Fe2SiO4 olivine is completely transformed into spinel at this pressure and temperature, the degree of Vs jump will reach 14%. This result indicates that even at high pressure and temperature conditions where actual phase transformation takes place in the earth's interior, a considerable amount of velocity jump reaching about 14% can be expected across the olivine-spinel phase boundary. In the earth's mantle, a seismic discontinuity of around 400km has been explained by the olivine-modified spinel transformation (RINGWOOD, 1970; AKIMOTO et al., 1976). Recently, FUKAO et al. (1982) have determined shear velocity structure in the mantle transition zone, and they concluded that the amount of the shear velocity jump across the 400km discontinuity is about 8%. This estimation is too low compared with the present determination of Vs jump associated with the olivine-spinel transformation, even though some correction is required for the olivine-modified spinel transformation instead of the olivine-spinel transformation. It is obvious that the simple olivine-mantle model is incompatible with the seismic velocity structure. To explain the difference, at the depth of about 400km in the mantle, the existence of a considerable amount of other silicate minerals such as orthopyroxene, clinopyroxene and garnet is necessary in addition to olivine. Recently, POIRIER (1981) has proposed a martensitic transformation model for the olivine-spinel transformation. He suggested that the phase transforma-

8 252 tion from olivine to spinel structure may take place by the gliding of (100) planes in the olivine structure, resulting in the change of the stacking of the close-packed oxygen ions from the approximately hexagonal stacking to the cubic close-packing as exists in the spinel structure. LACAM et al. (1980) and HAMAYA and AKIMOTO (1982) have succeeded in observing the crystallographic orientation relationship favorable to Poirier's model. Poirier also suggested that the shear modulus would decrease just before the transformation. If the decrease in shear modulus is fairly large, anomalous decrease in shear velocity would be observed even in the polycrystalline aggregate by means of the ultrasonic method. However, in the present experiment, no detectable change in Vs was observed at time of the transformation. Further, recent detailed microscopic observation of the surface structure of spinel crystals, which were transformed from single crystal olivine in Ni2SiO4, is not compatible with Poirier's model, because no dislocations were observed in the spinel crystals (KOMATSU et al., 1982). There still remains some uncertainty as to the transformation mechanism. We are grateful to Prof. S. Akimoto, Institute for Solid State Physics, the University of Tokyo, for discussion and critical reading the manuscript. We wish also to express our thanks to Dr. T. Yagi, Institute for Solid State Physics, the University of Tokyo, for useful suggestions in the course of the present experiment. REFERENCES AKIMOTO, S., The system MgO-FeO-SiO2 at high pressure and temperature-phase equilibria and elastic properties, Tectonophysics, 13, , AKIMOTO, S., Y. MATSUI, and Y. SYONO, High-pressure crystal chemistry of orthosilicates and the formation of the mantle transition zone, in The Physics and Chemistry of Minerals and Rocks, ed. R.G.J. Strens, pp , Wiley Interscience, London, AKIMOTO, S., T. YAGI, and K. INOUE, High temperature-pressure phase boundaries in silicate system using in-situ X-ray diffraction, in High Pressure Research, ed. M.H. Manghnani and S. Akimoto, pp , Academic Press, New York, BASSETT, W.A., H. SHIMIZU, and E.M. BRODY, Pressure dependence of elastic moduli of forsterite by Brillouin scattering in diamond cell, in High Pressure Research in Geophysics, ed. S. Akimoto and M.H. Manghnani, pp , Center for Academic Publications Japan, Tokyo, CHUNG, D.H., Elasticity and equations of state of olivines in the Mg2SiO4-Fe2SiO4 system, Geophys. T.R. Astron. Soc., 25, , FUKAO, Y., T. NAGAHASHI, and S. MORI, Shear velocity in the mantle transition zone, in High Pressure Research in Geophysics, ed. S. Akimoto and M.H. Manghnani, pp , Center for Academic Publications Japan, Tokyo, transformation: Growth of single crystal spinel from single crystal olivine in Ni2SiO4, in High Pressure Research in Geophysics, ed. S. Akimoto and M.H. Manghnani, pp , Center for Academic Publications Japan, Tokyo, ICHINOSE, K., M. WAKATSUKI, and T. AOKI, A new sliding type cubic anvil high pressure apparatus, Pressure Eng., J. Japan High Pressure Inst., 13, , 1975 (in Japanese).

9 Shear Wave Velocity Jump 253 ITO, H., H. MIZUTANI, K. ICHINOSE, and S. AKIMOTO, Ultrasonic wave velocity measurements in solids under high pressure using solid pressure media, in High Pressure Research, ed. M.H. Manghnani and S. Akimoto, pp , Academic Press, New York, KINOSHITA, H., N. HAMAYA, and H. FUJISAWA, Elastic properties of single-crystal NaCl under high pressures to 80kbar, J. Phys. Earth, 27, , KOMATSU, H., N. HAMAYA, and S. AKIMOTO, Evidences of nucleation-and-growth mechanism in Ni2SiO4, olivine-spinel transformation, submitted to Nature, 1982, KUMAZAWA, M. and O.L. ANDERSON, Elastic moduli, pressure derivatives, and temperature derivatives of single-crystal olivine and single-crystal forsterite, J. Geophys. Res., 74, , LACAM, A., M. MADON, and J.P. POIRIER, Olivine glass and spinel formed in a laser heated, diamond anvil high pressure cell: An investigation by transmission electron microscopy, Nature, 288, , LIEBERMANN, R.C., Compressional velocities of polycrystalline olivine, spinel and rutil minerals, Earth Planet. Sci. Lett., 17, , and silicates, Geophys. J.R. Astron. Soc., 42, , MAO, H.K., T. TAKAHASHI, W.A. BASSETT, J.S. WEAVER, and S. AKIMOTO, Effect of pressure and temperature on the molar volumes of wustite and of three (Fe, Mg)2SiO4 spinel solid solutions, J. Geophys. Res., 74, , MIZUTANI, H., Elasticity of mantle minerals and the constitution of the upper mantle, Ph. D. Thesis, the University of Tokyo, Tokyo, MIZUTANI, H., Y. HAMANO, Y. IDA, and S. AKIMOTO, Compressional-wave velocities of fayalite, Fe2SiO4 spinel, and coesite, J. Geophys. Res., 75, , POIRIER, J.P., On the kinetics of olivine-spinel transition, Phys. Earth Planet. Inter., 26, , RINGWOOD, A.E., Phase transformations and constitution of the mantle, Phys. Earth Planet. Inter., 3, , SUMINO, Y., The elastic constants of Mn2SiO4, Fe2SiO4 and Co2SiO4 and the elastic properties of olivine group minerals at high temperature, J. Phys. Earth, 27, , SUZUKI, I., K. SEYA, H. TAKEI, and Y. SUMINO, Thermal expansion of fayalite, Fe2SiO4, Phys. Chem. Miner., 7, 60-63, WAKATSUKI, M. and K. ICHINOSE, A wedge-type cubic anvil high pressure apparatus and its application to material synthesis research, in High Pressure Research in Geophysics, ed. S. Akimoto and M.H. Manghnani, pp , Center for Academic Publications Japan, Tokyo, 1982.

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