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Compressibility of the calcium aluminosilicate, CAS, phase to 44 GPa
Physics of the Earth and Planetary Interiors 150 (2005) 331–338
Compressibility of the calcium aluminosilicate, CAS, phase to 44 GPa Shigeaki Onoa,∗ , Tsuyoshi Iizukab , Takumi Kikegawac a
Institute for Frontier Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan b Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan c High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan Received 25 May 2004; received in revised form 2 December 2004; accepted 3 December 2004
Abstract In situ X-ray diffraction measurements on a calcium aluminosilicate (CAS) phase have been carried out using a laser-heated diamond anvil cell up to a pressure of 44 GPa, employing a synchrotron radiation source. CAS is the major mineral formed from sediments subducted into the Earth’s mantle. The sample was heated using a YAG laser after each pressure increment to relax the deviatoric stress in the sample. X-ray diffraction measurements were carried out at T = 300 K using an angle-dispersive technique. The pressure was calculated using an internal platinum metal pressure calibrant. The Birch–Murnaghan equation of state for the CAS phase obtained from the experimental unit cell parameters showed a density of ρ0 = 3.888 g/cm3 and a bulk modulus of K0 = 229 ± 9 GPa for K0 = 4.7 ± 0.7. When the first pressure derivative of the bulk modulus was fixed at K0 = 4, then the value of K0 = 239 ± 2 GPa. From the experimental compressibility, the density of the CAS phase was observed to be lower than the density of co-existing Al-bearing stishovite, calcium perovskite, calcium ferrite-type phases, and (Fe,Al)-bearing Mg-perovskite in subducted sediments in the lower mantle. Therefore, the density of subducted sediments in the lower mantle decreases with increasing mineral proportion of the CAS phase. © 2004 Elsevier B.V. All rights reserved. Keywords: CAS phase; Corundum; Equation of state; X-ray powder diffraction; High pressure; Subducted sediment
1. Introduction The phase relationships of sediments at high pressure and high temperatures are of geochemical interest ∗ Corresponding author. Tel.: +81 468 67 9763; fax: +81 468 67 9625. E-mail address: [email protected] (S. Ono).
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2004.12.001
because sediments are candidate components for creating the isotope heterogeneity of the Earth’s mantle (e.g., Hanyu and Kaneoka, 1997; Hofmann, 1997). The calcium aluminosilicate (CAS) phase, CaAl4 Si2 O11 , occurs in rocks formed from sediments and continental crust compositions at pressures corresponding to those found in the upper mantle and transition zone conditions. The formation of this CAS phase has been
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observed using multi-anvil high-pressure apparatus (Irifune et al., 1994), and it comprises some 5–10 wt.% of all minerals having sediment and continental crust compositions in the transition zone and the uppermost part of the lower mantle. Previous studies (Gautron et al., 1997, 1999) reported that this CAS phase has a hexagonal barium ferrite-type structure (space group P63 /mmc) in which some of the Si atoms are five-fold coordinated in a trigonal bipyramidal site. This is interesting, since it is known that silicates generally possess 4-fold or 6-fold coordinated Si atoms at both low and high pressures. Gautron et al. (1996) also detected this CAS phase in the transformation products of CaAl2 Si2 O8 (anorthite composition) at 14 GPa and 1500 ◦ C. The physical properties of a given phase in the subducted sediments may affect the dynamics of the subduction and material circulation in the mantle and, therefore, it is important to determine the compressibility of the CAS phase. Recently, naturally occurring CAS phase was discovered in a shocked Martian meteorite (Beck et al., 2004). The chemical composition of this natural CAS phase was not pure CaAl4 Si2 O11 , as it contained a significant concentration of sodium, and had a chemical composition of (Cax Na1−x )Al3+x Si3−x O11 . The pressure–temperature profile of the impact event that formed this CAS phase was not clear, and understanding of the physical properties and stability field of the CAS phase will contribute to the understanding of the formation of natural CAS phases such as that found in the shocked Martian meteorite. We used a laser-heated diamond anvil cell (LHDAC) and intense X-rays from a synchrotron radiation source that allowed us to acquire precise data on a sample under high pressure. We directly investigated the pressure–volume equation of state (EOS) of the CAS phase in subducted sediments over the pressure range 0.1 MPa–44 GPa, and we compared the density of the CAS phase with that of co-existing minerals in the mantle.
2. Experimental procedure A polycrystalline specimen of CAS phase and corundum was synthesized in a 1000 t multi-anvil apparatus (SEDI-1000) (see Takahashi et al., 1993), located at the Magma Factory at the Tokyo Institute of Technol-
ogy, Japan. An octahedral Cr2 O3 -doped MgO pressure medium was placed at the center of a tungsten carbide (WC) cubic anvil assembly employing pyrophyllite gaskets. A cylindrical rhenium metal heater was then inserted into the octahedral pressure medium, and this was enclosed within a LaCrO3 sleeve to provide thermal insulation. The powdered sample was loaded directly into the rhenium heater, which also served as a sample capsule. Before synthesizing the CAS phase and corundum, reagent-grade Al2 O3 , CaCO3 , and SiO2 powders were mixed and heated to 1000 ◦ C at ambient pressure. The chemical composition of the starting material was within the Al2 O3 and CAS phase composition. Then, the sample was ground and mixed with platinum powder, which was used as an internal pressure standard. The specimen used in this study was synthesized under a pressure of 20 GPa and a temperature of 1300 ◦ C for 30 min in the multi-anvil apparatus. The synthesized sample was examined using the microfocused X-ray diffraction technique at the Tokyo Institute of Technology, Japan. The experimental parameters of the microfocused X-ray diffraction were chromium radiation, 40 keV, 250 mA, 2 h counting times, and beam diameter of 100 m. The diffraction patterns confirmed that the sample contained the CAS phase, corundum, and platinum. Because the elastic parameters of corundum are well established, corundum was used to check the reliability of our experimental data. It is known that previous studies on the compressibility of high-pressure aluminous minerals provided controversial data. In the case of calcium-ferrite type phase of NaAlSiO4 , previous studies reported that the bulk modulus was in the range 185–275 GPa, which has a significant divergence of values (e.g., Sekine and Ahrens, 1992; Dubrovinsky et al., 2002; Guignot and Andrault, 2004). Therefore, the measured values of corundum in this study are an important check on the reliability of the CAS phase data. The synthesized sample was also examined using an electron probe microanalyzer (EPMA) at the Tokyo Institute of Technology, Japan. The experimental parameters used in the EPMA measurements were a 15 kV accelerating voltage, a 12 nA Faraday cup current, 20 s counting times, and an electron beam diameter of 1 m. The chemical composition of the synthesized CAS phase was Ca0.98 Al3.98 Si2.03 O11 , which was consistent with that reported by Gautron et al. (1996). The SiO2 and CaO content in the corundum were <0.3 wt.%. No chemical
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reaction between the sample and the capsule was observed. High-pressure X-ray diffraction experiments were carried out in the LHDAC. The synthesized sample was loaded into a 100 m diameter hole that had been pre-drilled into a rhenium gasket using an ArF excimer laser, MicroLas GeoLas 200 CQ, (Iizuka and Hirata, 2004). Argon was cryogenically loaded into the sample chamber, acting as a pressure transmitting and thermally insulating medium. The sample was heated after each change in pressure using a multimode continuous wave Nd:YAG laser to minimize any pressure inhomogeneity in the sample. The laser power and size of the heating spot were about 100 W and 50–100 m, respectively. An optical microscope was used to view the heated sample, and from the color of the sample’s emitted radiation the temperature was estimated to be about 2000 K. A typical duration of the sample annealing time was 5–10 min to produce data of sufficient quality for measuring the unit cell parameters. The sample was probed using angle-dispersive X-ray diffraction employing the BL13A synchrotron beam line at the photon factory (PF) of the High Energy Accelerator Research Organization (KEK), Japan (Ono et al., 2002a). The incident X-ray beam was monochro´˚ The X-ray beam matized to a wavelength of 0.4257 A. size was collimated to a diameter of about 50 m. The angle-dispersive X-ray diffraction patterns were obtained on an imaging plate X-ray data collection system (Rigaku RAXIS-IV, Japan). The observed intensities on the imaging plates were integrated as a function of 2θ using the Fit2d code software package (Hammersley, 1996) to obtain conventional, one-dimensional diffraction profiles. The diffraction peak positions were determined using a peak fitting program. The two intense platinum X-ray diffraction peaks (1 1 1 and 2 0 0) were observed, in addition to the diffraction peaks from CAS phase and corundum. The pressure was determined from the calculated platinum unit cell volume using the equation of state for platinum developed by Holmes et al. (1989). 3. Results The powder X-ray diffraction data of the CAS phase at ambient pressure revealed that it had a hexagonal barium ferrite-type structure (P63 /mmc), with unit cell di-
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´˚ and c = 12.728(26) A. ´˚ The mensions of a = 5.428(4) A observed and calculated X-ray diffraction patterns of the CAS phase are shown in Table 1. The volume and density of the CAS phase at ambient pressure were ´˚ 3 and 3.888(10) g/cm3 . The volume of CAS 324.7(8) A phase determined in this study was slightly higher than ´˚ 3 reported by Gautron et al. the value of 323.28(5) A (1999). Our sample was heated using the laser after each pressure increment, and the X-ray diffraction measurements were taken. The laser heating annealed the sample, and reduced any pressure gradient occurring across the sample, thus improving the quality of the X-ray diffraction data. Fig. 1 shows a typical X-ray diffraction pattern obtained. In addition to the diffraction peaks from the CAS phase and corundum, there were diffraction peaks arising from both the platinum internal pressure calibrant and the argon pressure transmitted medium. Small peaks ascribed to stishovite were also observed. The heating was performed at pressures >15 GPa, which corresponds to the stability field of the CAS phase, because we investigated the possibility that the CAS phase would dissociate into separate phases under metastable conditions. The entire sample was carefully heated by scanning with the laser beam to eliminate any potential reactions with either the argon pressure transmitting medium or the platinum pressure calibrant. A typical error in the pressure Table 1 Observed and calculated X-ray diffraction pattern of the CAS phase at 0 GPa hkl
˚´ dobs (A)
´˚ dcal (A)
dobs /dcal − 1
002 102 004 103 110 104 112 202 203 106 123 206 124 302 304
6.3648 3.7870 3.1746
6.3641 3.7810 3.1820 3.1495 2.7138 2.6350 2.4963 2.2047 2.0559 1.9336 1.6387 1.5747 1.5512 1.5214 1.4057
0.0001 0.0016 −0.0023
31 15 52
−0.0009 −0.0001 0.0010 0.0004 0.0000
21 79 33 28 100
0.0027 0.0007 −0.0028 −0.0006 0.0004
4 24 9 10 4
2.7115 2.6349 2.4989 2.2056 2.0559 1.6431 1.5758 1.5469 1.5205 1.4063
Iobs
Calculated d-spacings are based on hexagonal unit cell dimensions ´˚ and c = 12.728 A. ´˚ of a = 5.428 A
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Table 2 Lattice parameters and volumes of CAS phase and platinum to 44 GPa ´˚ a Platinum a (A)
P (GPa)b
´˚ CAS a (A)
´˚ c (A)
´˚ 3 ) Volume (A
3.923 (0) 3.856 (1) 3.836 (0) 3.815 (0) 3.795 (0) 3.781 (0) 3.771 (1)
0.0 15.9 21.7 28.2 35.1 40.4 44.0
5.428 (4) 5.371 (12) 5.334 (10) 5.306 (9) 5.264 (8) 5.247 (9) 5.223 (10)
12.728 (26) 12.262 (36) 12.159 (29) 12.073 (27) 12.030 (23) 11.981 (20) 11.956 (31)
324.7 (8) 306.3 (17) 299.6 (14) 294.4 (12) 288.7 (10) 285.7 (11) 282.4 (13)
Numbers in parentheses represent the error of lattice parameters. The data at 0.0 GPa were obtained after decompression. a Lattice parameter of platinum as internal pressure standard was calculated from two diffraction lines (1 1 1, 2 0 0) in all runs. b Pressure values from EOS of platinum (Holmes et al., 1989).
determination was <0.4 GPa, based on the lattice parameters calculated using different platinum diffraction lines at the same pressure. There was the possibility that the diffraction peaks of the sample would interfere with those of platinum at higher pressures, and an overlap of the diffraction peaks would lead to significant uncertainty in the pressure calibration. Therefore, the volume fraction of platinum powder used in this study was higher than that used in other studies. Thus, the platinum diffraction peaks were intense compared to those from the sample and the argon pressure transmitting medium (see Fig. 1). The intense platinum peaks enabled a reliable pressure determination. The effect of pressure on the unit cell parameters and volume of the CAS phase is shown in Table 2, and the unit cell parameters as a function of pressure are plotted
in Fig. 2. The c-axis was more compressible than the aaxis. The P–V data were fitted to the Birch–Murnaghan equation of state to determine the elastic parameters:
Fig. 1. Example of an X-ray diffraction pattern of a sample at 300 K. The peak identifications are as follows: C, corundum; H, CAS phase; P, platinum; A, argon; S, stishovite.
Fig. 2. Unit cell parameters of the CAS phase at 300 K. The data were acquired after laser heating to relieve the deviatoric stress in the sample after each pressure increment.
P = 1.5K0 (x−7 − x−5 )[1 + 0.75(K0 − 4)(x−2 − 1)] where x = (V/V0 )1/3 ; and V0 , K0 , and K0 are the volume, the isothermal bulk modulus, and the first pressure derivative of the isothermal bulk modulus, respectively. The isothermal bulk modulus and the first pressure derivative of the isothermal bulk modulus were determined to be K0 = 229(9) GPa and K0 = 4.7(7), respectively. The unit cell volume data as a function of pressure are plotted in Fig. 3. When the value of K0 was set to K0 = 4, then a value of K0 = 239(2) GPa was obtained. We did not observe any phase transition occurring in the CAS phase up to a pressure of
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Table 3 Lattice parameters and volume of corundum to 44 GPa P (GPa)a
´˚ a (A)
´˚ c (A)
´˚ 3 ) Volume (A
0.0 15.9 21.7 28.2 35.1 40.4 44.0
4.759 (1) 4.679 (1) 4.651 (2) 4.630 (2) 4.604 (4) 4.587 (3) 4.587 (8)
13.009 (5) 12.755 (5) 12.689 (10) 12.613 (8) 12.516 (16) 12.509 (12) 12.402 (31)
255.2 (2) 241.8 (1) 237.7 (3) 234.1 (2) 229.7 (5) 227.9 (4) 226.0 (9)
Numbers in parentheses represent the error of lattice parameters. The data at 0.0 GPa were obtained after decompression. a Pressure values from EOS of platinum (Holmes et al., 1989).
Fig. 3. Pressure–volume data for the CAS phase at 300 K. Dashed curve: third-order Birch–Murnaghan equation fit with K0 and K0 are 229 GPa and 4.7, respectively.
44 GPa, which was the maximum pressure used in this study. To assess the quality of the Birch–Murnaghan equation of state fit obtained from the plot of unit cell volume against pressure, the relationship between the Eulerian strain (f = 0.5[(V0 /V)2/3 − 1]) and the normalized pressure (F = P/[3f(2f + 1)5/2 ]) was plotted, and it shown in Fig. 4. The f–F plot provides a visual indication of
Fig. 4. Eulerian strain-normalized pressure (F–f) plot of the CAS phase data based on the Birch–Murnaghan equation of state. Dashed line represents the linear fit.
which higher order terms, such as K0 and K0 , are significant in the equation of state. The CAS phase data showed a slight positive slope (Fig. 4). This indicates that the pressure derivative of the bulk modulus (K0 ) was slightly higher than 4. Therefore, the value, K0 estimated to be 4.7(7), was consistent with the f–F plot analysis. The effect of pressure on the unit cell parameters and volume of corundum is also shown in Table 3, and the unit cell volume as a function of pressure is plotted in Fig. 5. From the corundum unit cell volume data, the bulk modulus was calculated to be K0 = 263(2) GPa when K0 was set to K0 = 4.
Fig. 5. Pressure–volume data for corundum at 300 K. Dashed curve: third-order Birch–Murnaghan equation fit.
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4. Discussion Although many researchers have investigated the compressibility of materials, there are many contradictions among the reported compressibility data. It is known that large differential stresses accumulate in a sample as the pressure increases in a diamond anvil cell, even for soft materials, such as noble gases, used as the pressure transmitting medium. These differential stresses often have a significant effect on the compressibility measurement of materials. Therefore, we used laser heating to relieve the differential stress on the sample in our study, because reliable equations of state of high-pressure minerals have been reported using this laser-heated annealing method (e.g., Ono et al., 2002a; Andrault et al., 2003). To assess the deviatoric stress in the sample chamber, the lattice parameters of platinum were measured using single lines and showed a random deviation from the average values obtained by fitting all the lines (Fig. 6). All the deviations were within ±0.1% of the fit value. This result shows that near hydrostatic conditions existed in the sample chamber in this study. In our study, excess Al2 O3 was added to the starting materials and the compressibilities of both the CAS phase and corundum (Al2 O3 ) could be measured simultaneously. As the compressibility of corundum is well known from previous studies, the compressibility of corundum was used to check the reliability of our heating method. Most previous studies have shown
Fig. 6. Platinum lattice parameters calculated using a single line (a(h k l) ) normalized to the average value obtained by fitting all the lines (afit ). The solid and open circles represent a(1 1 1) and a(2 0 0) , respectively.
that the compressibility of corundum is around 260 GPa (e.g., d’Amour et al., 1978; Finger and Hazen, 1979; Cohen, 1987; Richet et al., 1988; Thomson et al., 1996; Dubrovinsky et al., 1998). The value of the bulk modulus of K0 = 263 GPa determined by our experiments is consistent with the bulk modulus values of most of the previous studies. This indicated that the laserheated method used in our study lead to accurate results. Grevel et al. (2000) reported a lower value of the bulk modulus of corundum of K0 = 226 GPa compared to other studies, and this is likely to be due to the study of Grevel et al. (2000) involving higher experimental uncertainties. The annealing temperature in Grevel et al.’s study was not high enough to release the differential stress in the sample chamber, since the heating temperature used was <1273 K. We observed no phase transition of the CAS phase up to a pressure of 44 GPa, the maximum pressure studied. However, we did not demonstrate that CAS phase is stable under lower mantle conditions, because there is the possibility that the temperatures and durations used to heat the sample were not high or long enough to trigger a phase transition. Therefore, it will be necessary to carry out future experiments to investigate the stability of the CAS phase at higher temperatures and pressures corresponding to those in the lower mantle geotherm (i.e., >2000 K). The information on the CAS phase stability can contribute to the understanding of the mechanism of shock events, as in the case of the natural CAS phase discovered in the shocked Martian meteorites (Beck et al., 2004). Fig. 7 shows a comparison of the pressure-density relationship of the CAS phase with clinopyroxene, kyanite, majoritic garnet, K-hollandite, stishovite, calcium-ferrite type aluminous phase, and Caperovskite that all coexist in the subducted sediments in the transition zone and in the uppermost parts of the lower mantle (Irifune et al., 1994). Recently, a minor phase transition of Ca-perovskite has been reported, and it is likely that tetragonal Ca-perovskite exists only in low temperature regions, such as in the subducted slab of the lower mantle (Ono et al., 2004b). The densities of the above phases were calculated using an appropriate equation of state employing suitable thermoelastic parameters (see Table 4). It is known that the chemical composition of each phase in subducted sediment changes with increasing pressure and temperature (Irifune et al., 1994; Ono, 1998). However,
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Table 4 Parameters of the Birch–Murnaghan equation of state and densities of minerals of subducted sediment Minerals
K0 (GPa)
K0
ρ0
Cpx (Omphacite) Kyanite Majoritic garnet K-hollandite Al-bearing stishovite Calcium-ferrite type phase Al-bearing Mg-perovskite Cubic Ca-perovskite Tetragonal Ca-perovskite CAS
126 (Nishihara et al., 2003) 193 (Yang et al., 1997) 160 (Wang et al., 1998) 180 (Zhang et al., 1993) 282 (Ono et al., 2002b) 190 (Guignot and Andrault, 2004) 272 (Ono et al., 2004a) 236 (Shim et al., 2000) 248 (Ono et al., 2004b) 229 (this study)
4 (fixed) 4 (fixed) 4.9 4 (fixed) 4 (fixed) 4 (fixed) 4 (fixed) 3.9 4 (fixed) 4.7
3.26 (Nishihara et al., 2003) 3.67 (Yang et al., 1997) 3.71 (Ono et al., 2001) 3.91 (Zhang et al., 1993) 4.25 (Ono et al., 2002b) 4.00 (Guignot and Andrault, 2004) 4.35 (Guignot and Andrault, 2004) 4.23 (Shim et al., 2000) 4.25 (Ono et al., 2004b) 3.89 (this study)
K0 and K0 are the isothermal bulk modulus and the first derivative of the isothermal bulk modulus at 300 K. ρ0 is the density of mineral at 0 GPa and 300 K.
because of the lack of available experimental data, we assumed the chemical composition to be constant to estimate the density of phase. Although Irifune et al. (1994) reported on the chemical composition of phases in continental crust-like compositions at pressures up to 24 GPa, they used an iron-free bulk composition in their experiments, which was unrealistic for a natural system. Ono (1998) reported on the chemical composition of phases having the sediment composition. How-
Fig. 7. Comparison of the pressure–density relationship of the CAS phase and coexisting phases in the subducted sediment at 300 K. The densities were calculated using elastic parameters and the densities of the minerals at ambient pressure in the subducted sediment (Table 4). Phase abbreviations are clinopyroxene (omphacite), Cpx; Kyanite, Ky; Majoritic garnet, Mj; K-hollandite, Ho; Al-bearing stishovite, St; Calcium-ferrite type aluminous phase, CF; Al-bearing Mg-perovskite, MPv; Cubic Ca-perovskite, Cubic-CPv; Tetragonal Ca-perovskite, Tet-CPv; CAS phase, CAS.
ever, the maximum pressure reported by Ono (1998) was 15 GPa, and thus the reliable chemical composition of phases in subducted sediments at pressures in the transition zone and in the lower mantle were not reported. The CAS phase, kyanite, K-hollandite, and cubic and tetragonal Ca-perovskite were assumed to have pure compositions. The minor element content in the CAS phase, K-hollandite, and Ca-perovskite is very low in natural bulk compositions (Irifune et al., 1994; Ono, 1998; Ono et al., 2001). In contrast, kyanite can contain small concentrations of iron (Ono, 1998). Therefore, it is possible that the density of kyanite having a sediment composition can be denser than the estimated value used in this study. The density of the CAS phase has an intermediate value in the transition zone compared with other coexisting phases in subducted sediments. In contrast, the density of the CAS phase is lower than that of co-existing phases in the lower mantle. Therefore, the density of subducted sediments in the lower mantle may decrease with increasing mineral proportion of the CAS phase. Since the mineral proportion in natural subducted sediments shows considerable variation (depending on the temperature, pressure, and the bulk composition of the sediment), it is important in future studies to investigate the phase relationships of subducted sediments under lower mantle conditions.
Acknowledgements We thank E. Takahashi, T. Hirata, T. Suzuki and Y. Tatsumi for helps of this work. The manuscript was
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improved through reviews by D. Andrault, N. Miyajima, and D.C. Rubie. The synchrotron radiation experiments were performed at the PF with the approval of the High Energy Accelerator Research Organization (KEK) (Proposal No. 2003G187).
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Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219–229. Holmes, N.C., Moriarty, J.A., Gathers, G.R., Nellis, W.J., 1989. The equation of state of platinum to 660 GPa (6.6 mbar). J. Appl. Phys. 66, 2962–2967. Iizuka, T., Hirata, T., 2004. Simultaneous determinations of U-Pb age and REE abundances for zircons using ArF excimer laser ablation-ICPMS. Geochem. J. 38, 229–241. Irifune, T., Ringwood, A.E., Hibberson, W.O., 1994. Subduction of continental crust and terrigenous and pelagic sediments: an experimental study. Earth Planet Sci. Lett. 126, 351–368. Nishihara, Y., Takahashi, E., Matsukage, K., Kikegawa, T., 2003. Thermal equation of state of omphacite. Am. Miner. 88, 80– 86. Ono, S., 1998. Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions: implications for water transport in subduction zones. J. Geophys. Res. 103, 18253–18267. Ono, S., Ito, E., Katsura, T., 2001. Mineralogy of subducted basaltic crust (MORB) from 25 to 37 GPa, and chemical heterogeneity of the lower mantle. Earth Planet Sci. Lett. 190, 57–63. Ono, S., Hirose, K., Kikegawa, T., Saito, Y., 2002a. The compressibility of a natural composition calcium ferrite-type aluminous phase to 70 GPa. Phys. Earth Planet Inter. 131, 311–318. Ono, S., Suto, T., Hirose, K., Kuwayama, Y., Komabayashi, T., Kikegawa, T., 2002b. Equation of state of Al-bearing stishovite to 40 GPa at 300 K. Am. Miner. 87, 1486–1489. Ono, S., Kikegawa, T., Iizuka, T., 2004a. The equation of state of orthorhombic perovskite in a peridotitic mantle composition to 80 GPa: Implications for chemical composition of the lower mantle. Phys. Earth Planet Inter. 145, 67–74. Ono, S., Ohishi, Y., Mibe, K., 2004b. Phase transition of Caperovskite and stability of Al-bearing Mg-perovskite in the lower mantle. Am. Miner. 89, 1480–1485. Richet, P., Xu, J.A., Mao, H.K., 1988. Quasi-hydrostatic compression of ruby to 500 kbar. Phys. Chem. Miner. 16, 207–211. Sekine, T., Ahrens, T.J., 1992. Shock-induced transformations in the system NaAlSiO4 –SiO2 : a new interpretation. Phys. Chem. Miner. 18, 359–364. Shim, S.H., Duffy, T., Shen, G., 2000. The equation of state of CaSiO3 perovskite to 108 GPa at 300 K. Phys. Earth Planet Inter. 120, 327–338. Takahashi, E., Shimazaki, Y., Tsuzaki, Y., Yoshido, H., 1993. Melting study of peridotite KLB-1 to 6.5 GPa and the origin of basaltic magma. Phil. Trans. R. Soc. Lond. Ser. A 342, 105–120. Thomson, K.T., Wentzcovitch, R.M., Bukowinski, M.S.T., 1996. Polymorphs of alumina predicted by first principles: putting pressure on the ruby pressure scale. Science 274, 1880–1882. Wang, Y., Weidner, D.J., Zhang, J., Gwanrnesia, G.D., Liebermann, R., 1998. Thermal equation of state of garnets along the pyropemajorite join. Phys. Earth Planet Inter. 105, 59–71. Yang, H., Downs, R.T., Finger, L.W., Hazen, R.M., Prewitt, C.T., 1997. Compressibility and crystal structure of kyanite, Al2 SiO5 , at high pressure. Am. Miner. 82, 467–474. Zhang, J., Ko, J., Hazen, R.M., Prewitt, C.T., 1993. High-pressure crystal chemistry of KAlSi3 O8 hollandite. Am. Miner. 78, 493–499.
Compressibility of the calcium aluminosilicate, CAS, phase to 44 GPa Shigeaki Onoa,∗ , Tsuyoshi Iizukab , Takumi Kikegawac a
Institute for Frontier Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan b Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan c High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan Received 25 May 2004; received in revised form 2 December 2004; accepted 3 December 2004
Abstract In situ X-ray diffraction measurements on a calcium aluminosilicate (CAS) phase have been carried out using a laser-heated diamond anvil cell up to a pressure of 44 GPa, employing a synchrotron radiation source. CAS is the major mineral formed from sediments subducted into the Earth’s mantle. The sample was heated using a YAG laser after each pressure increment to relax the deviatoric stress in the sample. X-ray diffraction measurements were carried out at T = 300 K using an angle-dispersive technique. The pressure was calculated using an internal platinum metal pressure calibrant. The Birch–Murnaghan equation of state for the CAS phase obtained from the experimental unit cell parameters showed a density of ρ0 = 3.888 g/cm3 and a bulk modulus of K0 = 229 ± 9 GPa for K0 = 4.7 ± 0.7. When the first pressure derivative of the bulk modulus was fixed at K0 = 4, then the value of K0 = 239 ± 2 GPa. From the experimental compressibility, the density of the CAS phase was observed to be lower than the density of co-existing Al-bearing stishovite, calcium perovskite, calcium ferrite-type phases, and (Fe,Al)-bearing Mg-perovskite in subducted sediments in the lower mantle. Therefore, the density of subducted sediments in the lower mantle decreases with increasing mineral proportion of the CAS phase. © 2004 Elsevier B.V. All rights reserved. Keywords: CAS phase; Corundum; Equation of state; X-ray powder diffraction; High pressure; Subducted sediment
1. Introduction The phase relationships of sediments at high pressure and high temperatures are of geochemical interest ∗ Corresponding author. Tel.: +81 468 67 9763; fax: +81 468 67 9625. E-mail address: [email protected] (S. Ono).
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2004.12.001
because sediments are candidate components for creating the isotope heterogeneity of the Earth’s mantle (e.g., Hanyu and Kaneoka, 1997; Hofmann, 1997). The calcium aluminosilicate (CAS) phase, CaAl4 Si2 O11 , occurs in rocks formed from sediments and continental crust compositions at pressures corresponding to those found in the upper mantle and transition zone conditions. The formation of this CAS phase has been
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observed using multi-anvil high-pressure apparatus (Irifune et al., 1994), and it comprises some 5–10 wt.% of all minerals having sediment and continental crust compositions in the transition zone and the uppermost part of the lower mantle. Previous studies (Gautron et al., 1997, 1999) reported that this CAS phase has a hexagonal barium ferrite-type structure (space group P63 /mmc) in which some of the Si atoms are five-fold coordinated in a trigonal bipyramidal site. This is interesting, since it is known that silicates generally possess 4-fold or 6-fold coordinated Si atoms at both low and high pressures. Gautron et al. (1996) also detected this CAS phase in the transformation products of CaAl2 Si2 O8 (anorthite composition) at 14 GPa and 1500 ◦ C. The physical properties of a given phase in the subducted sediments may affect the dynamics of the subduction and material circulation in the mantle and, therefore, it is important to determine the compressibility of the CAS phase. Recently, naturally occurring CAS phase was discovered in a shocked Martian meteorite (Beck et al., 2004). The chemical composition of this natural CAS phase was not pure CaAl4 Si2 O11 , as it contained a significant concentration of sodium, and had a chemical composition of (Cax Na1−x )Al3+x Si3−x O11 . The pressure–temperature profile of the impact event that formed this CAS phase was not clear, and understanding of the physical properties and stability field of the CAS phase will contribute to the understanding of the formation of natural CAS phases such as that found in the shocked Martian meteorite. We used a laser-heated diamond anvil cell (LHDAC) and intense X-rays from a synchrotron radiation source that allowed us to acquire precise data on a sample under high pressure. We directly investigated the pressure–volume equation of state (EOS) of the CAS phase in subducted sediments over the pressure range 0.1 MPa–44 GPa, and we compared the density of the CAS phase with that of co-existing minerals in the mantle.
2. Experimental procedure A polycrystalline specimen of CAS phase and corundum was synthesized in a 1000 t multi-anvil apparatus (SEDI-1000) (see Takahashi et al., 1993), located at the Magma Factory at the Tokyo Institute of Technol-
ogy, Japan. An octahedral Cr2 O3 -doped MgO pressure medium was placed at the center of a tungsten carbide (WC) cubic anvil assembly employing pyrophyllite gaskets. A cylindrical rhenium metal heater was then inserted into the octahedral pressure medium, and this was enclosed within a LaCrO3 sleeve to provide thermal insulation. The powdered sample was loaded directly into the rhenium heater, which also served as a sample capsule. Before synthesizing the CAS phase and corundum, reagent-grade Al2 O3 , CaCO3 , and SiO2 powders were mixed and heated to 1000 ◦ C at ambient pressure. The chemical composition of the starting material was within the Al2 O3 and CAS phase composition. Then, the sample was ground and mixed with platinum powder, which was used as an internal pressure standard. The specimen used in this study was synthesized under a pressure of 20 GPa and a temperature of 1300 ◦ C for 30 min in the multi-anvil apparatus. The synthesized sample was examined using the microfocused X-ray diffraction technique at the Tokyo Institute of Technology, Japan. The experimental parameters of the microfocused X-ray diffraction were chromium radiation, 40 keV, 250 mA, 2 h counting times, and beam diameter of 100 m. The diffraction patterns confirmed that the sample contained the CAS phase, corundum, and platinum. Because the elastic parameters of corundum are well established, corundum was used to check the reliability of our experimental data. It is known that previous studies on the compressibility of high-pressure aluminous minerals provided controversial data. In the case of calcium-ferrite type phase of NaAlSiO4 , previous studies reported that the bulk modulus was in the range 185–275 GPa, which has a significant divergence of values (e.g., Sekine and Ahrens, 1992; Dubrovinsky et al., 2002; Guignot and Andrault, 2004). Therefore, the measured values of corundum in this study are an important check on the reliability of the CAS phase data. The synthesized sample was also examined using an electron probe microanalyzer (EPMA) at the Tokyo Institute of Technology, Japan. The experimental parameters used in the EPMA measurements were a 15 kV accelerating voltage, a 12 nA Faraday cup current, 20 s counting times, and an electron beam diameter of 1 m. The chemical composition of the synthesized CAS phase was Ca0.98 Al3.98 Si2.03 O11 , which was consistent with that reported by Gautron et al. (1996). The SiO2 and CaO content in the corundum were <0.3 wt.%. No chemical
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reaction between the sample and the capsule was observed. High-pressure X-ray diffraction experiments were carried out in the LHDAC. The synthesized sample was loaded into a 100 m diameter hole that had been pre-drilled into a rhenium gasket using an ArF excimer laser, MicroLas GeoLas 200 CQ, (Iizuka and Hirata, 2004). Argon was cryogenically loaded into the sample chamber, acting as a pressure transmitting and thermally insulating medium. The sample was heated after each change in pressure using a multimode continuous wave Nd:YAG laser to minimize any pressure inhomogeneity in the sample. The laser power and size of the heating spot were about 100 W and 50–100 m, respectively. An optical microscope was used to view the heated sample, and from the color of the sample’s emitted radiation the temperature was estimated to be about 2000 K. A typical duration of the sample annealing time was 5–10 min to produce data of sufficient quality for measuring the unit cell parameters. The sample was probed using angle-dispersive X-ray diffraction employing the BL13A synchrotron beam line at the photon factory (PF) of the High Energy Accelerator Research Organization (KEK), Japan (Ono et al., 2002a). The incident X-ray beam was monochro´˚ The X-ray beam matized to a wavelength of 0.4257 A. size was collimated to a diameter of about 50 m. The angle-dispersive X-ray diffraction patterns were obtained on an imaging plate X-ray data collection system (Rigaku RAXIS-IV, Japan). The observed intensities on the imaging plates were integrated as a function of 2θ using the Fit2d code software package (Hammersley, 1996) to obtain conventional, one-dimensional diffraction profiles. The diffraction peak positions were determined using a peak fitting program. The two intense platinum X-ray diffraction peaks (1 1 1 and 2 0 0) were observed, in addition to the diffraction peaks from CAS phase and corundum. The pressure was determined from the calculated platinum unit cell volume using the equation of state for platinum developed by Holmes et al. (1989). 3. Results The powder X-ray diffraction data of the CAS phase at ambient pressure revealed that it had a hexagonal barium ferrite-type structure (P63 /mmc), with unit cell di-
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´˚ and c = 12.728(26) A. ´˚ The mensions of a = 5.428(4) A observed and calculated X-ray diffraction patterns of the CAS phase are shown in Table 1. The volume and density of the CAS phase at ambient pressure were ´˚ 3 and 3.888(10) g/cm3 . The volume of CAS 324.7(8) A phase determined in this study was slightly higher than ´˚ 3 reported by Gautron et al. the value of 323.28(5) A (1999). Our sample was heated using the laser after each pressure increment, and the X-ray diffraction measurements were taken. The laser heating annealed the sample, and reduced any pressure gradient occurring across the sample, thus improving the quality of the X-ray diffraction data. Fig. 1 shows a typical X-ray diffraction pattern obtained. In addition to the diffraction peaks from the CAS phase and corundum, there were diffraction peaks arising from both the platinum internal pressure calibrant and the argon pressure transmitted medium. Small peaks ascribed to stishovite were also observed. The heating was performed at pressures >15 GPa, which corresponds to the stability field of the CAS phase, because we investigated the possibility that the CAS phase would dissociate into separate phases under metastable conditions. The entire sample was carefully heated by scanning with the laser beam to eliminate any potential reactions with either the argon pressure transmitting medium or the platinum pressure calibrant. A typical error in the pressure Table 1 Observed and calculated X-ray diffraction pattern of the CAS phase at 0 GPa hkl
˚´ dobs (A)
´˚ dcal (A)
dobs /dcal − 1
002 102 004 103 110 104 112 202 203 106 123 206 124 302 304
6.3648 3.7870 3.1746
6.3641 3.7810 3.1820 3.1495 2.7138 2.6350 2.4963 2.2047 2.0559 1.9336 1.6387 1.5747 1.5512 1.5214 1.4057
0.0001 0.0016 −0.0023
31 15 52
−0.0009 −0.0001 0.0010 0.0004 0.0000
21 79 33 28 100
0.0027 0.0007 −0.0028 −0.0006 0.0004
4 24 9 10 4
2.7115 2.6349 2.4989 2.2056 2.0559 1.6431 1.5758 1.5469 1.5205 1.4063
Iobs
Calculated d-spacings are based on hexagonal unit cell dimensions ´˚ and c = 12.728 A. ´˚ of a = 5.428 A
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Table 2 Lattice parameters and volumes of CAS phase and platinum to 44 GPa ´˚ a Platinum a (A)
P (GPa)b
´˚ CAS a (A)
´˚ c (A)
´˚ 3 ) Volume (A
3.923 (0) 3.856 (1) 3.836 (0) 3.815 (0) 3.795 (0) 3.781 (0) 3.771 (1)
0.0 15.9 21.7 28.2 35.1 40.4 44.0
5.428 (4) 5.371 (12) 5.334 (10) 5.306 (9) 5.264 (8) 5.247 (9) 5.223 (10)
12.728 (26) 12.262 (36) 12.159 (29) 12.073 (27) 12.030 (23) 11.981 (20) 11.956 (31)
324.7 (8) 306.3 (17) 299.6 (14) 294.4 (12) 288.7 (10) 285.7 (11) 282.4 (13)
Numbers in parentheses represent the error of lattice parameters. The data at 0.0 GPa were obtained after decompression. a Lattice parameter of platinum as internal pressure standard was calculated from two diffraction lines (1 1 1, 2 0 0) in all runs. b Pressure values from EOS of platinum (Holmes et al., 1989).
determination was <0.4 GPa, based on the lattice parameters calculated using different platinum diffraction lines at the same pressure. There was the possibility that the diffraction peaks of the sample would interfere with those of platinum at higher pressures, and an overlap of the diffraction peaks would lead to significant uncertainty in the pressure calibration. Therefore, the volume fraction of platinum powder used in this study was higher than that used in other studies. Thus, the platinum diffraction peaks were intense compared to those from the sample and the argon pressure transmitting medium (see Fig. 1). The intense platinum peaks enabled a reliable pressure determination. The effect of pressure on the unit cell parameters and volume of the CAS phase is shown in Table 2, and the unit cell parameters as a function of pressure are plotted
in Fig. 2. The c-axis was more compressible than the aaxis. The P–V data were fitted to the Birch–Murnaghan equation of state to determine the elastic parameters:
Fig. 1. Example of an X-ray diffraction pattern of a sample at 300 K. The peak identifications are as follows: C, corundum; H, CAS phase; P, platinum; A, argon; S, stishovite.
Fig. 2. Unit cell parameters of the CAS phase at 300 K. The data were acquired after laser heating to relieve the deviatoric stress in the sample after each pressure increment.
P = 1.5K0 (x−7 − x−5 )[1 + 0.75(K0 − 4)(x−2 − 1)] where x = (V/V0 )1/3 ; and V0 , K0 , and K0 are the volume, the isothermal bulk modulus, and the first pressure derivative of the isothermal bulk modulus, respectively. The isothermal bulk modulus and the first pressure derivative of the isothermal bulk modulus were determined to be K0 = 229(9) GPa and K0 = 4.7(7), respectively. The unit cell volume data as a function of pressure are plotted in Fig. 3. When the value of K0 was set to K0 = 4, then a value of K0 = 239(2) GPa was obtained. We did not observe any phase transition occurring in the CAS phase up to a pressure of
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Table 3 Lattice parameters and volume of corundum to 44 GPa P (GPa)a
´˚ a (A)
´˚ c (A)
´˚ 3 ) Volume (A
0.0 15.9 21.7 28.2 35.1 40.4 44.0
4.759 (1) 4.679 (1) 4.651 (2) 4.630 (2) 4.604 (4) 4.587 (3) 4.587 (8)
13.009 (5) 12.755 (5) 12.689 (10) 12.613 (8) 12.516 (16) 12.509 (12) 12.402 (31)
255.2 (2) 241.8 (1) 237.7 (3) 234.1 (2) 229.7 (5) 227.9 (4) 226.0 (9)
Numbers in parentheses represent the error of lattice parameters. The data at 0.0 GPa were obtained after decompression. a Pressure values from EOS of platinum (Holmes et al., 1989).
Fig. 3. Pressure–volume data for the CAS phase at 300 K. Dashed curve: third-order Birch–Murnaghan equation fit with K0 and K0 are 229 GPa and 4.7, respectively.
44 GPa, which was the maximum pressure used in this study. To assess the quality of the Birch–Murnaghan equation of state fit obtained from the plot of unit cell volume against pressure, the relationship between the Eulerian strain (f = 0.5[(V0 /V)2/3 − 1]) and the normalized pressure (F = P/[3f(2f + 1)5/2 ]) was plotted, and it shown in Fig. 4. The f–F plot provides a visual indication of
Fig. 4. Eulerian strain-normalized pressure (F–f) plot of the CAS phase data based on the Birch–Murnaghan equation of state. Dashed line represents the linear fit.
which higher order terms, such as K0 and K0 , are significant in the equation of state. The CAS phase data showed a slight positive slope (Fig. 4). This indicates that the pressure derivative of the bulk modulus (K0 ) was slightly higher than 4. Therefore, the value, K0 estimated to be 4.7(7), was consistent with the f–F plot analysis. The effect of pressure on the unit cell parameters and volume of corundum is also shown in Table 3, and the unit cell volume as a function of pressure is plotted in Fig. 5. From the corundum unit cell volume data, the bulk modulus was calculated to be K0 = 263(2) GPa when K0 was set to K0 = 4.
Fig. 5. Pressure–volume data for corundum at 300 K. Dashed curve: third-order Birch–Murnaghan equation fit.
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4. Discussion Although many researchers have investigated the compressibility of materials, there are many contradictions among the reported compressibility data. It is known that large differential stresses accumulate in a sample as the pressure increases in a diamond anvil cell, even for soft materials, such as noble gases, used as the pressure transmitting medium. These differential stresses often have a significant effect on the compressibility measurement of materials. Therefore, we used laser heating to relieve the differential stress on the sample in our study, because reliable equations of state of high-pressure minerals have been reported using this laser-heated annealing method (e.g., Ono et al., 2002a; Andrault et al., 2003). To assess the deviatoric stress in the sample chamber, the lattice parameters of platinum were measured using single lines and showed a random deviation from the average values obtained by fitting all the lines (Fig. 6). All the deviations were within ±0.1% of the fit value. This result shows that near hydrostatic conditions existed in the sample chamber in this study. In our study, excess Al2 O3 was added to the starting materials and the compressibilities of both the CAS phase and corundum (Al2 O3 ) could be measured simultaneously. As the compressibility of corundum is well known from previous studies, the compressibility of corundum was used to check the reliability of our heating method. Most previous studies have shown
Fig. 6. Platinum lattice parameters calculated using a single line (a(h k l) ) normalized to the average value obtained by fitting all the lines (afit ). The solid and open circles represent a(1 1 1) and a(2 0 0) , respectively.
that the compressibility of corundum is around 260 GPa (e.g., d’Amour et al., 1978; Finger and Hazen, 1979; Cohen, 1987; Richet et al., 1988; Thomson et al., 1996; Dubrovinsky et al., 1998). The value of the bulk modulus of K0 = 263 GPa determined by our experiments is consistent with the bulk modulus values of most of the previous studies. This indicated that the laserheated method used in our study lead to accurate results. Grevel et al. (2000) reported a lower value of the bulk modulus of corundum of K0 = 226 GPa compared to other studies, and this is likely to be due to the study of Grevel et al. (2000) involving higher experimental uncertainties. The annealing temperature in Grevel et al.’s study was not high enough to release the differential stress in the sample chamber, since the heating temperature used was <1273 K. We observed no phase transition of the CAS phase up to a pressure of 44 GPa, the maximum pressure studied. However, we did not demonstrate that CAS phase is stable under lower mantle conditions, because there is the possibility that the temperatures and durations used to heat the sample were not high or long enough to trigger a phase transition. Therefore, it will be necessary to carry out future experiments to investigate the stability of the CAS phase at higher temperatures and pressures corresponding to those in the lower mantle geotherm (i.e., >2000 K). The information on the CAS phase stability can contribute to the understanding of the mechanism of shock events, as in the case of the natural CAS phase discovered in the shocked Martian meteorites (Beck et al., 2004). Fig. 7 shows a comparison of the pressure-density relationship of the CAS phase with clinopyroxene, kyanite, majoritic garnet, K-hollandite, stishovite, calcium-ferrite type aluminous phase, and Caperovskite that all coexist in the subducted sediments in the transition zone and in the uppermost parts of the lower mantle (Irifune et al., 1994). Recently, a minor phase transition of Ca-perovskite has been reported, and it is likely that tetragonal Ca-perovskite exists only in low temperature regions, such as in the subducted slab of the lower mantle (Ono et al., 2004b). The densities of the above phases were calculated using an appropriate equation of state employing suitable thermoelastic parameters (see Table 4). It is known that the chemical composition of each phase in subducted sediment changes with increasing pressure and temperature (Irifune et al., 1994; Ono, 1998). However,
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Table 4 Parameters of the Birch–Murnaghan equation of state and densities of minerals of subducted sediment Minerals
K0 (GPa)
K0
ρ0
Cpx (Omphacite) Kyanite Majoritic garnet K-hollandite Al-bearing stishovite Calcium-ferrite type phase Al-bearing Mg-perovskite Cubic Ca-perovskite Tetragonal Ca-perovskite CAS
126 (Nishihara et al., 2003) 193 (Yang et al., 1997) 160 (Wang et al., 1998) 180 (Zhang et al., 1993) 282 (Ono et al., 2002b) 190 (Guignot and Andrault, 2004) 272 (Ono et al., 2004a) 236 (Shim et al., 2000) 248 (Ono et al., 2004b) 229 (this study)
4 (fixed) 4 (fixed) 4.9 4 (fixed) 4 (fixed) 4 (fixed) 4 (fixed) 3.9 4 (fixed) 4.7
3.26 (Nishihara et al., 2003) 3.67 (Yang et al., 1997) 3.71 (Ono et al., 2001) 3.91 (Zhang et al., 1993) 4.25 (Ono et al., 2002b) 4.00 (Guignot and Andrault, 2004) 4.35 (Guignot and Andrault, 2004) 4.23 (Shim et al., 2000) 4.25 (Ono et al., 2004b) 3.89 (this study)
K0 and K0 are the isothermal bulk modulus and the first derivative of the isothermal bulk modulus at 300 K. ρ0 is the density of mineral at 0 GPa and 300 K.
because of the lack of available experimental data, we assumed the chemical composition to be constant to estimate the density of phase. Although Irifune et al. (1994) reported on the chemical composition of phases in continental crust-like compositions at pressures up to 24 GPa, they used an iron-free bulk composition in their experiments, which was unrealistic for a natural system. Ono (1998) reported on the chemical composition of phases having the sediment composition. How-
Fig. 7. Comparison of the pressure–density relationship of the CAS phase and coexisting phases in the subducted sediment at 300 K. The densities were calculated using elastic parameters and the densities of the minerals at ambient pressure in the subducted sediment (Table 4). Phase abbreviations are clinopyroxene (omphacite), Cpx; Kyanite, Ky; Majoritic garnet, Mj; K-hollandite, Ho; Al-bearing stishovite, St; Calcium-ferrite type aluminous phase, CF; Al-bearing Mg-perovskite, MPv; Cubic Ca-perovskite, Cubic-CPv; Tetragonal Ca-perovskite, Tet-CPv; CAS phase, CAS.
ever, the maximum pressure reported by Ono (1998) was 15 GPa, and thus the reliable chemical composition of phases in subducted sediments at pressures in the transition zone and in the lower mantle were not reported. The CAS phase, kyanite, K-hollandite, and cubic and tetragonal Ca-perovskite were assumed to have pure compositions. The minor element content in the CAS phase, K-hollandite, and Ca-perovskite is very low in natural bulk compositions (Irifune et al., 1994; Ono, 1998; Ono et al., 2001). In contrast, kyanite can contain small concentrations of iron (Ono, 1998). Therefore, it is possible that the density of kyanite having a sediment composition can be denser than the estimated value used in this study. The density of the CAS phase has an intermediate value in the transition zone compared with other coexisting phases in subducted sediments. In contrast, the density of the CAS phase is lower than that of co-existing phases in the lower mantle. Therefore, the density of subducted sediments in the lower mantle may decrease with increasing mineral proportion of the CAS phase. Since the mineral proportion in natural subducted sediments shows considerable variation (depending on the temperature, pressure, and the bulk composition of the sediment), it is important in future studies to investigate the phase relationships of subducted sediments under lower mantle conditions.
Acknowledgements We thank E. Takahashi, T. Hirata, T. Suzuki and Y. Tatsumi for helps of this work. The manuscript was
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improved through reviews by D. Andrault, N. Miyajima, and D.C. Rubie. The synchrotron radiation experiments were performed at the PF with the approval of the High Energy Accelerator Research Organization (KEK) (Proposal No. 2003G187).
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