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The compressibility of a natural composition calcium ferrite-type aluminous phase to 70 GPa
Physics of the Earth and Planetary Interiors 131 (2002) 311–318
The compressibility of a natural composition calcium ferrite-type aluminous phase to 70 GPa Shigeaki Ono a,b,∗ , Kei Hirose b , Takumi Kikegawa c , Yoko Saito d a
b
Institute for Frontier Research on Earth Evolution, Japan Marine Science & Technology Center, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan 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 d College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajiyosui, Setagaya, Tokyo 156-8550, Japan Received 12 April 2002; received in revised form 14 June 2002
Abstract In situ X-ray diffraction (XRD) experiments of a calcium ferrite-type aluminous phase that is a sodium host mineral of subducted oceanic crusts into the Earth’s lower mantle have been carried out using a laser-heated diamond anvil cell (LHDAC), up to a pressure of 70 GPa with synchrotron radiation source at the Photon Factory (PF) in Japan. The sample was heated using a Nd:YAG laser at each pressure increment to relax the deviatoric stress in the sample. XRD measurements were carried out at 300 K using an angle-dispersive technique. The pressure was determined from an internal platinum pressure calibrant. A Birch–Murnaghan equation of state (EOS) was determined from the experimental unit cell parameters: volume V0 = 244.07 (±55) Å3 , density ρ0 = 4.143 g/cm3 , bulk modulus K0 = 253 (±14) GPa, and K0 = 3.6 (±0.6). When the first pressure derivative of the bulk modulus K0 was fixed at 4, the value of K0 = 243 (±2) GPa was obtained. The density of the calcium ferrite-type aluminous phase is lower than those of co-existing Mg-, Ca-perovskite, and hexagonal aluminous phase in subducted oceanic crusts. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Calcium ferrite-type aluminous phase; Equation of state; X-ray powder diffraction; High P–T; Subducting slab
1. Introduction The phase relationship of mid-oceanic ridge basalt (MORB) at high pressures and temperatures is of interest to the geophysical community because MORB is one of the rocks in the oceanic crust that seems to subduct into the lower mantle (e.g.Van der Hilst et al., 1997). An aluminous phase appears in rocks having the MORB composition at pressures corresponding ∗ Corresponding author. Tel.: +81-3-5734-2618; fax: +81-3-5734-3538. E-mail address: [email protected] (S. Ono).
to lower mantle conditions, and this phenomenon has been investigated by several groups using multi-anvil high pressure apparatus (Irifune and Ringwood, 1993; Hirose et al., 1999; Ono et al., 2001). These studies have reported that the aluminous phase has a calcium ferrite-type structure (Pnam) with orthorhombic symmetry. This aluminous phase amounts to some 15–25 wt.% of all minerals having the MORB composition in the lower mantle (Kesson et al., 1994; Ono et al., 2001). The physical properties of any phase present in the subducted slab may affect its dynamics and that of material circulation in the mantle. Therefore, it is important to determine the compressibility
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of the MORB aluminous phase. Recently, it was reported that the incorporation of Al2 O3 into Mgperovskite could significantly vary its bulk modulus (Zhang and Weidner, 1999; Daniel et al., 2001; Andrault et al., 2001). The mineral compositions in the natural mantle are often quite different from those of simple systems, and to understand fully the dynamics of the mantle, it is, therefore, necessary to know the compositional effect on the physical properties. In this study, we used a laser-heated diamond anvil cell (LHDAC) which made possible to acquire precise data on a sample under high pressure, and we also used intense X-rays from a synchrotron radiation source. We have directly investigated the stability and the pressure–volume equation of state (EOS) of a calcium ferrite-type aluminous phase in the subducted MORB over the pressure range from 0.1 MPa to 70 GPa, equivalent to the lower mantle conditions. In this paper, we compare the density of calcium ferrite-type aluminous phase with that of co-existing minerals in the lower mantle.
2. Experimental procedure A polycrystalline specimen of calcium ferrite-type aluminous phase was synthesized in a 1000 t 6–8-type multi-anvil apparatus (SEDI-1000; see Takahashi et al., 1993) at the Magma Factory at the Tokyo Institute of Technology. A synthetic gel was used to produce a reactive and homogeneous starting
material (Hamilton and Henderson, 1968). The gel was prepared as follows. Reagent-grade Fe, MgO, CaCO3 , Na2 CO3 , and K2 CO3 were dissolved in nitric acid and mixed with solutions of (C2 H5 O)4 Si, (C3 H9 O)4 Ti and Al(NO3 )3 . The gel was precipitated from the mixed solution and dried. The dried gel was ground and heated to 1000 ◦ C under controlled oxygen fugacity. The specimen used in this study was synthesized under a pressure of 25 GPa and a temperature of 1800 ◦ C for 30 min. Table 1 shows the chemical composition of the specimen in this study, and those of calcium ferrite-type aluminous phases in the subducted MORB reported by previous studies. High pressure X-ray diffraction (XRD) experiments were performed using a LHDAC high pressure apparatus. The synthesized calcium ferrite-type aluminous phase was loaded into a 100 m hole that had been drilled in a rhenium gasket pre-indented to a thickness of 50 m. Fine NaCl powder was used as the pressure transmission medium. Platinum foil was placed in the sample chamber to provide an internal pressure calibrant. The sample was heated after each pressure change with a multi-mode continuous wave Nd:YAG laser using a one-sided laser heating technique to reduce any pressure inhomogeneity in the sample (Fig. 1). The size of the heating spot was about 100 m. The sample was probed using angle-dispersive XRD using the synchrotron beam line BL13A at the Photon Factory (PF) of the High Energy Accelerator Research Organization (KEK), Japan. The incident X-ray beam was monochromatized
Table 1 Chemical compositions of the starting material and the calcium ferrite-type aluminous phase reported by previous studies This study
Irifune and Ringwood (1993)
Hirose et al. (1999)
Ono et al. (2001)
SiO2 (wt.%) TiO2 (wt.%) Al2 O3 (wt.%) FeOa (wt.%) MnO (wt.%) MgO (wt.%) CaO (wt.%) Na2 O (wt.%) K2 O (wt.%) Cr2 O3 (wt.%)
26.5 0.8 36.5 7.6 – 9.6 0.8 18.2 – –
23.42 0.38 46.67 3.10 – 18.53 3.10 4.71 – –
26.96 0.92 32.78 16.19 0.05 10.74 1.87 11.15 0.00 0.10
27.5 0.7 40.1 7.9 10.0 0.8 12.1 0.0 –
Total (wt.%) Pressure (GPa)
100.0 25
99.91 27
100.75 27
99.1 30
Calcium ferrite-type aluminous phase made from synthetic gel in this study. The chemical composition of this study was analyzed by electron microprobe analysis (EPMA). a All iron has been calculated as FeO.
S. Ono et al. / Physics of the Earth and Planetary Interiors 131 (2002) 311–318
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Fig. 1. Schematic diagram of the experimental design of in situ X-ray diffraction measurement at BL13A, PF. Abbreviations are lens (L) and mirror (M).
to a wavelength of 0.4266 Å. The X-ray beam size was collimated to a diameter of about 50 m. Angledispersive XRD patterns were obtained on an imaging plate. The observed intensities on the imaging plates were integrated as a function of 2θ using the Fit2d code (Hammersley, 1996) in order to obtain conventional, one-dimensional diffraction profiles. The diffraction peak positions were determined using a peak fitting program. Two XRD lines from platinum, the (1 1 1) and (2 0 0) peaks, were observed in addition to any diffraction lines from the calcium ferrite-type aluminous phase. The pressure was determined from the measured platinum unit cell volume using the EOS for platinum developed by Holmes et al. (1989).
The sample was heated by laser after each pressure increment before the XRD measurement. The laser heating annealed the sample and reduced any pressure gradient across the sample. This improved the quality of the XRD data (Fig. 2). The laser beam was carefully scanned to heat the entire sample in order to prevent any reaction with the NaCl pressure medium and the
3. Results and discussion The powder XRD data of the calcium ferrite-type aluminous phase at room pressure before compression revealed that this phase has an orthorhombic cell (Pnam) with unit cell dimensions of a = 8.660 (2) Å, b = 10.128 (2) Å and c = 2.786 (1) Å. The observed and calculated XRD patterns of the calcium ferrite-type aluminous phase are compared in Table 2. The calcium ferrite-type aluminous phase in the MORB has a unit cell volume, V0 = 244.34 (12) Å3 , which is higher than that of MgAl2 O4 as reported by Irifune et al. (1991), where V0 = 240.00 Å3 . The density of our sample, ρ0 = 4.143 g/cm3 is also higher than the MgAl2 O4 density of ρ0 = 3.937 g/cm3 .
Fig. 2. An example of X-ray diffraction pattern of the sample at high pressure. Abbreviations of peaks are as follows: C, calcium ferrite-type aluminous phase; P, platinum; A, B1-type sodium chloride; B, B2-type sodium chloride; R, Rhenium gasket.
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Table 2 Observed and calculated X-ray diffraction pattern of calcium ferrite-type aluminous phase before compression (0.1 MPa)
Table 3 Lattice parameters and volumes of calcium ferrite-type aluminous phase to 70 GPa
hkl
dobs (Å)
dcal (Å)
d (Å)
Iobs
P (GPa)
a (Å)
020 200 220 130 310 011 111 040 320 121 201 330 240 400 410 131 420 150 311 231 321 141 250 331 241 401 411 350 160 440 421 260 251 431 360 600
5.0657 4.3262 3.2911 3.1470 2.7774 2.6853 2.5626 2.5305 2.5067 2.3468 2.3468 2.1933
5.0641 4.3301 3.2911 3.1455 2.7762 2.6859 2.5654 2.5321 2.5079 2.3492 2.3427 2.1940 2.1858 2.1651 2.1172 2.0854 1.9907 1.9724 1.9664 1.9247 1.8638 1.8313 1.8348 1.7236 1.7196 1.7095 1.6856 1.6581 1.6569 1.6455 1.6197 1.5728 1.5323 1.5251 1.4572 1.4434
0.0003 −0.0009 0.0000 0.0005 0.0004 −0.0002 −0.0011 −0.0006 −0.0005 −0.0010 0.0017 −0.0003
10 35 15 <5 <5 1 <5 70 100 65 65 5
0.0001 5.5 6.5 9.7 19.2 31.4 47.3 57.2 65.5 70.1
8.640 8.630 8.631 8.559 8.468 8.445 8.211 8.140 8.128 8.132
0.0003 0.0000 −0.0005
10 85 30
0.0004 0.0004 0.0002 0.0003 −0.0016
75 10 5 20 20
−0.0001 −0.0008 0.0001 0.0001 0.0008
85 30 25 5 5
−0.0001 0.0000
<5 20
0.0000 0.0001 0.0003
<5 15 15
2.1179 2.0855 1.9898 1.9671 1.9254 1.8643 1.8319 1.8319 1.7195 1.7081 1.6858 1.6582 1.6582 1.6195 1.5728 1.5251 1.4573 1.4437
Calculated d-spacings and volume are based on orthorhombic unit cell dimensions of a = 8.660 (2) Å, b = 10.128 (2) Å, c = 2.786 (1) Å and 244.34 (12) Å3 .
platinum pressure calibrant. The typical error in pressure was less than 0.5 GPa based on the lattice parameters as calculated from different platinum diffraction lines at the same pressure. However, by using different EOS for the platinum pressure calibrant, the difference in experimental pressure was greater than 1 GPa at higher pressures (e.g. Holmes et al., 1989; Jamieson et al., 1982). Therefore, the uncertainty of pressure in the present study is taken to be greater than 1 GPa.
b (Å) (11) (17) (10) (11) (14) (20) (9) (17) (10) (30)
10.115 10.012 10.013 10.014 9.884 9.780 9.640 9.550 9.475 9.434
Volume (Å3 )
c (Å) (12) (16) (9) (10) (11) (17) (10) (11) (17) (28)
2.793 2.761 2.759 2.756 2.723 2.675 2.665 2.639 2.630 2.626
(4) (7) (3) (3) (3) (4) (3) (4) (3) (6)
244.07 238.53 238.43 236.23 227.90 220.91 210.95 205.16 202.57 201.44
(55) (85) (43) (48) (51) (72) (41) (57) (51) (105)
Numbers in parentheses represent the error of lattice parameters. The data at 0.0001 GPa was obtained after decompression.
The effect of pressure on the unit cell parameters and volume of the calcium ferrite-type aluminous phase is tabulated in Table 3. The unit cell parameters as a function of pressure are plotted in Fig. 3. The pressure dependence of the a-axis can be represented by a = 8.654 − 0.008P , the b-axis by b = 10.084 − 0.009P , and the c-axis by c = 2.774 − 0.002P , where P is the pressure in GPa and the cell axis lengths are in Å. The c-axis is the less compressible than the a- and b-axis’s. The lattice parameters of the calcium ferrite-type aluminous phase have a linear dependence on the pressure over the range studied. To determine the elastic parameters, the P–V data were fitted to the Birch–Murnaghan EOS (Birch, 1947): P = 1.5 K0 (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 bulk modulus, K0 , was determined to be 253 (14) GPa and K0 = 3.6 (0.6). The unit cell volume data as a function of pressure are plotted in Fig. 4. When K0 was set to K0 = 4, the value of K0 = 243 (2) GPa was obtained. This value is generally consistent with that of the MgAl2 O4 end-member phase reported in a previous study (Table 4). Therefore, the compositional effect on the compressibility of the calcium ferrite-type aluminous phase is not large. In the present study, we did not observe a phase transition of the calcium ferrite-type aluminous phase
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315
Fig. 3. Unit cell parameters of calcium ferrite-type aluminous phase. The dashed lines show the results of the linear fit.
up to a pressure of 70 GPa, the maximum pressure studied. However, we could not demonstrate that calcium ferrite-type aluminous phase is stable at the entire lower mantle conditions, because there is the
possibility that the temperatures used to heat the sample were not high enough to trigger a phase transition. In this study, the heating temperatures were kept low to prevent reaction between the sample and the
Fig. 4. Pressure–volume data for calcium ferrite-type aluminous phase at 300 K. The powder X-ray diffraction results are shown by the circles (compression) and squares (decompression). Dashed curve: third-order Birch–Murnaghan equation fit.
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Table 4 Parameters of the Birch–Murnaghan equation of state derived for the calcium ferrite-type aluminous phase from the experimental studies K0
K0
Compositions
References
253 (14) 243 (2) 241 (3)
3.6 (0.6) 4 (fixed) 4 (fixed)
Natural Natural MgAl2 O4
This study This study Yutani et al. (1997)
K0 and K0 are the isothermal bulk modulus and the first derivative of the isothermal bulk modulus at 300 K. The error in parentheses after each number represents the standard deviations.
pressure medium. Therefore, it is necessary to perform future experiments at higher temperatures corresponding to the lower mantle geotherm (i.e. >2000 K). In Fig. 5, we compare the pressure–density relationship of the calcium ferrite-type aluminous phase with those of Mg-, Ca-perovskite, stishovite, CaCl2 -type silica phase, and the hexagonal aluminous phase that all co-exist in the subducted MORB. Densities of the phases were calculated using an appropriate EOS with suitable thermoelastic parameters (see Table 5). It is known that the compressibility of Mg-perovskite changes with chemical composition.
Fig. 5 shows three density profiles of Mg-perovskite based on different compressibilities reported in previous studies (Fiquet et al., 2000; Zhang and Weidner, 1999; Andrault et al., 2001). Although there is a large difference in density among these phases, the Mg-perovskite is denser than the other phases. To estimate the densities of the phases precisely, it is necessary to consider the change in chemical composition of each phase in the lower mantle. As it has been reported that the chemical composition change of minerals caused by the pressure change is small (Kesson et al., 1994; Ono et al., 2001), we did not consider it in our density estimation. The density of the calcium ferrite-type aluminous phase is lower than those of the Mg-, Ca-perovskite and hexagonal aluminous phase in the lower mantle. It is known that natural subducted oceanic crust has a considerable variation in its chemical composition. Changes of whole rock composition can change the proportion of the minerals in the subducted oceanic crusts. The proportion of calcium ferrite-type aluminous phase seems to be most sensitive to the sodium content compared with other elements, and it increases with increasing sodium content, because the calcium ferrite-type
Fig. 5. Comparison of pressure–density relations for calcium ferrite-type aluminous phase and co-existing phases in the subducted MORB at room temperature. The densities were calculated using the chemical compositions of minerals in the subducted MORB (Ono et al., 2001; Hirose and Fei, 2002) and the parameters of the equations of state of minerals reported in Table 5. Phase abbreviations as in Table 5.
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Table 5 Parameters of the Birch–Murnaghan equation of state and densities of minerals of subducted oceanic crust Minerals
K0
K0
ρ0
References
CF Mg-pv1 Mg-pv2 Mg-pv3 Ca-pv St CC HAP
253 259 236 275.5 236 282 291 198
3.6 3.7 4 4 3.9 4 4.29 4
4.138 4.399 4.399 4.399 4.211 4.253 4.253 4.145
This study Fiquet et al. (2000); Ono et al. (2001) Ono et al. (2001); Zhang and Weidner (1999) Ono et al. (2001); Andrault et al. (2001) Ono et al. (2001); Shim et al. (2000) Ono et al. (2002a) Ono et al. (2001); Andrault et al. (1998) Ono et al. (2002b)
K0 and K0 are the isothermal bulk modulus and the first derivative of the isothermal bulk modulus at 300 K. ρ 0 is the density of mineral at 0.0001 GPa and 300 K. Abbreviations are: calcium ferrite-type structure, CF; MgSiO3 perovskite, Mg-pv1 ; Al0.1 (MgSi)0.95 O3 perovskite, Mg-pv2 ; Al0.22 (MgSi)0.89 O3 perovskite, Mg-pv3 ; CaSiO3 perovskite, Ca-pv; Al-bearing SiO2 stishovite, St; CaCl2 -type structure SiO2 phase, CC; hexagonal aluminous phase, HAP.
aluminous phase is the sodium host phase. Therefore, the density of subducted oceanic crust may decrease with increasing sodium content, because the density of calcium ferrite-type phase is lower than those of the Mg-, Ca-perovskite, and hexagonal aluminous co-existing phases.
Acknowledgements We thank E. Takahashi, T. Suto, T. Kurashina, T. Kawamoto, K. Mibe, M. Isshiki, and Y. Ohishi for help during experiments. The synchrotron radiation experiments were performed at the PF with the approval of the High Energy Accelerator Research Organization (KEK) (proposal no. 2001G222).
References Andrault, D., Fiquet, G., Guyot, F., Hanfland, M., 1998. Pressure-induced Landau-type transition in stishovite. Science 282, 720–724. Andrault, D., Bolfan-Casanova, N., Guignot, N., 2001. Equation of state of lower mantle (Al, Fe)-MgSiO3 perovskite. Earth Planet. Sci. Lett. 193, 501–508. Birch, F., 1947. Finite elastic strain of cubic crystals. Phys. Rev. 71, 809–924. Daniel, I., Cardon, H., Fiquet, G., Guyot, F., Mezouar, M., 2001. Equation of state of Al-bearing perovskite to lower mantle pressure conditions. Geophys. Res. Lett. 28, 3789–3792. Fiquet, G., Dewaele, A., Andrault, D., Kunz, M., Bihan, T.L., 2000. Thermoelastic properties and crystal structure of MgSiO3
perovskite at lower mantle pressure and temperature conditions. Geophys. Res. Lett. 27, 21–24. Hamilton, D.H., Henderson, C.M.B., 1968. The preparation of silicate composition by a gelling method. Miner. Mag. 36, 832– 838. Hammersley, J., 1996. Fit2d Report. European Synchrotron Radiation Facility, Grenoble, France. Hirose, K., Fei, Y., 2002. Subsolidus and melting phase relations of basaltic composition in the uppermost lower mantle. Geochim. Cosmochim. Acta. 66, 2099–2108. Hirose, K., Fei, Y., Ma, Y., Mao, H.K., 1999. The fate of subducted basaltic crust in the Earth’s lower mantle. Nature 397, 53–56. 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. Irifune, T., Ringwood, A.E., 1993. Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600–800 km in the mantle. Earth Planet. Sci. Lett. 117, 101–110. Irifune, T., Fujino, K., Ohtani, E., 1991. A new high pressure form of MgAl2 O4 . Nature 349, 409–411. Jamieson, J.C., Fritz, J.N., Manghnani, M.H., 1982. Pressure measurement at high temperature in X-ray diffraction studies: gold as a primary standard. In: Akimoto, S., Manghnani, M.H. (Eds.), High-Pressure Research in Geophysics. Center for Academic Publishing, Tokyo, pp. 27–48. Kesson, S.E., Fitz Gerald, J.D., Shelley, J.M., 1994. Mineral chemistry and density of subducted basaltic crust at lower-mantle pressures. Nature 372, 767–769. 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., Suto, T., Hirose, K., Kuwayama, Y., Komabayashi, T., Kikegawa, T., 2002a. Equation of state of Al-bearing stishovite to 40 GPa at 300 K. Am. Miner., in press. Ono, S., Hirose, K., Isshiki, M., Mibe, K., Saito, Y., 2002b. Equation of state of hexagonal aluminous phase of natural
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composition to 63 GPa at 300 K. Phys. Chem. Miner., in press. Shim, S.H., Duffy, T., Shen, G., 2000. The equation of state of CaSiO3 perovskite to 108 GPa at 300 K. Phys. Earth Plant. 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. Philos. Trans. R. Soc. London Ser. A 342, 105–120.
Van der Hilst, R., Widiyantoro, S., Engdahl, E.R., 1997. Evidence for deep mantle circulation from global tomography. Nature 386, 578–584. Yutani, M., Yagi, T., Yusa, H., Irifune, T., 1997. Compressibility of calcium ferrite-type MgAl2 O4 . Phys. Chem. Miner. 24, 340–344. Zhang, J., Weidner, D.J., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784.
The compressibility of a natural composition calcium ferrite-type aluminous phase to 70 GPa Shigeaki Ono a,b,∗ , Kei Hirose b , Takumi Kikegawa c , Yoko Saito d a
b
Institute for Frontier Research on Earth Evolution, Japan Marine Science & Technology Center, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan 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 d College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajiyosui, Setagaya, Tokyo 156-8550, Japan Received 12 April 2002; received in revised form 14 June 2002
Abstract In situ X-ray diffraction (XRD) experiments of a calcium ferrite-type aluminous phase that is a sodium host mineral of subducted oceanic crusts into the Earth’s lower mantle have been carried out using a laser-heated diamond anvil cell (LHDAC), up to a pressure of 70 GPa with synchrotron radiation source at the Photon Factory (PF) in Japan. The sample was heated using a Nd:YAG laser at each pressure increment to relax the deviatoric stress in the sample. XRD measurements were carried out at 300 K using an angle-dispersive technique. The pressure was determined from an internal platinum pressure calibrant. A Birch–Murnaghan equation of state (EOS) was determined from the experimental unit cell parameters: volume V0 = 244.07 (±55) Å3 , density ρ0 = 4.143 g/cm3 , bulk modulus K0 = 253 (±14) GPa, and K0 = 3.6 (±0.6). When the first pressure derivative of the bulk modulus K0 was fixed at 4, the value of K0 = 243 (±2) GPa was obtained. The density of the calcium ferrite-type aluminous phase is lower than those of co-existing Mg-, Ca-perovskite, and hexagonal aluminous phase in subducted oceanic crusts. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Calcium ferrite-type aluminous phase; Equation of state; X-ray powder diffraction; High P–T; Subducting slab
1. Introduction The phase relationship of mid-oceanic ridge basalt (MORB) at high pressures and temperatures is of interest to the geophysical community because MORB is one of the rocks in the oceanic crust that seems to subduct into the lower mantle (e.g.Van der Hilst et al., 1997). An aluminous phase appears in rocks having the MORB composition at pressures corresponding ∗ Corresponding author. Tel.: +81-3-5734-2618; fax: +81-3-5734-3538. E-mail address: [email protected] (S. Ono).
to lower mantle conditions, and this phenomenon has been investigated by several groups using multi-anvil high pressure apparatus (Irifune and Ringwood, 1993; Hirose et al., 1999; Ono et al., 2001). These studies have reported that the aluminous phase has a calcium ferrite-type structure (Pnam) with orthorhombic symmetry. This aluminous phase amounts to some 15–25 wt.% of all minerals having the MORB composition in the lower mantle (Kesson et al., 1994; Ono et al., 2001). The physical properties of any phase present in the subducted slab may affect its dynamics and that of material circulation in the mantle. Therefore, it is important to determine the compressibility
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S. Ono et al. / Physics of the Earth and Planetary Interiors 131 (2002) 311–318
of the MORB aluminous phase. Recently, it was reported that the incorporation of Al2 O3 into Mgperovskite could significantly vary its bulk modulus (Zhang and Weidner, 1999; Daniel et al., 2001; Andrault et al., 2001). The mineral compositions in the natural mantle are often quite different from those of simple systems, and to understand fully the dynamics of the mantle, it is, therefore, necessary to know the compositional effect on the physical properties. In this study, we used a laser-heated diamond anvil cell (LHDAC) which made possible to acquire precise data on a sample under high pressure, and we also used intense X-rays from a synchrotron radiation source. We have directly investigated the stability and the pressure–volume equation of state (EOS) of a calcium ferrite-type aluminous phase in the subducted MORB over the pressure range from 0.1 MPa to 70 GPa, equivalent to the lower mantle conditions. In this paper, we compare the density of calcium ferrite-type aluminous phase with that of co-existing minerals in the lower mantle.
2. Experimental procedure A polycrystalline specimen of calcium ferrite-type aluminous phase was synthesized in a 1000 t 6–8-type multi-anvil apparatus (SEDI-1000; see Takahashi et al., 1993) at the Magma Factory at the Tokyo Institute of Technology. A synthetic gel was used to produce a reactive and homogeneous starting
material (Hamilton and Henderson, 1968). The gel was prepared as follows. Reagent-grade Fe, MgO, CaCO3 , Na2 CO3 , and K2 CO3 were dissolved in nitric acid and mixed with solutions of (C2 H5 O)4 Si, (C3 H9 O)4 Ti and Al(NO3 )3 . The gel was precipitated from the mixed solution and dried. The dried gel was ground and heated to 1000 ◦ C under controlled oxygen fugacity. The specimen used in this study was synthesized under a pressure of 25 GPa and a temperature of 1800 ◦ C for 30 min. Table 1 shows the chemical composition of the specimen in this study, and those of calcium ferrite-type aluminous phases in the subducted MORB reported by previous studies. High pressure X-ray diffraction (XRD) experiments were performed using a LHDAC high pressure apparatus. The synthesized calcium ferrite-type aluminous phase was loaded into a 100 m hole that had been drilled in a rhenium gasket pre-indented to a thickness of 50 m. Fine NaCl powder was used as the pressure transmission medium. Platinum foil was placed in the sample chamber to provide an internal pressure calibrant. The sample was heated after each pressure change with a multi-mode continuous wave Nd:YAG laser using a one-sided laser heating technique to reduce any pressure inhomogeneity in the sample (Fig. 1). The size of the heating spot was about 100 m. The sample was probed using angle-dispersive XRD using the synchrotron beam line BL13A at the Photon Factory (PF) of the High Energy Accelerator Research Organization (KEK), Japan. The incident X-ray beam was monochromatized
Table 1 Chemical compositions of the starting material and the calcium ferrite-type aluminous phase reported by previous studies This study
Irifune and Ringwood (1993)
Hirose et al. (1999)
Ono et al. (2001)
SiO2 (wt.%) TiO2 (wt.%) Al2 O3 (wt.%) FeOa (wt.%) MnO (wt.%) MgO (wt.%) CaO (wt.%) Na2 O (wt.%) K2 O (wt.%) Cr2 O3 (wt.%)
26.5 0.8 36.5 7.6 – 9.6 0.8 18.2 – –
23.42 0.38 46.67 3.10 – 18.53 3.10 4.71 – –
26.96 0.92 32.78 16.19 0.05 10.74 1.87 11.15 0.00 0.10
27.5 0.7 40.1 7.9 10.0 0.8 12.1 0.0 –
Total (wt.%) Pressure (GPa)
100.0 25
99.91 27
100.75 27
99.1 30
Calcium ferrite-type aluminous phase made from synthetic gel in this study. The chemical composition of this study was analyzed by electron microprobe analysis (EPMA). a All iron has been calculated as FeO.
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Fig. 1. Schematic diagram of the experimental design of in situ X-ray diffraction measurement at BL13A, PF. Abbreviations are lens (L) and mirror (M).
to a wavelength of 0.4266 Å. The X-ray beam size was collimated to a diameter of about 50 m. Angledispersive XRD patterns were obtained on an imaging plate. The observed intensities on the imaging plates were integrated as a function of 2θ using the Fit2d code (Hammersley, 1996) in order to obtain conventional, one-dimensional diffraction profiles. The diffraction peak positions were determined using a peak fitting program. Two XRD lines from platinum, the (1 1 1) and (2 0 0) peaks, were observed in addition to any diffraction lines from the calcium ferrite-type aluminous phase. The pressure was determined from the measured platinum unit cell volume using the EOS for platinum developed by Holmes et al. (1989).
The sample was heated by laser after each pressure increment before the XRD measurement. The laser heating annealed the sample and reduced any pressure gradient across the sample. This improved the quality of the XRD data (Fig. 2). The laser beam was carefully scanned to heat the entire sample in order to prevent any reaction with the NaCl pressure medium and the
3. Results and discussion The powder XRD data of the calcium ferrite-type aluminous phase at room pressure before compression revealed that this phase has an orthorhombic cell (Pnam) with unit cell dimensions of a = 8.660 (2) Å, b = 10.128 (2) Å and c = 2.786 (1) Å. The observed and calculated XRD patterns of the calcium ferrite-type aluminous phase are compared in Table 2. The calcium ferrite-type aluminous phase in the MORB has a unit cell volume, V0 = 244.34 (12) Å3 , which is higher than that of MgAl2 O4 as reported by Irifune et al. (1991), where V0 = 240.00 Å3 . The density of our sample, ρ0 = 4.143 g/cm3 is also higher than the MgAl2 O4 density of ρ0 = 3.937 g/cm3 .
Fig. 2. An example of X-ray diffraction pattern of the sample at high pressure. Abbreviations of peaks are as follows: C, calcium ferrite-type aluminous phase; P, platinum; A, B1-type sodium chloride; B, B2-type sodium chloride; R, Rhenium gasket.
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Table 2 Observed and calculated X-ray diffraction pattern of calcium ferrite-type aluminous phase before compression (0.1 MPa)
Table 3 Lattice parameters and volumes of calcium ferrite-type aluminous phase to 70 GPa
hkl
dobs (Å)
dcal (Å)
d (Å)
Iobs
P (GPa)
a (Å)
020 200 220 130 310 011 111 040 320 121 201 330 240 400 410 131 420 150 311 231 321 141 250 331 241 401 411 350 160 440 421 260 251 431 360 600
5.0657 4.3262 3.2911 3.1470 2.7774 2.6853 2.5626 2.5305 2.5067 2.3468 2.3468 2.1933
5.0641 4.3301 3.2911 3.1455 2.7762 2.6859 2.5654 2.5321 2.5079 2.3492 2.3427 2.1940 2.1858 2.1651 2.1172 2.0854 1.9907 1.9724 1.9664 1.9247 1.8638 1.8313 1.8348 1.7236 1.7196 1.7095 1.6856 1.6581 1.6569 1.6455 1.6197 1.5728 1.5323 1.5251 1.4572 1.4434
0.0003 −0.0009 0.0000 0.0005 0.0004 −0.0002 −0.0011 −0.0006 −0.0005 −0.0010 0.0017 −0.0003
10 35 15 <5 <5 1 <5 70 100 65 65 5
0.0001 5.5 6.5 9.7 19.2 31.4 47.3 57.2 65.5 70.1
8.640 8.630 8.631 8.559 8.468 8.445 8.211 8.140 8.128 8.132
0.0003 0.0000 −0.0005
10 85 30
0.0004 0.0004 0.0002 0.0003 −0.0016
75 10 5 20 20
−0.0001 −0.0008 0.0001 0.0001 0.0008
85 30 25 5 5
−0.0001 0.0000
<5 20
0.0000 0.0001 0.0003
<5 15 15
2.1179 2.0855 1.9898 1.9671 1.9254 1.8643 1.8319 1.8319 1.7195 1.7081 1.6858 1.6582 1.6582 1.6195 1.5728 1.5251 1.4573 1.4437
Calculated d-spacings and volume are based on orthorhombic unit cell dimensions of a = 8.660 (2) Å, b = 10.128 (2) Å, c = 2.786 (1) Å and 244.34 (12) Å3 .
platinum pressure calibrant. The typical error in pressure was less than 0.5 GPa based on the lattice parameters as calculated from different platinum diffraction lines at the same pressure. However, by using different EOS for the platinum pressure calibrant, the difference in experimental pressure was greater than 1 GPa at higher pressures (e.g. Holmes et al., 1989; Jamieson et al., 1982). Therefore, the uncertainty of pressure in the present study is taken to be greater than 1 GPa.
b (Å) (11) (17) (10) (11) (14) (20) (9) (17) (10) (30)
10.115 10.012 10.013 10.014 9.884 9.780 9.640 9.550 9.475 9.434
Volume (Å3 )
c (Å) (12) (16) (9) (10) (11) (17) (10) (11) (17) (28)
2.793 2.761 2.759 2.756 2.723 2.675 2.665 2.639 2.630 2.626
(4) (7) (3) (3) (3) (4) (3) (4) (3) (6)
244.07 238.53 238.43 236.23 227.90 220.91 210.95 205.16 202.57 201.44
(55) (85) (43) (48) (51) (72) (41) (57) (51) (105)
Numbers in parentheses represent the error of lattice parameters. The data at 0.0001 GPa was obtained after decompression.
The effect of pressure on the unit cell parameters and volume of the calcium ferrite-type aluminous phase is tabulated in Table 3. The unit cell parameters as a function of pressure are plotted in Fig. 3. The pressure dependence of the a-axis can be represented by a = 8.654 − 0.008P , the b-axis by b = 10.084 − 0.009P , and the c-axis by c = 2.774 − 0.002P , where P is the pressure in GPa and the cell axis lengths are in Å. The c-axis is the less compressible than the a- and b-axis’s. The lattice parameters of the calcium ferrite-type aluminous phase have a linear dependence on the pressure over the range studied. To determine the elastic parameters, the P–V data were fitted to the Birch–Murnaghan EOS (Birch, 1947): P = 1.5 K0 (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 bulk modulus, K0 , was determined to be 253 (14) GPa and K0 = 3.6 (0.6). The unit cell volume data as a function of pressure are plotted in Fig. 4. When K0 was set to K0 = 4, the value of K0 = 243 (2) GPa was obtained. This value is generally consistent with that of the MgAl2 O4 end-member phase reported in a previous study (Table 4). Therefore, the compositional effect on the compressibility of the calcium ferrite-type aluminous phase is not large. In the present study, we did not observe a phase transition of the calcium ferrite-type aluminous phase
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315
Fig. 3. Unit cell parameters of calcium ferrite-type aluminous phase. The dashed lines show the results of the linear fit.
up to a pressure of 70 GPa, the maximum pressure studied. However, we could not demonstrate that calcium ferrite-type aluminous phase is stable at the entire lower mantle conditions, because there is the
possibility that the temperatures used to heat the sample were not high enough to trigger a phase transition. In this study, the heating temperatures were kept low to prevent reaction between the sample and the
Fig. 4. Pressure–volume data for calcium ferrite-type aluminous phase at 300 K. The powder X-ray diffraction results are shown by the circles (compression) and squares (decompression). Dashed curve: third-order Birch–Murnaghan equation fit.
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Table 4 Parameters of the Birch–Murnaghan equation of state derived for the calcium ferrite-type aluminous phase from the experimental studies K0
K0
Compositions
References
253 (14) 243 (2) 241 (3)
3.6 (0.6) 4 (fixed) 4 (fixed)
Natural Natural MgAl2 O4
This study This study Yutani et al. (1997)
K0 and K0 are the isothermal bulk modulus and the first derivative of the isothermal bulk modulus at 300 K. The error in parentheses after each number represents the standard deviations.
pressure medium. Therefore, it is necessary to perform future experiments at higher temperatures corresponding to the lower mantle geotherm (i.e. >2000 K). In Fig. 5, we compare the pressure–density relationship of the calcium ferrite-type aluminous phase with those of Mg-, Ca-perovskite, stishovite, CaCl2 -type silica phase, and the hexagonal aluminous phase that all co-exist in the subducted MORB. Densities of the phases were calculated using an appropriate EOS with suitable thermoelastic parameters (see Table 5). It is known that the compressibility of Mg-perovskite changes with chemical composition.
Fig. 5 shows three density profiles of Mg-perovskite based on different compressibilities reported in previous studies (Fiquet et al., 2000; Zhang and Weidner, 1999; Andrault et al., 2001). Although there is a large difference in density among these phases, the Mg-perovskite is denser than the other phases. To estimate the densities of the phases precisely, it is necessary to consider the change in chemical composition of each phase in the lower mantle. As it has been reported that the chemical composition change of minerals caused by the pressure change is small (Kesson et al., 1994; Ono et al., 2001), we did not consider it in our density estimation. The density of the calcium ferrite-type aluminous phase is lower than those of the Mg-, Ca-perovskite and hexagonal aluminous phase in the lower mantle. It is known that natural subducted oceanic crust has a considerable variation in its chemical composition. Changes of whole rock composition can change the proportion of the minerals in the subducted oceanic crusts. The proportion of calcium ferrite-type aluminous phase seems to be most sensitive to the sodium content compared with other elements, and it increases with increasing sodium content, because the calcium ferrite-type
Fig. 5. Comparison of pressure–density relations for calcium ferrite-type aluminous phase and co-existing phases in the subducted MORB at room temperature. The densities were calculated using the chemical compositions of minerals in the subducted MORB (Ono et al., 2001; Hirose and Fei, 2002) and the parameters of the equations of state of minerals reported in Table 5. Phase abbreviations as in Table 5.
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Table 5 Parameters of the Birch–Murnaghan equation of state and densities of minerals of subducted oceanic crust Minerals
K0
K0
ρ0
References
CF Mg-pv1 Mg-pv2 Mg-pv3 Ca-pv St CC HAP
253 259 236 275.5 236 282 291 198
3.6 3.7 4 4 3.9 4 4.29 4
4.138 4.399 4.399 4.399 4.211 4.253 4.253 4.145
This study Fiquet et al. (2000); Ono et al. (2001) Ono et al. (2001); Zhang and Weidner (1999) Ono et al. (2001); Andrault et al. (2001) Ono et al. (2001); Shim et al. (2000) Ono et al. (2002a) Ono et al. (2001); Andrault et al. (1998) Ono et al. (2002b)
K0 and K0 are the isothermal bulk modulus and the first derivative of the isothermal bulk modulus at 300 K. ρ 0 is the density of mineral at 0.0001 GPa and 300 K. Abbreviations are: calcium ferrite-type structure, CF; MgSiO3 perovskite, Mg-pv1 ; Al0.1 (MgSi)0.95 O3 perovskite, Mg-pv2 ; Al0.22 (MgSi)0.89 O3 perovskite, Mg-pv3 ; CaSiO3 perovskite, Ca-pv; Al-bearing SiO2 stishovite, St; CaCl2 -type structure SiO2 phase, CC; hexagonal aluminous phase, HAP.
aluminous phase is the sodium host phase. Therefore, the density of subducted oceanic crust may decrease with increasing sodium content, because the density of calcium ferrite-type phase is lower than those of the Mg-, Ca-perovskite, and hexagonal aluminous co-existing phases.
Acknowledgements We thank E. Takahashi, T. Suto, T. Kurashina, T. Kawamoto, K. Mibe, M. Isshiki, and Y. Ohishi for help during experiments. The synchrotron radiation experiments were performed at the PF with the approval of the High Energy Accelerator Research Organization (KEK) (proposal no. 2001G222).
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