- Email: [email protected]
Approach to the mineralogy of the lower mantle by a combined method of a laser-heated diamond anvil cell experiment and analytical electron microscopy
Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
Approach to the mineralogy of the lower mantle by a combined method of a laser-heated diamond anvil cell experiment and analytical electron microscopy Kiyoshi Fujino a,∗ , Yohei Sasaki a , Toyohisa Komori a , Hisayuki Ogawa a , Nobuyoshi Miyajima b , Nagayoshi Sata b , Takehiko Yagi b a
Division of Earth and Planetary Sciences, Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo 060-0810, Japan b Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan Received 27 February 2003; received in revised form 25 August 2003; accepted 17 November 2003
Abstract A combined method of a laser-heated diamond anvil cell (DAC) experiment and analytical electron microscopy to study the mineralogy of the lower mantle was explored. Particularly, the use of ultramicrotomy to make ultrathin foils for analytical transmission electron microscopy (ATEM) of the ultrahigh pressure materials was explored with the aim of obtaining the exact chemical compositions of those materials with submicron size. Ultrathin foils with a thickness down to about 30 nm were obtained for ultrahigh pressure materials synthesized by a laser-heated DAC, although some areas of the foils were lost. Reliable chemical compositions were obtained with these foils using the experimentally obtained k-factors. The preliminary results on the mutual solubilities between (Mg,Fe)SiO3 and CaSiO3 perovskites with this method indicate that the (Mg,Fe) solubility in CaSiO3 perovskite increases dramatically (up to 0.34 cations per foumula unit at 78 GPa and 1900 ◦ C) with increasing the bulk iron content and/or pressure, while the Ca solubility in coexisting (Mg,Fe)SiO3 perovskite remains very limited (0.01–0.02 cations per formula unit) under the same conditions. © 2004 Elsevier B.V. All rights reserved. Keywords: Diamond anvil cell; Analytical transmission electron microscopy; Ultramicrotomy; Lower mantle
1. Introduction Laser-heated diamond anvil cell (DAC) experiments are indispensable for the study of the mineralogy of the lower mantle. However, the materials synthesized by laser-heated DAC experiments are very small in amount and very fine in grain size, submicron in most cases. For these reasons, it is very difficult to determine both structures and compositions of respective ∗ Corresponding author. Tel.: +81-11-706-2728; fax: +81-11-706-4650. E-mail address: [email protected] (K. Fujino).
product phases, even if we could identify the existing phases by synchrotron X-ray radiation. Analytical transmission electron microscopy (ATEM) provides us with both structures and compositions of respective phases simultaneously at the nm scale. Therefore, a combined method of a laser-heated DAC experiment and ATEM is a very powerful tool for the study of the mineralogy of the lower mantle. With such a perspective, many studies have been performed (Guyot et al., 1988; Madon et al., 1989; Kesson et al., 1995; Fujino et al., 1998; Miyajima et al., 1999). We have recently installed a newly designed YLF laser-heated DAC system in our laboratory and are now further exploring
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2003.11.013
216
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
the advantages of this combined method. The details of the DAC system will be presented elsewhere, and in this paper we mainly report on the use of ATEM in studying ultrahigh pressure materials. Although ATEM is very powerful for the study of ultrahigh pressure materials, there are several problems that have to be resolved. The first is that the specimens of ultrahigh pressure phases are in general very sensitive to electron beam irradiation and easily become amorphous. In particular, silicate perovskites are very sensitive. We are trying to overcome this problem by the use of a high-sensitivity and high-resolution TV camera system for TEM. With this system, we are expecting to observe TEM images and also electron diffraction patterns of the ultrahigh pressure phases under a very weak electron beam. This is now under way and the results will be presented elsewhere. The second problem is that in widely used ion-thinning to make ATEM specimens, elements are selectively removed from the surface area of the specimens during ion thinning in some cases, particularly in ultrahigh pressure materials, and analyzed compositions are different from the real values (Fujino et al., 1998). To overcome this problem, we have explored the use of ultramicrotomy for ultrahigh pressure materials because it leaves the chemistry unchanged. In the following, we mainly report the details of using ultramicrotomy on ultrahigh pressure materials, and also report some preliminary results on the mineralogy of the lower mantle.
2. Experimental 2.1. Ultrahigh pressure materials examined Materials used for the investigation of ultramicrotomy are Ca(Si,Ti)O3 , (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites and the details are summarized in Table 1 together with the high pressure synthesis conditions. Runs 1803 and 1814 were chosen from samples of high pressure and high temperature experiments performed on the system CaSiO3 -CaTiO3 at 5.7–13.2 GPa and 1500 ◦ C using a multianvil cell apparatus at the Institute for the Study of the Earth’s Interior, Okayama University. Runs A-13, B-02, A-06, A-14 and C-05 are selected samples from the system CaMgSi2 O6 (Di)-CaFeSi2 O6 (Hd) synthesized at 25–80 GPa and 1700–2000 ◦ C by laser-heated diamond anvil cell apparatus at the Institute for Solid State Physics, University of Tokyo and at Hokkaido University. In the DAC experiments, diamonds with 450 and 300 m culets were used for generating pressures of 25–30 and 70–80 GPa, respectively. A powder of a synthetic pyroxene solid solution starting material was loaded into the hole of a stainless steel or rhenium gasket and was sandwiched by NaCl in the diamond cell. Platinum powder was mixed with the CaMgSi2 O6 sample in order to absorb laser radiation for heating. Pressure was measured by the ruby fluorescence technique (Mao et al., 1978). Typical pressure differences between the center and the edge
Table 1 Synthesis conditions of the materials examined Pressure (GPa)
Temperature (◦ C)
Duration (min)
Product phases
CaSiO3 –CaTiO3 1803 CaSi0.8 Ti0.2 O3 1814 CaSi0.8 Ti0.2 O3
8.3 11.3
1500 1500
30 30
Pv + Wal Pv + Tit + Lar
CaMgSi2 O6 (Di)–CaFeSi2 O6 (Hd) A-13 Di100 Hd0 B-02 Di75 Hd25 A-06 Di50 Hd50 A-14 Di50 Hd50 C-05 Di50 Hd50
29.7 31.5 29.5 26.2 78.0
1930 1800 1800 1820 1900
10 10 10 10 10
Mg-Pva + Ca-Pva Mg-Pv + Mw + St + Ca-Pva Mg-Pv + Mw + St + Ca-Pva Mg-Pv + Mw + St + Ca-Pva Mwa + SiO2 b + Ca-Pva
Run no.
Starting material
Pv: Ca(Si,Ti)O3 perovskite, Wal: walstromite-structured CaSiO3 , Tit: titanite solid solution, Lar: larnite (-Ca2 SiO4 ), Mg-Pv: (Mg,Fe)SiO3 perovskite, Ca-Pv: CaSiO3 perovskite, Mw: magnesiowustite, St: stishovite. a Identified by ATEM. b Unknown phase of SiO . 2
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
of the sample are about 5 GPa. Pressures after heating are listed in Table 1. The sample was heated from both sides by a YAG laser beam focused to 70–80 m in multimode (A-13, A-06, A-14 and C-05) or a YLF laser beam focused to about 30 m in TEM01 mode (B-02) and temperature of the sample was measured spectroradiometrically. The experimental details and comprehensive results of high pressure phase relations and structure variations of perovskite phases in both systems will be presented in separate papers. 2.2. Ultramicrotomy Ultramicrotomy was originally developed in biology to section biological materials into ultrathin foils for TEM observation, and is now applied to high molecular compounds and metallic materials using a diamond knife (Williams and Carter, 1996). In ultramicrotomy the sample, embedded in polymeric resin, is moved across a knife blade while being advanced stepwise (Fig. 1). In our case, the multianvil cell samples were cut into cubes with edges of 100 m and laser-heated DAC samples were thin disks of 100 m diameter and 20–30 m in thickness. The samples were embedded in polymeric resin Quetol 812 (Nisshin EM Co. Ltd.) using micro-molds and hardened at 70 ◦ C for 72 h. Samples were sectioned by an ultramicrotome (Reichert-Nissen ultracuts N) at the Electron Microscope Center of the Faculty of Agriculture, Hokkaido University. A resin block including a sample was cured and trimmed by a glass
Fig. 1. Schematic drawing of an ultramicrotome.
217
knife so that the cutting surface becomes 100–300 m across and was then sectioned by a diamond knife of 45◦ . The sectioned thin flakes floated off onto water in a trough (Fig. 1), and the flakes which contain the central part of the heated sample were collected on a grid with a carbon film. The sectioned flakes were additionally carbon-coated. 2.3. TEM observations and quantitative chemical analysis TEM observations and quantitative chemical analyses were performed using an analytical electron microscope (JEOL, JEM-2010) at Hokkaido University, operated at 200 kV. Quantitative chemical analyses were made with a NORAN Instruments/Voyager II energy-dispersive analytical system attached to the JEM 2010 electron microscope. Silicate perovskites are very sensitive to electron beam irradiation, and in measuring the characteristic X-ray intensities, elements are selectively removed from the specimens when the electron beam density is stronger than some critical value (Fujino et al., 1998). Therefore, TEM observations and chemical analyses were made with a low electron beam current of 4 pA/cm2 on the small fluorescent screen, and the characteristic X-ray intensities were measured with an electron beam of 100–150 nm diameter and a counting time of 60–120 s. Characteristic X-ray intensities were measured in the central region of the thin foils so that the X-ray Intensities were obtained from the heated area of the sample. The k-factors (Cliff and Lorimer, 1975) used for the calculation of quantitative chemical analyses were experimentally obtained using the synthetic standard materials CaSiO3 (wollastonite), CaTiO3 (perovskite), Mg2 SiO4 (forsterite) and Fe2 SiO4 (fayalite). They were crushed in a pestle and mortar in petroleum ether and were placed on a grid with a holey carbon film. Kij -factors were experimentally obtained from the measured intensity ratios Ii /Ij and known concentration ratios Ci /Cj of the paired elements i and j in the standard materials (kij = (Ci /Cj )/(Ii /Ij )). The kACa -factors (where A = Si, Ti, Ca) used for Ca(Si,Ti)O3 perovskite were obtained as functions of total X-ray counts of Ca and the kASi -factors (where A = Mg, Fe, Ca, Si) used for (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites were obtained as functions of total X-ray counts of Si.
218
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
The experimentally obtained k-factors for our ATEM operating at 200 kV with an ultra-thin window and take-off angle of 25 ◦ were expressed as the following regression equations. For Ca(Si,Ti)O3 perovskite kTiCa = 0.540(5) + 9(5) × 10−7 ICa
with sufficient quality). Numbers in parentheses of the coefficients of the above equations denote one estimated standard deviation (E.S.D.) of the least significant digit. In the above equations, kSiCa is not equal to 1/kCaSi because the respective k-factors were obtained from the different data sets.
kSiCa = 0.423(16) + 1.2(17) × 10−6 ICa For (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites kCaSi = 2.45(18) − 2.6(20) × 10−5 ISi kMgSi = 1.14(2) − 5(20) × 10−7 ISi kFeSi = 1.80(7) − 9.9(4) × 10
−5
ISi
where ICa and ISi are the total counts of characteristic X-rays of Ca and Si, respectively, of the standard materials, and roughly correspond to the foil’s thickness. Assignment of k-factors by those intensities includes the correction for X-ray absorption (although standard materials MgSiO3 and FeSiO3 would have been better for this purpose, we could not get those materials
3. Results and discussion 3.1. Ultramicrotomy of ultrahigh pressure materials With our ultramicrotome, aggregates of submicron ultrahigh pressure materials could be sectioned into ultrathin foils down to 30 nm thickness. However, this was actually the distance over which the diamond knife was advanced stepwise, and the actual thickness of the foils may be slightly larger, considering that the resin block that contained the sample was not sectioned for every stroke of the diamond knife. Fig. 2 shows the electron micrographs of sectioned foils obtained using the ultramicrotome. Some areas of the
Fig. 2. Electron micrographs of foils prepared by the ultramicrotome. Foils are collected on a grid with a carbon film. (a) Foil of a mutianvil cell sample (sample no. 1814), (b) foil of a laser-heated DAC sample (sample no. A-14).
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
sectioned foils were often lost. With the multianvil cell samples (Fig. 2(a), No. 1814) the sectioned aggregates are fairly concentrated and coherent, while in the case of the DAC samples (Fig. 2(b), No. A-14) aggregates are rather dispersed. One of the reasons for the dispersion of the DAC samples is that we used NaCl as a pressure medium and the NaCl surrounding the specimen in the sectioned foil dissolved into water in the trough. This problem can be overcome in future by using other materials as pressure medium, which are not soluble in water. As seen in Fig. 2, the grain sizes of the multianvil cell samples are about 100–500 nm, while those of the DAC samples are much smaller, about 20–100 nm, and both are far below the grain size that can be measured by EPMA. There seem to be some cracks or deformation textures in the multianvil cell samples. Therefore, use of both ion-thinning for structure and texture observations and ultramicrotomy for chemical analyses on the same samples, will be desirable for the preparation of ATEM foils. 3.2. Quantitative chemical analyses of perovskites For the specimens prepared by ultramicrotomy, quantitative chemical analyses were performed by measuring the characteristic X-ray intensities of the product phases and calculating the compositions using the experimentally obtained k-factors. Analyses were quantified in terms of oxides and all iron was treated as Fe2+ . The equations listed in Section 2.3. were used to derive the k-factors appropriate for the unknown
219
samples based on the total X-ray counts of Ca or Si of the unknown samples. The estimated E.S.D.s of the obtained k-factors, depending on the measured X-ray intensities of Ca or Si, are σ(kTiCa ) =∼ 0.005, σ(kSiCa ) =∼ 0.017 (ICa =∼ 2600), and σ(kCaSi ) = 0.19–0.21, σ(kMgSi ) = 0.02, σ(kFeSi ) = 0.15–0.27 (ISi = 3000–6000). Relatively large σ(kCaSi ) and σ(kFeSi ) values seem to originate from the scatter of the measured X-ray intensity ratios of the paired elements in standard materials. Rough estimates of a relative error, σ(Ci /Cj )/(Ci /Cj ), based on σ(kij )/kij , σ(Ii )/Ii , and σ(Ij )/Ij (Williams and Carter, 1996) were 3–5% for Ca(Si,Ti)O3 perovskite (i = Si, Ti, j = Ca) and 3–12% for (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites (i = Mg, Fe, Ca, j = Si). Among the experimentally obtained k-factors, those for Ca and Fe deviated significantly from the theoretical ones (around 1) installed in the program. Although k-factors are not universal but are sensitive to the respective analytical system, so far it is not clear why the k-factors of our system deviate so much from the theoretical values. The calculated compositions, using these k-factors, of the perovskite phases in some of the samples of Table 1 are presented in Table 2. These are the averages of several values measured on the same sample, excluding ones that deviated strongly from the stoichiometry of perovskite. The analyzed cation numbers based on three oxygens per formula unit are all close to the ideal chemical formula of perovskite, and the obtained composition of Ca(Si,Ti)O3 perovskite (No. 1803) is nearly the same as that obtained by EPMA (25 mol% of CaSiO3 ). Therefore, ultramicrotomy,
Table 2 Compositions analyzed by ATEM Run no. Starting composition Phase Pressure (GPa)
1803 CaSi0.8 Ti0.2 O3 Pv 8.3
A-13 Di100 Hd0 Mg-Pv 29.7
B-02 Di75 Hd25 Mg-Pv 31.5
A-13 Di100 Hd0 Ca-Pv 29.7
A-06 Di50 Hd50 Ca-Pv 29.5
C-05 Di50 Hd50 Ca-Pv 78.0
Cation number (O = 3) Mg Si Ca Ti Fe
– 0.244 0.998 0.757 –
0.958 1.021 0.009 – –
0.828 1.038 0.003 – 0.094
0.036 1.059 0.847 – –
0.092 1.040 0.741 – 0.087
0.182 0.986 0.689 – 0.158
1.999
1.988
1.963
1.942
1.960
2.015
Total
Pv: Ca(Si,Ti)O3 perovskite, Mg-Pv: (Mg,Fe)SiO3 perovskite, Ca-Pv: CaSiO3 perovskite.
220
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
combined with the experimentally obtained k-factors, gives us reliable chemical compositions. 3.3. Preliminary results on the solubility relations between (Mg,Fe)SiO3 and CaSiO3 perovskites It is important to know the mutual solubilities between (Mg,Fe)SiO3 perovskite and CaSiO3 perovskite under lower mantle conditions to understand the mineral compositions and dynamics of the lower mantle. From high pressure phase transformation experiments on diopside (CaMgSi2 O6 ), Irifune et al. (2000) reported very limited mutual solubilities between MgSiO3 and CaSiO3 perovskites; the Ca solubility in MgSiO3 perovskite was reported to be less than about 0.02 cations per formula unit based on three oxygens and the Mg solubility in CaSiO3 perovskite was less than about 0.04 cations at 25–27 GPa and 1000–1900 ◦ C. However, the solubilities of those cations at higher pressures or in the presence of iron containing phases has been unclear. In our recent laser-heated DAC experiments on the system CaMgSi2 O6 (Di)–CaFeSi2 O6 (Hd) at 25 to 80 GPa and 1700–2000 ◦ C with synchrotron X-ray radiation identification of the product phases, orthorhombic (Mg,Fe)SiO3 perovskite and cubic CaSiO3 perovskite coexisted in this system for compositions ranging from Di100 Hd0 to around Di45 Hd55 , although the exact structural states of both perovskites at around 70–80 GPa have not yet been ascertained. In Table 1, the product phases that were identified by synchrotron X-ray radiation and ATEM are summarized. Although we have not yet identified (Mg,Fe)SiO3 perovskite in C-05, analyses of the other samples at similar conditions suggest that (Mg,Fe)SiO3 perovskite should also be present in C-05. Table 2 shows the compositions of (Mg,Fe)SiO3 and CaSiO3 perovskites of selected samples analyzed by ATEM although we have not yet fully succeeded in obtaining the compositions of both perovskite phases in some samples. For the iron-free sample (A-13) at about 30 GPa, the Ca content of MgSiO3 perovskite is around 0.01 cations per formula unit based on three oxygens, and it remained nearly the same with increasing iron content (B-02) and/or pressure. On the other hand, the Mg content of CaSiO3 perovskite in A-13 is around 0.04 cations per formula unit, a little higher than the Ca solubility in MgSiO3 perovskite.
But the (Mg,Fe) content in CaSiO3 perovskite increases significantly to nearly 0.18 cations per formula unit with increasing iron content at 30 GPa (A-06), and it further increases to nearly 0.34 cations at 78 GPa (C-05). The interesting point is that nearly the same amounts of Mg and Fe enter into CaSiO3 perovskite in the iron-bearing system. The above results indicate that both (Mg,Fe)SiO3 and CaSiO3 perovskites can coexist in the deeper part of the lower mantle, considering the small amount of CaSiO3 perovskite compared to the large amount of (Mg,Fe) SiO3 perovskite in the lower mantle.
4. Summary Ultramicrotomy for preparing ATEM foils down to a thickness of about 30 nm was successfully applied to ultrahigh pressure materials, although some area of the foils were lost during the preparation. Experimentally obtained k-factors using standard materials are indispensable for the calculation of reliable chemical compositions for the ATEM foils. Preliminary results on (Mg,Fe)SiO3 and CaSiO3 perovskites indicate that Mg and Fe solubilities in CaSiO3 perovskite dramatically increase in the Fe-bearing system and/or with increasing pressure (up to about 0.34 cations of (Mg+Fe) per foumula unit at around 80 GPa), while the Ca solubility in (Mg,Fe)SiO3 perovskite remains very limited.
Acknowledgements We thank E. Ito for the permission to use samples synthesized at ISEI, Okayama University and T. Kikegawa for his help at Photon Factory (proposal no. 2001G044). We also thank Falko Langenhorst and Denis Andrault for their constructive comments. This study is supported by the Grant-in-Aid for Research (#12304030 and #1212621) from the Ministry of Education, Culture, Sports, Science and Technology. References Cliff, G., Lorimer, G.W., 1975. The quantitative analysis of thin specimens. J. Microsc. 129, 233–247. Fujino, K., Miyajima, N., Yagi, T., Kondo, T., Funamori, N., 1998. Analytical electron microscopy of the garnet-perovskite
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221 transformation in a laser-heated diamond anvil cell. In: Manghnami, M., Yagi, T. (Eds.), Properties of Earth and Planetary Materials at High Pressure and Temperature. Am. Geophys. Union, Geophys. Monogr. 101, Washington, DC, pp. 409–417. Guyot, F., Madon, M., Peyronneau, J., Poirier, J.P., 1988. X-ray microanalysis of high-pressure/high-temperature phases synthesized from natural olivine in a diamond–anvil cell. Earth Planet. Sci. Lett. 90, 52–64. Irifune, T., Miyashita, M., Inoue, T., 2000. High-pressure phase transformation in CaMgSi2 O6 and implications for origin of ultra-deep diamond inclusions. Geophys. Res. Lett. 27, 3541– 3544. Kesson, S.E., Fitz Gerald, J.D., Shelley, J.M.G., Withers, R.L., 1995. Phase relations, structure and crystal chemistry of some
221
aluminous silicate perovskites. Earth Planet. Sci. Lett. 134, 187–201. Madon, M., Guyot, F., Peyronneau, J., Poirier, J.P., 1989. Electron microscopy of high-pressure phases synthesized from natural olivine in diamond anvil cell. Phys. Chem. Miner. 16, 320–330. Mao, H.K., Bell, P.M., Shaner, J.W., Steinberg, D.J., 1978. Specific volume measurements of Cu, Mo, Pd and Ag and calibration of the ruby R1 fluorescence pressure gauge from 0.06 to 1 Mbar. J. Appl. Phys. 49, 3276–3283. Miyajima, N., Fujino, K., Funamori, N., Kondo, T., Yagi, T., 1999. Garnet-perovskite transformation under conditions of the Earth’s lower mantle: an analytical transmission electron microscopy study. Phys. Earth Planet. Int. 116, 117–131. Williams, D.B., Carter, C.B., 1996. Transmission Electron Microscopy. Plenum Press, New York.
Approach to the mineralogy of the lower mantle by a combined method of a laser-heated diamond anvil cell experiment and analytical electron microscopy Kiyoshi Fujino a,∗ , Yohei Sasaki a , Toyohisa Komori a , Hisayuki Ogawa a , Nobuyoshi Miyajima b , Nagayoshi Sata b , Takehiko Yagi b a
Division of Earth and Planetary Sciences, Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo 060-0810, Japan b Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan Received 27 February 2003; received in revised form 25 August 2003; accepted 17 November 2003
Abstract A combined method of a laser-heated diamond anvil cell (DAC) experiment and analytical electron microscopy to study the mineralogy of the lower mantle was explored. Particularly, the use of ultramicrotomy to make ultrathin foils for analytical transmission electron microscopy (ATEM) of the ultrahigh pressure materials was explored with the aim of obtaining the exact chemical compositions of those materials with submicron size. Ultrathin foils with a thickness down to about 30 nm were obtained for ultrahigh pressure materials synthesized by a laser-heated DAC, although some areas of the foils were lost. Reliable chemical compositions were obtained with these foils using the experimentally obtained k-factors. The preliminary results on the mutual solubilities between (Mg,Fe)SiO3 and CaSiO3 perovskites with this method indicate that the (Mg,Fe) solubility in CaSiO3 perovskite increases dramatically (up to 0.34 cations per foumula unit at 78 GPa and 1900 ◦ C) with increasing the bulk iron content and/or pressure, while the Ca solubility in coexisting (Mg,Fe)SiO3 perovskite remains very limited (0.01–0.02 cations per formula unit) under the same conditions. © 2004 Elsevier B.V. All rights reserved. Keywords: Diamond anvil cell; Analytical transmission electron microscopy; Ultramicrotomy; Lower mantle
1. Introduction Laser-heated diamond anvil cell (DAC) experiments are indispensable for the study of the mineralogy of the lower mantle. However, the materials synthesized by laser-heated DAC experiments are very small in amount and very fine in grain size, submicron in most cases. For these reasons, it is very difficult to determine both structures and compositions of respective ∗ Corresponding author. Tel.: +81-11-706-2728; fax: +81-11-706-4650. E-mail address: [email protected] (K. Fujino).
product phases, even if we could identify the existing phases by synchrotron X-ray radiation. Analytical transmission electron microscopy (ATEM) provides us with both structures and compositions of respective phases simultaneously at the nm scale. Therefore, a combined method of a laser-heated DAC experiment and ATEM is a very powerful tool for the study of the mineralogy of the lower mantle. With such a perspective, many studies have been performed (Guyot et al., 1988; Madon et al., 1989; Kesson et al., 1995; Fujino et al., 1998; Miyajima et al., 1999). We have recently installed a newly designed YLF laser-heated DAC system in our laboratory and are now further exploring
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2003.11.013
216
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
the advantages of this combined method. The details of the DAC system will be presented elsewhere, and in this paper we mainly report on the use of ATEM in studying ultrahigh pressure materials. Although ATEM is very powerful for the study of ultrahigh pressure materials, there are several problems that have to be resolved. The first is that the specimens of ultrahigh pressure phases are in general very sensitive to electron beam irradiation and easily become amorphous. In particular, silicate perovskites are very sensitive. We are trying to overcome this problem by the use of a high-sensitivity and high-resolution TV camera system for TEM. With this system, we are expecting to observe TEM images and also electron diffraction patterns of the ultrahigh pressure phases under a very weak electron beam. This is now under way and the results will be presented elsewhere. The second problem is that in widely used ion-thinning to make ATEM specimens, elements are selectively removed from the surface area of the specimens during ion thinning in some cases, particularly in ultrahigh pressure materials, and analyzed compositions are different from the real values (Fujino et al., 1998). To overcome this problem, we have explored the use of ultramicrotomy for ultrahigh pressure materials because it leaves the chemistry unchanged. In the following, we mainly report the details of using ultramicrotomy on ultrahigh pressure materials, and also report some preliminary results on the mineralogy of the lower mantle.
2. Experimental 2.1. Ultrahigh pressure materials examined Materials used for the investigation of ultramicrotomy are Ca(Si,Ti)O3 , (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites and the details are summarized in Table 1 together with the high pressure synthesis conditions. Runs 1803 and 1814 were chosen from samples of high pressure and high temperature experiments performed on the system CaSiO3 -CaTiO3 at 5.7–13.2 GPa and 1500 ◦ C using a multianvil cell apparatus at the Institute for the Study of the Earth’s Interior, Okayama University. Runs A-13, B-02, A-06, A-14 and C-05 are selected samples from the system CaMgSi2 O6 (Di)-CaFeSi2 O6 (Hd) synthesized at 25–80 GPa and 1700–2000 ◦ C by laser-heated diamond anvil cell apparatus at the Institute for Solid State Physics, University of Tokyo and at Hokkaido University. In the DAC experiments, diamonds with 450 and 300 m culets were used for generating pressures of 25–30 and 70–80 GPa, respectively. A powder of a synthetic pyroxene solid solution starting material was loaded into the hole of a stainless steel or rhenium gasket and was sandwiched by NaCl in the diamond cell. Platinum powder was mixed with the CaMgSi2 O6 sample in order to absorb laser radiation for heating. Pressure was measured by the ruby fluorescence technique (Mao et al., 1978). Typical pressure differences between the center and the edge
Table 1 Synthesis conditions of the materials examined Pressure (GPa)
Temperature (◦ C)
Duration (min)
Product phases
CaSiO3 –CaTiO3 1803 CaSi0.8 Ti0.2 O3 1814 CaSi0.8 Ti0.2 O3
8.3 11.3
1500 1500
30 30
Pv + Wal Pv + Tit + Lar
CaMgSi2 O6 (Di)–CaFeSi2 O6 (Hd) A-13 Di100 Hd0 B-02 Di75 Hd25 A-06 Di50 Hd50 A-14 Di50 Hd50 C-05 Di50 Hd50
29.7 31.5 29.5 26.2 78.0
1930 1800 1800 1820 1900
10 10 10 10 10
Mg-Pva + Ca-Pva Mg-Pv + Mw + St + Ca-Pva Mg-Pv + Mw + St + Ca-Pva Mg-Pv + Mw + St + Ca-Pva Mwa + SiO2 b + Ca-Pva
Run no.
Starting material
Pv: Ca(Si,Ti)O3 perovskite, Wal: walstromite-structured CaSiO3 , Tit: titanite solid solution, Lar: larnite (-Ca2 SiO4 ), Mg-Pv: (Mg,Fe)SiO3 perovskite, Ca-Pv: CaSiO3 perovskite, Mw: magnesiowustite, St: stishovite. a Identified by ATEM. b Unknown phase of SiO . 2
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
of the sample are about 5 GPa. Pressures after heating are listed in Table 1. The sample was heated from both sides by a YAG laser beam focused to 70–80 m in multimode (A-13, A-06, A-14 and C-05) or a YLF laser beam focused to about 30 m in TEM01 mode (B-02) and temperature of the sample was measured spectroradiometrically. The experimental details and comprehensive results of high pressure phase relations and structure variations of perovskite phases in both systems will be presented in separate papers. 2.2. Ultramicrotomy Ultramicrotomy was originally developed in biology to section biological materials into ultrathin foils for TEM observation, and is now applied to high molecular compounds and metallic materials using a diamond knife (Williams and Carter, 1996). In ultramicrotomy the sample, embedded in polymeric resin, is moved across a knife blade while being advanced stepwise (Fig. 1). In our case, the multianvil cell samples were cut into cubes with edges of 100 m and laser-heated DAC samples were thin disks of 100 m diameter and 20–30 m in thickness. The samples were embedded in polymeric resin Quetol 812 (Nisshin EM Co. Ltd.) using micro-molds and hardened at 70 ◦ C for 72 h. Samples were sectioned by an ultramicrotome (Reichert-Nissen ultracuts N) at the Electron Microscope Center of the Faculty of Agriculture, Hokkaido University. A resin block including a sample was cured and trimmed by a glass
Fig. 1. Schematic drawing of an ultramicrotome.
217
knife so that the cutting surface becomes 100–300 m across and was then sectioned by a diamond knife of 45◦ . The sectioned thin flakes floated off onto water in a trough (Fig. 1), and the flakes which contain the central part of the heated sample were collected on a grid with a carbon film. The sectioned flakes were additionally carbon-coated. 2.3. TEM observations and quantitative chemical analysis TEM observations and quantitative chemical analyses were performed using an analytical electron microscope (JEOL, JEM-2010) at Hokkaido University, operated at 200 kV. Quantitative chemical analyses were made with a NORAN Instruments/Voyager II energy-dispersive analytical system attached to the JEM 2010 electron microscope. Silicate perovskites are very sensitive to electron beam irradiation, and in measuring the characteristic X-ray intensities, elements are selectively removed from the specimens when the electron beam density is stronger than some critical value (Fujino et al., 1998). Therefore, TEM observations and chemical analyses were made with a low electron beam current of 4 pA/cm2 on the small fluorescent screen, and the characteristic X-ray intensities were measured with an electron beam of 100–150 nm diameter and a counting time of 60–120 s. Characteristic X-ray intensities were measured in the central region of the thin foils so that the X-ray Intensities were obtained from the heated area of the sample. The k-factors (Cliff and Lorimer, 1975) used for the calculation of quantitative chemical analyses were experimentally obtained using the synthetic standard materials CaSiO3 (wollastonite), CaTiO3 (perovskite), Mg2 SiO4 (forsterite) and Fe2 SiO4 (fayalite). They were crushed in a pestle and mortar in petroleum ether and were placed on a grid with a holey carbon film. Kij -factors were experimentally obtained from the measured intensity ratios Ii /Ij and known concentration ratios Ci /Cj of the paired elements i and j in the standard materials (kij = (Ci /Cj )/(Ii /Ij )). The kACa -factors (where A = Si, Ti, Ca) used for Ca(Si,Ti)O3 perovskite were obtained as functions of total X-ray counts of Ca and the kASi -factors (where A = Mg, Fe, Ca, Si) used for (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites were obtained as functions of total X-ray counts of Si.
218
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
The experimentally obtained k-factors for our ATEM operating at 200 kV with an ultra-thin window and take-off angle of 25 ◦ were expressed as the following regression equations. For Ca(Si,Ti)O3 perovskite kTiCa = 0.540(5) + 9(5) × 10−7 ICa
with sufficient quality). Numbers in parentheses of the coefficients of the above equations denote one estimated standard deviation (E.S.D.) of the least significant digit. In the above equations, kSiCa is not equal to 1/kCaSi because the respective k-factors were obtained from the different data sets.
kSiCa = 0.423(16) + 1.2(17) × 10−6 ICa For (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites kCaSi = 2.45(18) − 2.6(20) × 10−5 ISi kMgSi = 1.14(2) − 5(20) × 10−7 ISi kFeSi = 1.80(7) − 9.9(4) × 10
−5
ISi
where ICa and ISi are the total counts of characteristic X-rays of Ca and Si, respectively, of the standard materials, and roughly correspond to the foil’s thickness. Assignment of k-factors by those intensities includes the correction for X-ray absorption (although standard materials MgSiO3 and FeSiO3 would have been better for this purpose, we could not get those materials
3. Results and discussion 3.1. Ultramicrotomy of ultrahigh pressure materials With our ultramicrotome, aggregates of submicron ultrahigh pressure materials could be sectioned into ultrathin foils down to 30 nm thickness. However, this was actually the distance over which the diamond knife was advanced stepwise, and the actual thickness of the foils may be slightly larger, considering that the resin block that contained the sample was not sectioned for every stroke of the diamond knife. Fig. 2 shows the electron micrographs of sectioned foils obtained using the ultramicrotome. Some areas of the
Fig. 2. Electron micrographs of foils prepared by the ultramicrotome. Foils are collected on a grid with a carbon film. (a) Foil of a mutianvil cell sample (sample no. 1814), (b) foil of a laser-heated DAC sample (sample no. A-14).
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
sectioned foils were often lost. With the multianvil cell samples (Fig. 2(a), No. 1814) the sectioned aggregates are fairly concentrated and coherent, while in the case of the DAC samples (Fig. 2(b), No. A-14) aggregates are rather dispersed. One of the reasons for the dispersion of the DAC samples is that we used NaCl as a pressure medium and the NaCl surrounding the specimen in the sectioned foil dissolved into water in the trough. This problem can be overcome in future by using other materials as pressure medium, which are not soluble in water. As seen in Fig. 2, the grain sizes of the multianvil cell samples are about 100–500 nm, while those of the DAC samples are much smaller, about 20–100 nm, and both are far below the grain size that can be measured by EPMA. There seem to be some cracks or deformation textures in the multianvil cell samples. Therefore, use of both ion-thinning for structure and texture observations and ultramicrotomy for chemical analyses on the same samples, will be desirable for the preparation of ATEM foils. 3.2. Quantitative chemical analyses of perovskites For the specimens prepared by ultramicrotomy, quantitative chemical analyses were performed by measuring the characteristic X-ray intensities of the product phases and calculating the compositions using the experimentally obtained k-factors. Analyses were quantified in terms of oxides and all iron was treated as Fe2+ . The equations listed in Section 2.3. were used to derive the k-factors appropriate for the unknown
219
samples based on the total X-ray counts of Ca or Si of the unknown samples. The estimated E.S.D.s of the obtained k-factors, depending on the measured X-ray intensities of Ca or Si, are σ(kTiCa ) =∼ 0.005, σ(kSiCa ) =∼ 0.017 (ICa =∼ 2600), and σ(kCaSi ) = 0.19–0.21, σ(kMgSi ) = 0.02, σ(kFeSi ) = 0.15–0.27 (ISi = 3000–6000). Relatively large σ(kCaSi ) and σ(kFeSi ) values seem to originate from the scatter of the measured X-ray intensity ratios of the paired elements in standard materials. Rough estimates of a relative error, σ(Ci /Cj )/(Ci /Cj ), based on σ(kij )/kij , σ(Ii )/Ii , and σ(Ij )/Ij (Williams and Carter, 1996) were 3–5% for Ca(Si,Ti)O3 perovskite (i = Si, Ti, j = Ca) and 3–12% for (Mg,Fe)SiO3 and (Ca,Mg,Fe)SiO3 perovskites (i = Mg, Fe, Ca, j = Si). Among the experimentally obtained k-factors, those for Ca and Fe deviated significantly from the theoretical ones (around 1) installed in the program. Although k-factors are not universal but are sensitive to the respective analytical system, so far it is not clear why the k-factors of our system deviate so much from the theoretical values. The calculated compositions, using these k-factors, of the perovskite phases in some of the samples of Table 1 are presented in Table 2. These are the averages of several values measured on the same sample, excluding ones that deviated strongly from the stoichiometry of perovskite. The analyzed cation numbers based on three oxygens per formula unit are all close to the ideal chemical formula of perovskite, and the obtained composition of Ca(Si,Ti)O3 perovskite (No. 1803) is nearly the same as that obtained by EPMA (25 mol% of CaSiO3 ). Therefore, ultramicrotomy,
Table 2 Compositions analyzed by ATEM Run no. Starting composition Phase Pressure (GPa)
1803 CaSi0.8 Ti0.2 O3 Pv 8.3
A-13 Di100 Hd0 Mg-Pv 29.7
B-02 Di75 Hd25 Mg-Pv 31.5
A-13 Di100 Hd0 Ca-Pv 29.7
A-06 Di50 Hd50 Ca-Pv 29.5
C-05 Di50 Hd50 Ca-Pv 78.0
Cation number (O = 3) Mg Si Ca Ti Fe
– 0.244 0.998 0.757 –
0.958 1.021 0.009 – –
0.828 1.038 0.003 – 0.094
0.036 1.059 0.847 – –
0.092 1.040 0.741 – 0.087
0.182 0.986 0.689 – 0.158
1.999
1.988
1.963
1.942
1.960
2.015
Total
Pv: Ca(Si,Ti)O3 perovskite, Mg-Pv: (Mg,Fe)SiO3 perovskite, Ca-Pv: CaSiO3 perovskite.
220
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221
combined with the experimentally obtained k-factors, gives us reliable chemical compositions. 3.3. Preliminary results on the solubility relations between (Mg,Fe)SiO3 and CaSiO3 perovskites It is important to know the mutual solubilities between (Mg,Fe)SiO3 perovskite and CaSiO3 perovskite under lower mantle conditions to understand the mineral compositions and dynamics of the lower mantle. From high pressure phase transformation experiments on diopside (CaMgSi2 O6 ), Irifune et al. (2000) reported very limited mutual solubilities between MgSiO3 and CaSiO3 perovskites; the Ca solubility in MgSiO3 perovskite was reported to be less than about 0.02 cations per formula unit based on three oxygens and the Mg solubility in CaSiO3 perovskite was less than about 0.04 cations at 25–27 GPa and 1000–1900 ◦ C. However, the solubilities of those cations at higher pressures or in the presence of iron containing phases has been unclear. In our recent laser-heated DAC experiments on the system CaMgSi2 O6 (Di)–CaFeSi2 O6 (Hd) at 25 to 80 GPa and 1700–2000 ◦ C with synchrotron X-ray radiation identification of the product phases, orthorhombic (Mg,Fe)SiO3 perovskite and cubic CaSiO3 perovskite coexisted in this system for compositions ranging from Di100 Hd0 to around Di45 Hd55 , although the exact structural states of both perovskites at around 70–80 GPa have not yet been ascertained. In Table 1, the product phases that were identified by synchrotron X-ray radiation and ATEM are summarized. Although we have not yet identified (Mg,Fe)SiO3 perovskite in C-05, analyses of the other samples at similar conditions suggest that (Mg,Fe)SiO3 perovskite should also be present in C-05. Table 2 shows the compositions of (Mg,Fe)SiO3 and CaSiO3 perovskites of selected samples analyzed by ATEM although we have not yet fully succeeded in obtaining the compositions of both perovskite phases in some samples. For the iron-free sample (A-13) at about 30 GPa, the Ca content of MgSiO3 perovskite is around 0.01 cations per formula unit based on three oxygens, and it remained nearly the same with increasing iron content (B-02) and/or pressure. On the other hand, the Mg content of CaSiO3 perovskite in A-13 is around 0.04 cations per formula unit, a little higher than the Ca solubility in MgSiO3 perovskite.
But the (Mg,Fe) content in CaSiO3 perovskite increases significantly to nearly 0.18 cations per formula unit with increasing iron content at 30 GPa (A-06), and it further increases to nearly 0.34 cations at 78 GPa (C-05). The interesting point is that nearly the same amounts of Mg and Fe enter into CaSiO3 perovskite in the iron-bearing system. The above results indicate that both (Mg,Fe)SiO3 and CaSiO3 perovskites can coexist in the deeper part of the lower mantle, considering the small amount of CaSiO3 perovskite compared to the large amount of (Mg,Fe) SiO3 perovskite in the lower mantle.
4. Summary Ultramicrotomy for preparing ATEM foils down to a thickness of about 30 nm was successfully applied to ultrahigh pressure materials, although some area of the foils were lost during the preparation. Experimentally obtained k-factors using standard materials are indispensable for the calculation of reliable chemical compositions for the ATEM foils. Preliminary results on (Mg,Fe)SiO3 and CaSiO3 perovskites indicate that Mg and Fe solubilities in CaSiO3 perovskite dramatically increase in the Fe-bearing system and/or with increasing pressure (up to about 0.34 cations of (Mg+Fe) per foumula unit at around 80 GPa), while the Ca solubility in (Mg,Fe)SiO3 perovskite remains very limited.
Acknowledgements We thank E. Ito for the permission to use samples synthesized at ISEI, Okayama University and T. Kikegawa for his help at Photon Factory (proposal no. 2001G044). We also thank Falko Langenhorst and Denis Andrault for their constructive comments. This study is supported by the Grant-in-Aid for Research (#12304030 and #1212621) from the Ministry of Education, Culture, Sports, Science and Technology. References Cliff, G., Lorimer, G.W., 1975. The quantitative analysis of thin specimens. J. Microsc. 129, 233–247. Fujino, K., Miyajima, N., Yagi, T., Kondo, T., Funamori, N., 1998. Analytical electron microscopy of the garnet-perovskite
K. Fujino et al. / Physics of the Earth and Planetary Interiors 143–144 (2004) 215–221 transformation in a laser-heated diamond anvil cell. In: Manghnami, M., Yagi, T. (Eds.), Properties of Earth and Planetary Materials at High Pressure and Temperature. Am. Geophys. Union, Geophys. Monogr. 101, Washington, DC, pp. 409–417. Guyot, F., Madon, M., Peyronneau, J., Poirier, J.P., 1988. X-ray microanalysis of high-pressure/high-temperature phases synthesized from natural olivine in a diamond–anvil cell. Earth Planet. Sci. Lett. 90, 52–64. Irifune, T., Miyashita, M., Inoue, T., 2000. High-pressure phase transformation in CaMgSi2 O6 and implications for origin of ultra-deep diamond inclusions. Geophys. Res. Lett. 27, 3541– 3544. Kesson, S.E., Fitz Gerald, J.D., Shelley, J.M.G., Withers, R.L., 1995. Phase relations, structure and crystal chemistry of some
221
aluminous silicate perovskites. Earth Planet. Sci. Lett. 134, 187–201. Madon, M., Guyot, F., Peyronneau, J., Poirier, J.P., 1989. Electron microscopy of high-pressure phases synthesized from natural olivine in diamond anvil cell. Phys. Chem. Miner. 16, 320–330. Mao, H.K., Bell, P.M., Shaner, J.W., Steinberg, D.J., 1978. Specific volume measurements of Cu, Mo, Pd and Ag and calibration of the ruby R1 fluorescence pressure gauge from 0.06 to 1 Mbar. J. Appl. Phys. 49, 3276–3283. Miyajima, N., Fujino, K., Funamori, N., Kondo, T., Yagi, T., 1999. Garnet-perovskite transformation under conditions of the Earth’s lower mantle: an analytical transmission electron microscopy study. Phys. Earth Planet. Int. 116, 117–131. Williams, D.B., Carter, C.B., 1996. Transmission Electron Microscopy. Plenum Press, New York.