| Citation: | Tomohiro Ohuchi, Takaaki Kawazoe, Norimasa Nishiyama, Yu Nishihara, Tetsuo Irifune. Technical Development of Simple Shear Deformation Experiments Using a Deformation-DIA Apparatus. Journal of Earth Science, 2010, 21(5): 523-531. doi: 10.1007/s12583-010-0110-4
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Plastic deformation of mantle minerals plays an important role in controlling the dynamics in the earth's mantle. The lattice-preferred orientation (LPO) of mantle minerals developed by dislocation creep is known to be the cause of the anisotropic elastic properties of mantle materials. If a relationship between the LPO of minerals and the deformation geometry is known from laboratory studies, then one can infer deformation conditions from observed deformation microstructures and/or seismic anisotropy of rocks (e.g., Jung et al., 2006). High-pressure simple-shear deformation techniques, which were developed by Zhang and Karato (1995) and Karato and Rubie (1997), have been used for the evaluation of the LPO of minerals because the shear direction and shear plane are unique in the simple shear design. Thus, simple shear deformation experiments have been conducted to investigate the relationship between the LPO of minerals and deformation conditions (such as pressure, temperature, and dissolved water content in minerals).
Recently, Raterron et al. (2007) conducted a series of deformation experiments on forsterite single crystals at pressures of 2.1–7.5 GPa and temperatures of 1 373–1 677 K using a deformation-DIA apparatus and reported that the dominant slip direction changes from b=[100] to [001] at > 7 GPa (see also Raterron et al., 2009). Their results suggest the possibility of a pressure-induced fabric transition of olivine. However, the influence of pressure on the LPO of olivine and other minerals has been evaluated at pressures of < 4 GPa, which is the range of generated pressures using a Griggs apparatus. To evaluate the possibility of a pressure-induced fabric transition of olivine, Couvy et al. (2004) performed simple-shear deformation experiments on olivine (in the style of a stress-relaxation test, Karato and Rubie, 1997) at 11 GPa and 1 673 K using a multianvil apparatus. It has been known, however, that the exact values of stress at which much of LPO develops are unknown in stress-relaxation tests (Karato et al., 2008), and strain rates cannot be controlled in these types of experiments. Although deformation experiments in the uniaxial geometry have been conducted at high pressures (e.g., 9.6 GPa, Li et al., 2006) under the strain-rate-controlled conditions using a deformation-DIA apparatus, the maximum pressure for the simple-shear deformation experiments using a deformation-DIA apparatus has been limited to 3 GPa (Walte et al., 2007). Thus, the pressure-induced fabric transition of olivine and other minerals has not been fully evaluated.
It has been reported that water has significant effects on the LPO of olivine, which is the dominant mineral in the earth's upper mantle (e.g., Jung and Karato, 2001a). The fabric transition is controlled by the relative strength of slip systems, and the relative strength of slip systems of olivine is changed by the presence of a small amount of dissolved water (Mackwell et al., 1985). The effects of water on the LPO of olivine have been evaluated at pressures of ≤2 GPa. It is known that the amount of dissolved water in olivine and other mantle minerals increases with pressure (e.g., Kohlstedt et al., 1996). Thus, the effects of water on the LPO of minerals are expected to be important under high-pressure conditions.
In order to explore the pressure-induced fabric transition of minerals and the effect of water on the LPO of minerals at high pressures, we developed a new cell assembly for the multi-anvil assembly 6-6 (MA 6-6) system combined with a deformation-DIA apparatus (Nishiyama et al., 2008). In this article, we report the details on the performance of the cell assembly from pressure generation tests and deformation experiments.
High-pressure generation tests and deformation experiments were conducted using a deformation-DIA type cubic-anvil apparatus at Ehime University. The design of the apparatus is based on Wang et al. (2003). The MA 6-6 system consists of six second-stage tungsten carbide anvils and the anvil guide (Nishiyama et al., 2008). The MA 6-6 system with the truncated edge length (TEL) of the second-stage tungsten carbide anvils (Fujilloy-TF05, Fuji Die Co. Ltd.) of 5 mm was adopted for these experiments. A semi-sintered cobaltdoped magnesia (Mg, Co)O cube with an edge length of 7 mm was used as the pressure medium. Preformed gaskets were not used in the experiments.
In order to determine the relationship between generated pressure and applied press load, the phase transitions of Bi (Ⅰ–Ⅱ transition at 2.55 GPa and Ⅲ–Ⅳ at 7.7 GPa) were used as pressure fixed points. Resistance of a thin strip of Bi (2×0.2 mm2 and 0.05 mm in thickness) was measured at room temperature using a two-wire method through the top and bottom anvils to detect phase transitions.
The generated pressures at high temperatures were also evaluated by a quench method using phase transitions in Fe2SiO4 (α-γ transition, Yagi et al., 1987), (Mg, Fe)2SiO4 (α-γ transition, Frost, 2003) and SiO2 (quartz-coesite, coesite-stishovite transitions: Zhang et al., 1996; Bose and Ganguly, 1995). Fine-grained powder of Fe2SiO4 glass, (Mg, Fe)2SiO4 glass, and quartz was packed into inner Re-capsules separately, and then the Re capsules were placed into a Pt capsule. The cell assembly used for the sintering of Fe2SiO4, (Mg, Fe)2SiO4 and SiO2 powder is identical to that for the deformation experiments, except for the inside of the Pt capsule (Fig. 1). The entire cell assembly was compressed to a desired load (≤1.6 MN) isotropically at a rate of 0.5 MN/h, and then the temperature was raised to 1 300–1 673 K (1 673 K, most of sintering experiments; 1 300 and 1 573 K, sintering of SiO2 powder at the press load of 1.6 and 0.3 MN, respectively). The samples were annealed at 1 673 K for 30–60 min and then quenched. The run products were recovered from the cell assembly and cut with a low-speed saw. The phases in the recovered samples were identified by Raman spectroscopy. Chemical compositions of olivine and ringwoodite were measured using a JEOL JSM-7000F field emission scanning electron microscope (FE-SEM) equipped with an Oxford Inca EDX system at Ehime University under the operating conditions of 15-kV accelerating voltage and 0.5-nA probe current.
Shear deformation experiments were conducted at pressures of P=5.2–7.6 GPa (0.6–1.1 MN), temperature T=1 473–1 573 K, and shear strain rates of 4.0×10-5–7.5×10-5 s-1 under water-saturated conditions (Table 1). The cross-sectional view of the cell assembly used for deformation experiments is shown in Fig. 1. A graphite heater is located at the inner bore of a tubular LaCrO3 thermal insulator. Copper and molybdenum electrodes, hard alumina pistons, and machinable alumina rods were placed along the direction of the axial differential stress (σ1–σ3). The machinable alumina rods were made from the Norton crushable alumina © having ~50% of porosity. The use of the machinable alumina rods with 1.45-mm length (corresponding to 21% of the edge length of the semi-sintered cobalt-doped magnesia cube) resulted in very little deformation of the samples during the compression stage. A sectioned sample of olivine aggregates or olivine+4 wt.% MORB glass aggregates having a thickness of 230–270 μm was placed into a platinum capsule and then sandwiched between two alumina or tungsten pistons. The samples were synthesized from fine-grained powder of San Carlos olivine (Fo90) and gel powder having the chemical composition of MORB (SiO2: 50.82 wt.%; Al2O3: 16.08 wt.%; FeO: 7.68 wt.%; MgO: 10.49 wt.%; CaO: 13.05 wt.%, and Na2O: 1.87 wt.%) at 2 GPa and 1 373 K (T for the synthesis of olivine aggregates) or 1 673 K (T for the synthesis of olivine+4 wt.% MORB glass aggregates) using a Kawai-type multi-anvil highpressure apparatus (Orange 3000) at Ehime University. Deformation experiments of olivine+4 wt.% MORB samples were conducted using tungsten pistons so as to avoid the chemical reaction between alumina pistons and the samples. Even though the alumina pistons were coated with platinum (thickness of a few hundred nanometer), the chemical reaction between alumina pistons and the olivine+4wt.% MORB samples could not have been prevented. The platinum capsule was surrounded by a hexagonal boron nitride (hBN) or MgO sleeve. The entire assembly was dried at 383 K for ~12 h in an oven before each experiment. The desired amount of distilled water (~3 wt.% of the sample), which would be enough for achieving watersaturated conditions, was added to the inside of the platinum capsule using a microsyringe just before performing the high-pressure experiments. No water was added to the samples for the experiments under dry conditions. A platinum lid placed at the top of the platinum capsule forms a watertight seal with the open capsule during cold pressurization(e.g., Ayers et al., 1992).
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Pressure was first raised to the desired value at a rate of 0.5 MN/h, and then temperature was increased at a rate of ~25 K/min. Temperature was monitored by a W97Re3-W75Re25 thermocouple placed along one of the diagonal directions of the cubic (Mg, Co)O pressure media. The thermocouple wires were in direct contact with the graphite heater so that the hot junction of the thermocouple was maintained via the graphite heater. Although the ends of the thermocouple wires were in contact with the platinum capsule, the platinum capsule did not work as the hot junction at high temperatures probably due to loose connection between the thermocouple wires and the platinum capsule. The temperature gradient between the central part and the edge of the sample, which was measured with two thermocouples at 5.2 GPa, was less than 50 K at temperatures of ≤1 673 K. After the temperature reached the desired value, the sample was annealed for 1 h. Then the upper and lower anvils were advanced at a constant rate by operating the deformation rams. Shear strain was measured by the rotation of a nickel strain-marker, which was initially placed perpendicularly to the shear direction. The strain rate was calculated under the assumption that the strainmaker rotated at a constant rate during the deformation experiment. The uncertainty in the strain rate, which resulted from the shape of the strain-marker, was within 15%. In order to demonstrate that no sample deformation occurred before the operation of the deformation rams, three experiments were conducted without the sample deformation process.
The differential stress on the sample was measured with the dislocation density piezometer. We used the dislocation density of olivine to infer the stress magnitude using an SEM technique, where the total length of the dislocation lines per unit volume is measured (Karato and Jung, 2003; Jung and Karato, 2001b). The dislocation density was measured in each olivine polycrystalline sample after oxidation at 1 173 K for 1 h. The oxidized dislocations were observed by the back-scattered electron (BSE) images using an FE-SEM. The detailed processes of the dislocation density measurements are shown in Ohuchi et al. (2010). The uncertainties of the stress estimation are about ±10%–15% from the calibrations and the heterogeneity of the dislocations in a sample (Jung and Karato, 2001b). The relationship between applied stress and dislocation density can be described empirically as follows
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(1) |
where ρ is the density of free dislocations; β and m are constants; and σ is the axial differential stress (=σ1–σ3) (Kohlstedt et al., 1976). Substituting the obtained values of ρ and the reported values of the constants (log10β=9.21±0.16 and m=1.39±0.07 in the case of σ in MPa and ρ in m-2: Ohuchi et al., ) into equation (1), we calculated the values of σ.
The run products were recovered from the capsule assembly and cut with a low-speed saw. These were then impregnated with epoxy under a vacuum and polished using 1.0-μm alumina powder followed by 0.06-μm colloidal silica suspension. The BSE images of the deformed samples were observed with an FE-SEM.
To determine the water content in the recovered samples, the unpolarized infrared absorption spectra of the polycrystalline samples were obtained from the doubly polished sections of the samples that were 90–150 μm thick. All of the measurements were carried out in air by putting the sections on a BaF plate and using a PerkinElmer Spectrum One Fourier- transform infrared spectrometer (FTIR). An aperture size of 50×50 μm2 was used for all of the measurements. The infrared spectra were taken at 4–5 points in each sample. The bulk water contents in the samples were determined by integrating the infrared absorption spectra from 3 100 to 3 700 cm-1 on the basis of the extinction coefficient calibration of Paterson (1982).
Figure 2 shows the results of the pressure calibration experiments at room temperature (resistance measurements of Bi) and T=1 673 K (phase transitions of Fe2SiO4, (Mg, Fe)2SiO4 and SiO2). Sharp decreases in resistance of Bi were observed at 0.265 and 1.328 MN, which correspond to the Ⅰ–Ⅱ and Ⅲ–Ⅴ transitions, respectively. A phase transition of fayalite to ringwoodite was observed at the press load of 0.75–0.90 MN. Quartz-coesite transition was observed at the press load of 0.3–0.6 MN and 1 573–1 673 K. Coexistence of coesite and stishovite was observed at the press load of 1.6 MN and 1 300 K, showing that the sample pressure was ~8.8 GPa at 1 300 K (Zhang et al., 1996). The Fe/(Mg+Fe) ratios (in mol) in coexisting olivine and ringwoodite in the (Mg, Fe)2SiO4 sample sintered at the press load of 1.1 MN and 1 673 K were obtained to be ~0.72 and ~0.94, respectively. Based on the olivine-ringwoodite phase diagram (Frost, 2003), the sample pressure at the press load of 1.1 MN and 1 673 K is expected to be 7.7±0.4 GPa. The generated pressures at T=1 300–1 673 K are higher than those at room temperature (the difference is within 0.7 GPa) at the same press load.
Figure 3a shows a representative relationship between the displacement of the anvils and time in a deformation run conducted at 0.6 MN (=5.2 GPa) and 1 573 K. The time dependency of the change of distance between the upper and lower guide blocks is also shown in Fig. 3a. The upper and lower anvils were advanced to the desired positions with a constant velocity, being associated with the retreat of the four side anvils and the guide blocks. The applied load was kept constant during deformation runs (Fig. 3b).
Figure 4a shows a BSE image of olivine aggregates without the deformation process (i.e., without the operation of deformation rams). No rotation (or very small rotation) of the strain marker is observed in the undeformed samples (Fig. 4a, Table 1), showing that shear deformation of samples hardly occurred before the deformation process (shear strain γ=0–0.07: see Table 1). This result demonstrates that the simpleshear strain of the deformed samples can be appropriately obtained from the rotation of the strain marker. Shortening of the samples (12%–29% of uniaxial strain) was observed during the compression process (Table 1). The sample shortening is comparable to the reported values which were obtained using a Griggs-type apparatus with a solid confining media (9%–29% of uniaxial strain, Ohuchi et al., 2010). Fig. 4b shows the BSE images of a deformed olivine aggregate. The rotation angle of the strain marker in the deformed sample shown in Fig. 4b is 51º, corresponding to shear strain γ=1.2. Because shortening of alumina/ tungsten pistons also occurred during the deformation experiments, the shear strain measured from the strain marker rotation was smaller than the shear strain calculated from the shortening of the cell assembly (former ones were 54%–91% of latter ones: Table 1).
Figure 5 shows representative dislocation microstructures of the samples. Although the deformation rams were not operated during the annealing experiments, many dislocations were observed in the undeformed sample annealed for short duration (1 min, Fig. 5a). The dislocations were formed during the compression stage. The dislocation densities in the undeformed samples annealed for a longer duration are low (Figs. 5b and 5c, Table 1), resulting from grain boundary migration and subgrain formation during the annealing process. Thus, the samples need to be annealed for > 30 min before sample deformation in order to examine the dislocation microstructures formed in the sample deformation stage. Many dislocations and formation of subboundaries are observed (Fig. 5d), showing evidence of dislocation creep. The differential stress on the sample measured with the dislocation density piezometer is summarized in Table 1.
The average concentrations of water dissolved in the recovered samples, as obtained by infrared spectroscopy, are summarized in Table 1. Representative unpolarized infrared spectra of the olivine samples annealed under dry and wet conditions are shown in Fig. 6. The infrared beam was strongly absorbed in the wavenumbers ranging from 3 530 to 3 650 cm-1 in the wet samples. The peaks resulting from O-H stretching bands are hardly observed in the dry samples. Water contents in the olivine samples under dry and wet conditions were in the range of 46–227 and 1 544– 1 730 ppm H/Si, respectively. Water contents in the M0134 and M0143 samples are at least 3 times lower than the reported values of the maximum water solubility in olivine (4 700–8 060 ppm H/Si at 5–6.5 GPa and 1 373 K: Kohlstedt et al., 1995). This discrepancy can be attributed to lower water activity in the melt-bearing rocks (e.g., Litasov et al., 2009) in the case of olivine+4 wt.% MORB sample (M0134). A very short annealing duration (1 min) would be the cause of the low water content in the M0143 sample.
Our experimental results show that deformation experiments in simple shear style can be conducted under a wide range of upper mantle conditions using the combination of a deformation-DIA apparatus and the MA 6-6 system. Recently, Kawazoe et al. (2010) successfully conducted the deformation of wadsleyite aggregates at 16–18 GPa and 1 700 K using a deformation-DIA apparatus and the MA 6-6 system with TEL 3 mm. Our cell assembly design is applicable to the MA 6-6 system with TEL 3 mm. Thus, the effect of pressure, temperature, and water fugacity on the rheological properties of rocks forming deeper parts of the earth's mantle (upper mantle to transition zone) can be explored.
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Figures(6) / Tables(1)
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Tomohiro Ohuchi, Takaaki Kawazoe, Norimasa Nishiyama, Yu Nishihara, Tetsuo Irifune. Technical Development of Simple Shear Deformation Experiments Using a Deformation-DIA Apparatus. Journal of Earth Science, 2010, 21(5): 523-531. doi: 10.1007/s12583-010-0110-4
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