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Survival of pyropic garnet in subducting plates
Physics of the Earth and Planetary Interiors 170 (2008) 274–280
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Survival of pyropic garnet in subducting plates Masayuki Nishi a,∗ , Takumi Kato b , Tomoaki Kubo b , Takumi Kikegawa c a
Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan c Photon Factory, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan b
a r t i c l e
i n f o
Article history: Received 29 June 2007 Received in revised form 1 March 2008 Accepted 26 March 2008 Keywords: Majoritic garnet Majorite Transition zone Subducting plate Kinetics
a b s t r a c t An experimental study has been conducted to clarify the conditions and kinetic aspects of the pyroxene–garnet transformation by using synthetic polycrystals (two pyroxenes + garnet) in the CaO–MgO–Al2 O3 –SiO2 system. The run products recovered from the in situ X-ray diffraction experiments in a double-stage multi-anvil system at 15.5–20.4 GPa and 800–1600 ◦ C were carefully analyzed by scanning electron microscopy. Our results show that the pyroxene–garnet transformation proceeds by overgrowth of garnet absorbing the surrounding pyroxene components above 1550 ◦ C. At 18.4 GPa and 1320 ◦ C, pyroxenes (enstatite + diopside) directly decomposed to the assemblages (wadsleyite + stishovite + Ca-perovskite) without dissolving in garnet. The pyroxene–garnet transformation requires much higher temperatures than those needed for transformation of olivine to wadsleyite (∼1000 ◦ C) and of pyroxenes to their high-pressure phases (∼1300 ◦ C) in laboratory time (∼103 s) and grain (∼10−5 m) scales. Therefore, the pyroxene–garnet transformation is likely to be kinetically inhibited at low temperatures in the subducting plate, and the pyropic garnet could survive in the transition zone instead of equilibrium majoritic garnet. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Untransformed metastable minerals possibly exist under the low-temperature conditions of the subducting plates (e.g., Sung and Burns, 1976; Rubie and Ross, 1994). Their presence has significant implications for the dynamics of plate subduction, by changing the density relation and mechanical properties (e.g., Rubie, 1984; Karato et al., 2001; Bina et al., 2001). In order to clarify the survival of metastable minerals, several kinetic studies have been extensively carried out to date. Most of the studies, under transition zone conditions, are focused on high-pressure transformations in olivine. For example, the kinetics of the olivine to wadsleyite transformation have been examined using high-pressure experiments (e.g., Kubo et al., 1998a,b), and the depths of the transformation occurrence were estimated in slab models with various thermal structures (e.g., Rubie and Ross, 1994; Kirby et al., 1996; Mosenfelder et al., 2001; Marton et al., 2005). Pyroxenes (enstatite and diopside) and garnet are the secondary abundant minerals in the Earth’s upper mantle. In the subducting plate, pyroxenes and garnet constitute about 40 vol.% in the peridotitic layer, and almost 100 vol.% in the basaltic layer (Irifune and
∗ Corresponding author. Tel.: +81 92 642 2652. E-mail address: [email protected] (M. Nishi). 0031-9201/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2008.03.013
Ringwood, 1987). Pyroxene components progressively dissolve into the garnet structure with increasing pressure, mainly at 14–16 GPa in the transition zone, resulting in a single phase of aluminumdeficient garnet (majoritic garnet) becoming stable above 16 GPa under the equilibrium state in typical models of mantle compositions (Ringwood, 1967; Akaogi and Akimoto, 1977; Irifune, 1987; Gasparik, 1992). This reaction is called the pyroxene–garnet transformation. According to Hogrefe et al. (1994), the transformation of enstatite to wadsleyite + stishovite requires a higher temperature than the olivine to wadsleyite transformation, and enstatite transforms directly to the ilmenite structure above 20 GPa at low temperatures. They proposed that the direct transformation of enstatite to ilmenite causes a large decrease in the volume, which increases the stresses and changes the buoyancy force in the subducting plate. However, as mentioned above, the pyroxene components are gradually dissolved into the garnet structure above 16 GPa in equilibrium. Therefore, it is important to investigate the kinetics of the pyroxene–garnet transformation to clarify the possible presence of metastable phases under subduction zone conditions. In spite of these issues, there have been very few studies on the mechanisms and kinetics of the pyroxene–garnet transformation. In this paper, we report experimental results on the pyroxene–garnet transformation in the CaO–MgO–Al2 O3 –SiO2
M. Nishi et al. / Physics of the Earth and Planetary Interiors 170 (2008) 274–280
system. The in situ X-ray diffraction method was applied to observe the transformation process. Textural changes in the recovered sample were carefully analyzed to obtain a consistent interpretation with the X-ray observations. The implications of the present results for metastable phase transformations in the subducting plates are briefly discussed.
2. Experimental procedures We synthesized polycrystals of pyroxenes (enstatite + diopside) + garnet as starting materials for the transformation experiments from a glass corresponding to the simplified composition of the mantle pyrolite–minus–olivine (Irifune, 1987). The chemical composition of the glass is 52.6 wt.% SiO2 , 11.9 wt.% Al2 O3 , 25.9 wt.% MgO, and 9.7 wt.% CaO. FeO was replaced by MgO in order to avoid the complexity arising from the valence state of Fe. The sample assembly for the synthesis experiments is composed of a fired pyrophyllite pressure medium, Mo electrodes, and a graphite heater which also served as the sample capsule. The temperature was monitored with a W3%Re–W25%Re thermocouple. The synthesis experiments were carried out at 3.5 GPa and 1180 ◦ C for 180 min using a MAX-90 multi-anvil high-pressure apparatus installed at Kyushu University. The recovered cylindrical samples were cut into disks with 1.2 mm diameter and 350-m thickness and these were used in the transformation experiments. In situ X-ray diffraction experiments were carried out to observe the transformation process by using a MAX-III multi-anvil high-pressure apparatus installed at the Photon Factory (BL14C2, KEK-PF), Tsukuba, Japan. Pressure was generated by the doublestage method. One sintered diamond and seven WC cubes with 14 mm edge length were used as the second-stage anvils. The truncated edge length of the anvil is 3 mm. The sample assembly for the transformation experiment is composed of a sintered (Mg,Co)O pressure medium, a cylindrical LaCrO3 heater, Mo electrodes, and a graphite sample capsule. The starting material and pressure marker (MgO and Au) were enclosed in a capsule with a graphite separation disk of 30-m thickness. Graphite and MgO rods were placed along the X-ray beam path through the pressure medium and the heater in order to increase X-ray transmissivity. The pressure medium and heater were dried at 900 ◦ C, whereas the sample and capsule were dried at 120 ◦ C in air for a few hours prior to the high-pressure experiments. Temperature was monitored with a W3%Re–W25%Re thermocouple located in the furnace, and pressure was calculated based on the equation of state of MgO (Speziale et al., 2001). White X-rays from synchrotron radiation were used as the incident X-ray beam which was collimated to 0.1 mm × 0.2 mm by a slit just before the high-pressure apparatus. The diffracted beam was collected through a 0.05 mm horizontal collimator by the energy dispersive method with a fixed 2 angle of 5.0◦ using a Ge solidstate detector. The 2 value was calibrated precisely on the basis of the unit cell parameters of MgO and Au observed under ambient conditions before and after the experiments. The sample was pressurized to about 16–21 GPa at room temperature and heated to 800 ◦ C at a constant oil pressure. There were no signs of transformation after several tens of minutes at 800 ◦ C in all the runs. Then, the temperature was rapidly increased to the target value, and X-ray diffraction data was collected every 30–600 s. After the experiments, the recovered samples were processed into polished thin sections. Textures and chemical compositions of the phases were examined by Scanning Electron Microscopy (SEM, JEOL JSM-5800) equipped with an Energy Dispersive X-ray Spectrometer (EDS) at Kyushu University. Unpolarized infrared absorption spectra of the polycrystalline recovered samples were obtained in air using Fourier-transform
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infrared spectrometry (FTIR, JASCO FTIR-4100 combined with IRT3000). The samples were double-side polished polycrystalline disks with a thickness of 130–170 m. The aperture size was 100 m × 100 m. The water content in the samples was determined on the basis of the Paterson calibration (Paterson, 1982). Background corrections of the absorbance spectra were carried out by a linear fit of the baseline defined by the spectra of the OH-free region. An anisotropy factor of 1/3 was used assuming a random orientation of the polycrystalline samples. 3. Results and discussion Five transformation experiments were carried out at 15.5–20.4 GPa and 800–1600 ◦ C lasting from 3–90 min. The experimental conditions and equilibrium boundaries of the constituent phases reported in the present study are summarized in Fig. 1. Diffraction profiles obtained at high pressures and high temperatures are shown in Fig. 2A–D. The product phase assemblages at various conditions were identified using X-ray diffraction profiles with an acquisition time longer than 10 min. The time-resolved sequences of the X-ray diffraction data were not good enough for quantitative analysis of the transformation kinetics because of the weak intensities and overlapping of peaks among the multiple phases. Table 1 lists the experimental conditions, phase assemblages, the average atomic numbers of Si relative to 12 oxygens (the Si number) of garnet, average grain sizes of garnet, and the water content of the samples. Orthoenstatite (enstatite in orthorhombic form) would actually have transformed to high-pressure clinoenstatite above 7 GPa (e.g., Shinmei et al., 1999; Ulmer and Stalder, 2001). Ca-perovskite was amorphized during the release of the pressure. Table 2 shows the chemical compositions of product garnets at various conditions with a grain size of more than about 5 m. Back-scattered electron images (BEI) of the starting material and recovered samples are shown in Fig. 3A–H. The starting material shows a uniform microstructure, in which round-shaped
Fig. 1. Pressure–temperature diagram showing the experimental conditions and results. Solid circles show that the pyroxene–garnet transformation proceeded partially. Open circles show virtually no reaction. The open diamond shows that pyroxenes decompose to stishovite + wadsleyite + Ca-perovskite without the pyroxene–garnet transformation. In pyrolite composition under an equilibrium state, pyroxene components completely dissolve into garnet above 16 GPa forming majoritic garmet (Mj) (bold line; Irifune, 1987), and exsolution of a small amount of Ca-perovskite from majoritic garnet occurs above 20 GPa (gray bold line; Irifune, 1987). The dotted and dashed thin lines indicate equilibrium phase boundaries of MgSiO3 enstatite (En) to wadsleyite (Wd) + stishovite (St) (Gasparik, 1990) and CaMgSi2 O6 diopside (Di) to wadsleyite + stishovite + Ca-perovskite (Capv) (Akaogi et al., 2004).
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Table 1 Experimental conditions and results Run no.
Pressure (GPa)
Temperature (◦ C)
Start. MA23 MA11 MA12 MA22 MA21
3.5 19.9 15.5a 17.1 18.4 20.4
1180 800 1300 1550 1320 1600
Duration (min) 180 20 3 60 90 80
Present phases
Si number
Grain size (m)
OH contents (wt. ppm H2 O)
Ga, Di, En Ga, Di, En Ga, Di, En Ga(Mj), Di, En(tr) Ga, St, Capv, Wd Ga(Mj), Capv, St(tr)
3.13 (0.03) 3.12 (0.02) 3.14 (0.03) 3.17 (0.05) 3.14 (0.04) 3.31 (0.20)
6.7 (2.0) 5.9 (2.9) 6.7 (2.0) 7.9 (2.6) 6.6 (1.9) 9.3 (3.9)
3010 (600) – 3510 (40) 660 (50) 3840 (210) 1510 (240)
Ga, garnet; Di, diopside; En, enstatite; Mj, majoritic garnet; St, stishovite; Capv, Ca-perovskite; Wd, wadsleyite; (tr), trace amount. The Si number is the average atomic number of Si relative to 12 oxygens. Three-dimensional average grain sizes were calculated by using Schwartz–Saltykov method (Saltykov, 1958) under the assumption that the spherical grains are randomly distributed. The numbers in the parentheses are the standard deviation of 10–30 analyses (the Si number), 100–400 analyses (grain size), and 3–7 analyses (OH contents). Pressure was calculated based on the equation of state of MgO (Speziale et al., 2001). Estimated uncertainty is ±0.2 GPa. a We measured unit cell volume of MgO up to 800 ◦ C in this run for the pressure calculation. Above 800 ◦ C, pressure was estimated based on the similar P–T path in other runs.
Fig. 2. In situ X-ray diffraction observations of the pyroxene–garnet transformation. (A) Starting material; (B) Run MA12, 17.1 GPa and 1550 ◦ C for 60 min; (C) Run MA22, 18.4 GPa and 1320 ◦ C for 90 min; (D) Run MA21, 20.4 GPa and 1600 ◦ C for 80 min.
garnet grains are surrounded by pyroxenes with smaller grain sizes (Fig. 3A and B). After heating at 15.5 GPa and 1300 ◦ C for 3 min (Run MA11), no significant changes were observed in the X-ray diffraction patterns. The volume fraction, the average grain size, and the chemical composition of garnet in the recovered samples remained unchanged compared to those of the starting material (Tables 1 and 2).
After heating at 17.1 GPa and 1550 ◦ C for 60 min (Run MA12), the X-ray diffraction pattern showed an increase in the peak intensities corresponding to garnet (Fig. 2B). The Si number and the average grain size of the garnet slightly increased (Tables 1 and 2) from those of the starting material. Most pyroxenes remained unreacted in the recovered sample (Fig. 3C and D). Although the equilibrium assemblage in this condition should be a single phase of majoritic garnet (Fig. 1), the pyroxene–garnet transformation proceeded slightly. After heating at 18.4 GPa and 1320 ◦ C for 90 min (Run MA22), decomposition of pyroxenes to stishovite + wadsleyite + Caperovskite was observed in the X-ray diffraction profile (Fig. 2C). On the other hand, the volume fraction, grain size, and chemical composition of garnet did not change from those of the starting material (Fig. 3E and F, Tables 1 and 2). These results suggest that the decomposition of pyroxenes to their respective high-pressure phases proceeded much faster than the pyroxene–garnet transformation. For the 20.4 GPa and 1600 ◦ C run (Run MA21), the X-ray diffraction pattern indicated that pyroxenes rapidly decomposed to their high-pressure phases during elevation of the temperature. The pyroxene–garnet transformation proceeded significantly during 80 min heating at this condition, which was confirmed by the decrease in the peak intensities of the transformed high-pressure phases compared to those of garnet (Fig. 2D). The average grain size of garnet clearly increased in the recovered sample (Table 1, Fig. 3G and H), suggesting that the pyroxene–garnet transformation proceeds by overgrowth of the original garnet. In addition, the direct conversion of enstatite into Al-deficient majorite may also
Table 2 Chemical compositions of garnet Run no.
Start. mater.
MA23
MA11
MA12
MA22
MA21
Duration (min) Temperature (◦ C) Pressure (GPa)
180 1180 3.5
20 800 19.9
3 1300 15.5
60 1550 17.1
90 1320 18.4
80 1600 20.4
Wt. (%) SiO2 Al2 O3 MgO CaO Total Cation based on 12 oxygens Si Al Mg Ca Total
45.6 (0.5) 22.0 (0.7) 24.1 (0.4) 8.2 (0.3) 100.1
45.0 (0.4) 22.2 (0.4) 23.6 (0.2) 7.9 (0.5)
45.1 (0.4) 21.7 (0.6) 24.1 (0.3) 7.6 (0.4)
46.1 (0.7) 20.9 (1.3) 25.0 (0.7) 7.7 (0.7)
45.5 (0.7) 21.7 (0.9) 24.1 (0.8) 8.0 (0.8)
48.2 (3.1) 18.1 (4.3) 26.0 (2.3) 7.2 (0.8)
98.66
98.49
99.69
99.21
99.42
3.13 (0.04) 1.78 (0.05) 2.47 (0.03) 0.64 (0.03)
3.12 (0.02) 1.82 (0.03) 2.44 (0.03) 0.58 (0.04)
3.14 (0.03) 1.78 (0.05) 2.50 (0.03) 0.57 (0.03)
3.17 (0.05) 1.69 (0.10) 2.56 (0.06) 0.57 (0.06)
3.14 (0.04) 1.76 (0.07) 2.48 (0.08) 0.59 (0.06)
3.31 (0.19) 1.46 (0.36) 2.66 (0.22) 0.53 (0.06)
7.98
7.97
7.98
7.99
7.98
7.96
The numbers in the parentheses are the standard deviation of 10–30 analyses.
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Fig. 3. Back-scattered electron images showing textures of the starting material synthesized at 3.5 GPa and 1180 ◦ C for 180 min heating (A and B), and samples recovered from transformation experiments at various conditions: (C and D) Run MA12, 17.1 GPa and 1550 ◦ C for 60 min; (E and F) Run MA22, 18.4 GPa and 1320 ◦ C for 90 min; (G and H) Run MA21, 20.4 GPa and 1600 ◦ C for 80 min. At relatively high temperature conditions (C, D, G, and H), the volume fraction of garnet grains increased. Pyroxenes decomposed rapidly to their high-pressure phases at high-pressure conditions above the stability limit of pyroxenes (∼17 GPa) (E, F, G, and H). Although it is difficult to quantify the chemical compositions of the decomposed assemblages of pyroxenes (wadsleyite, Ca-perovskite, and stishovite) because of fine grains, the presence of these phases were inferred from the additional elemental mapping images of Ca, Mg, Si, and Al.
occur because the MgSiO3 majorite becomes stable above 16 GPa and 1600 ◦ C (e.g., Gasparik, 1996). We measured the unpolarized infrared absorption spectra of the starting material and the recovered samples using microFTIR to constrain the hydrous conditions of the experiments. The representative infrared spectra of the starting material and the
recovered sample of MA21 are shown in Fig. 4. FTIR spectra of the starting material show a broad absorption band in the range of 2500–3700 cm−1 with a relatively sharp peak around 3450 cm−1 . These features have also been observed in the previous study on cubic garnet (Bolfan-Cassanova et al., 2000). Small absorption peaks around 2900 cm−1 may be derived from acetone treatment
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Fig. 4. Unpolarized infrared spectra of the starting material and the recovered sample of Run MA22 (18.4 GPa and 1320 ◦ C for 90 min heating).
during sample preparation (e.g., Kubo et al., 2004). FTIR spectra of the recovered sample of run MA21 display a sharp absorption band around 3120 cm−1 . This peak is likely derived from the OH vibration of stishovite (Panero et al., 2003). The water content of the starting material and the recovered samples is in the range of 700–4000 wt. ppm H2 O (Table 1). Samples recovered from higher temperature conditions (Run MA12, MA21) had lower water content than the starting material. This result suggests that dehydration occurred mainly above 1550 ◦ C for 60–90 min. Fig. 5 shows the Si numbers and average grain sizes of garnet as a function of temperature. Al3+ ions in the garnet are replaced by M2+ and Si4+ ions (M = Mg + Ca) by the pyroxene–garnet transformation. In the equilibrium state, the Si number of garnet is 3.0 (equal to that of the ideal M3 Al2 Si3 O12 garnet) at low pressures. It increases with pressure above about 5 GPa and remains constant at around 3.5 between 16 and 20 GPa in the stability field of a single phase of majoritic garnet. The temperature dependence on the equilibrium Si content in garnet is relatively small in pyrolitic compositions (Irifune, 1987). Most of the present experiments have been carried out in the stability field of a single phase of majoritic garnet. Small
Fig. 5. Plots of atomic numbers of Si relative to 12 oxygens (A) and the grain sizes (B) of recovered garnets as a function of temperature. The Si number and the average grain size of the starting material garnet (probably containing small inclusions of enstatite) are 3.13 and 6.7 m, respectively (dotted lines). The gray area shows the Si number of equilibrium majoritic garnet (3.4–3.5) in the pyrolite model composition. The numbers indicate the run durations (min).
Fig. 6. Plots of the Si number with the Ca/(Mg + Ca) ratio of majoritic garnets formed in the run of MA21 (solid circles) and of the garnet in the starting material (open squares). Majoritic garnet with high Si number contains larger amounts of enstatite components compared to the equilibrium single phase of majoritic garnet calculated from the bulk composition (solid star sign). The open star sign indicates calculated chemical composition of majoritic garnet assuming coexistence with Ca-perovskite of 6 vol.%.
amounts of pyroxene and Ca-perovskite may coexist with majoritic garnet in equilibrium in the lowest and highest pressure conditions of the present experiments, respectively. However, because of their limited volume fractions, their presence only slightly reduces the Si number to about 3.4. Therefore, the Si number is a useful indicator of the progression of the pyroxene–garnet transformation. Fig. 5 shows the correlation between the Si number and the grain size of garnet. The garnet of the starting material has a relatively high Si number (3.07–3.15) compared to ideal garnet, probably because of the analytical interference by beam-overlapping of the small pyroxene inclusions. The Si number and the grain size increase only at high temperatures of 1550–1600 ◦ C in the time range of 60–90 min. This suggests that the pyroxene–garnet transformation requires a temperature higher than ∼1550 ◦ C in the time and grain scales of the present experimental condition. Intra-grain zonal heterogeneity may exist in garnet, as a result of the slow volume diffusion in garnet. However, we could find no evidence for grain chemical zonation or for a systematic relation between the Si number and the grain size. This is probably because the heterogeneous outer growth rim is limited to within the detection limit of a few m. Dissolution of pyroxene inclusions into garnet would also occur together with the separate grain interactions. Fig. 6 shows the compositional variation of the Si number and the Ca/(Mg + Ca) ratios in majoritic garnet recovered at 20.4 GPa and 1600 ◦ C (Run MA21, solid circles). The constituent minerals (garnet, enstatite and diopside) in the starting material are expressed by the reversed triangle in this figure. The compositions of garnet in this run product lie on the trend line between the original garnet and enstatite. The Si-rich majoritic garnet clearly has a higher Mg component compared to the expected equilibrium single phase of majoritic garnet calculated from the bulk composition (solid star sign). The compositional trend of garnet displayed in Fig. 6 could be interpreted to be the result of a combination of three transformation processes: (1) Pyropic garnet preferentially has absorbed the enstatite components relative to the diopside components immediately after two pyroxenes had transformed to their respective high-pressure phases (wadsleyite + stishovite and Ca-perovskite) at the initial stage of the pyroxene–garnet transformation, due to
M. Nishi et al. / Physics of the Earth and Planetary Interiors 170 (2008) 274–280
faster atomic diffusion of Mg relative to that of Ca in garnet (e.g., Schwandt et al., 1996; Carlson, 2006; Vielzeuf et al., 2007). (2) Some MgSiO3 majorite grains possibly have been formed from the direct conversion of enstatite, and the expected bimodal compositional distribution could have been smeared out by the reaction with pyropic garnet afterwards. (3) Some Ca-perovskite transformed from pyroxene may have existed without reaction of garnet because a small amount of Ca-perovskite could be a stable phase at this condition (Irifune and Ringwood, 1987). The composition of majoritic garnet shifts to a Mg-rich composition by coexisting the equilibrium Ca-perovskite (∼6 vol.%), as indicated by the open star sign in Fig. 6. In any of these cases, the elementary mechanism in the initial stage of the pyroxene–garnet transformation is likely to be the growth of the pyropic garnet controlled by the diffusion. The pyroxene–garnet transformation requires long-distance diffusion of atoms comparable to the grain size scale. It is known that cation diffusion is slow in garnet (e.g., Carlson, 2006; Vielzeuf et al., 2007). Therefore, it is likely that the transformation is kinetically inhibited by slow chemical diffusion in garnet although the quantitative diffusion kinetics of Si and Al has not been known. Exact rate-controlling process applicable to this transformation remains for future research. The present study demonstrated that the pyroxene–garnet transformation requires a much higher temperature (∼1550 ◦ C) than the olivine to wadsleyite transformation (∼1000 ◦ C) (Kubo et al., 2004), the enstatite to ilmenite transformation (∼1250 ◦ C) (Hogrefe et al., 1994), and the pyroxenes to wadsleyite + stishovite + Ca-perovskite transformation (∼1300 ◦ C of this study) in laboratory time (∼103 s) and grain size (∼10−5 m) scales. The temperatures within the cold subducting plates are estimated to be as low as 600 ◦ C even at a depth of 500 km corresponding to 16 GPa. It has been proposed that metastable olivine survives to depths of 500 km or more in the subducting slabs (Mosenfelder et al., 2001). Our experimental results suggest that the pyroxene–garnet transformation is much more kinetically inhibited than the transformations of olivine to wadsleyite and pyroxenes to their high-pressure phases. Moreover, as pyroxene and garnet coexist with a large amount of olivine and they are separated by olivine grains in the peridotite layer of the slabs, direct reaction between pyroxene and garnet is possibly prevented by the olivine. This also causes a delay in the pyroxene–garnet transformation. Therefore, the nonequilibrium phase of pyropic garnet could be present in the mantle transition zone in the subducting plate. The volume fraction of garnet increases slowly by overgrowth absorbing the surrounding pyroxenes. The remnant pyroxene phases, which kinetically failed to be accommodated in the garnet, transform to their own high-pressure phases while keeping their original chemical compositions. The transformation of pyroxenes (enstatite and diopside) to their high-pressure phases (wadsleyite, stishovite, and Ca-perovskite) could become major transformations in the subducting plate. In the coldest central part of the subducting plates, enstatite may directly transform to ilmenite as suggested by Hogrefe et al. (1994). In these cases, grain size reduction due to the transformation of pyroxenes may lead to rheological weakening of the subducting plates by the same mechanism as that of the olivinespinel transformation (Rubie, 1984; Riedel and Karato, 1997; Karato et al., 2001). This effect would be more important in the basaltic layer because pyroxene and garnet are major constituents. In addition, the survival of pyropic garnets affects the depth of the garnet–perovskite transformation, because the depth increases with an increase in the aluminum content of majoritic garnet (Irifune et al., 1996; Akaogi and Ito, 1999; Kubo and Akaogi, 2000). If the pyropic garnet survives in a metastable form, it then decomposes into four phases of Mg-perovskite, Ca-perovskite, stishovite, and aluminous phase (Oguri et al., 2000). Thus, the metastable min-
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eralogy in the peridotite layer is largely affected by the kinetics of the pyroxene–garnet transformation. The present study shows the importance of slow kinetics of the pyroxene–garnet transformation in the subducting slabs. Further quantitative determination of the kinetic parameters and their temperature dependence are needed for modeling the mineralogy and density of the subducting plate in the transition zone conditions. Moreover, the pyroxene–garnet transformation is a major transformation in the basaltic (MORB) layer. It is very important to investigate the kinetics of this transformation in the MORB composition including FeO and other minor components. Also, it has been suggested that water has great influence on the transformation kinetics (e.g., Hosoya et al., 2005). Although the present experiments were carried out under relatively hydrous conditions (∼700–4000 wt. ppm H2 O), the pyroxene–garnet transformation was very slow and difficult to be completed. It is indispensable to examine the effects of water on the pyroxene–garnet transformation kinetics for better understanding the realistic mineral constitution in the cold subducting plates. Acknowledgements We thank A. Tominaga for assistance with in situ X-ray diffraction experiments and S. Uehara and A. Shimojuku for their helpful discussions and useful comments. We also acknowledge D.C. Rubie, E Ohtani, and two anonymous referees for valuable comments. In situ X-ray diffraction experiments were carried out using the MAXIII system at BL14C of the Photon Factory (Proposal no. 2004G225). This work was supported by a Front Researcher Development program of Kyushu University, and by a Grant-in-Aid for Scientific Research to T.K. from the Japan Society for the Promotion of Science. References Akaogi, M., Akimoto, S., 1977. Pyroxene–garnet solid-solution equilibria in the systems Mg4 Si4 O12 –Mg3 Al2 Si3 O12 and Fe4 SiO12 –Fe3 Al2 Si3 O12 at high pressures and temperatures. Phys. Earth Planet. Inter. 15, 90–106. Akaogi, M., Ito, E., 1999. Calorimetric study on majorite-perovskite transition in the system Mg4 Si4 O12 –Mg3 Al2 Si3 O12 : transition boundaries with positive pressuretemperature slopes. Phys. Earth Planet. Inter. 114, 3–4. Akaogi, M., Yano, M., Tejima, Y., Iijima, M., Kojitani, H., 2004. High-pressure transitions of diopside and wollastonite: phase equilibria and thermochemistry of CaMgSi2 O6 , CaSiO3 and CaSi2 O5 –CaTiSiO5 system. Phys. Earth Planet. Inter. 143, 145–156. Bina, C.R., Stein, S., Marton, F.C., Van Ark, E.M., 2001. Implications of slab mineralogy for subduction dynamics. Phys. Earth Planet. Inter. 127, 51–66. Bolfan-Cassanova, N., Keppler, H., Rubie, D.C., 2000. Water partitioning between nominally anhydrous minerals in the MgO–SiO2 –H2 O system up to 24 GPa: implications for the distribution of water in the Earth’s mantle. Earth Planet. Sci. Lett. 182, 209–221. Carlson, W.D., 2006. Rates of Fe, Mg, Mn, and Ca diffusion in garnet. Am. Miner. 91, 1–11. Gasparik, T., 1996. Melting experiments on the enstatite-diopside join at 70–224 kbar, including the melting of diopside. Contrib. Mineral. Petrol. 124, 139–153. Gasparik, T., 1992. Melting experiments on the enstatite–pyrope join at 80–152 kbar. J. Geophys. Res. 97, 15181–15188. Gasparik, T., 1990. Phase relations in the transition zone. J. Geophys. Res. 95, 15751–15769. Hogrefe, A., Rubie, D.C., Sharp, T.G., Seifert, F., 1994. Metastability of enstatite in deep subducting lithosphere. Nature 372, 351–353. Hosoya, T., Kubo, T., Ohtani, E., Sano, A., Funakoshi, K.-I., 2005. Water controls the fields of metastable olivine in cold subducting slabs. Geophys. Res. Lett. 31, L17305, doi:10.1029/2005GL023398. Irifune, T., 1987. An experimental investigation of the pyroxene–garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle. Phys. Earth Planet. Inter. 45, 324–336. Irifune, T., Ringwood, A.E., 1987. Phase transformations in primitive MORB and pyrolite compositions to 25 GPa and some geophysical implications. In: Manghnani, M.H., Shono, Y. (Eds.), High-Pressure Research in Mineral Physics, Terrapub, Tokyo, pp. 231–242. Irifune, T., Koizumi, T., Ando, J., 1996. An experimental study of the garnet–perovskite transformation in the system MgSiO3 –Mg3 Al2 Si3 O12 . Phys. Earth Planet. Inter. 96, 147–157.
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Karato, S., Rindel, M.R., Yuen, D.A., 2001. Rheological structure and deformation of subducted slabs in the mantle transition zone: implications for mantle circulation and deep earthquakes. Phys. Earth Planet. Inter. 127, 83–108. Kirby, S.H., Stein, S., Okal, E.A., Rubie, D.C., 1996. Metestable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere. Rev. Geophys. 34 (2), 261–306. Kubo, T., Ohtani, E., Kato, T., Shinmei, T., Fujino, K., 1998a. Experimental investigation of the ␣– transformation of San Carlos olivine single crystal. Phys. Chem. Miner. 26, 1–6. Kubo, T., Ohtani, E., Kato, T., Morishima, H., Yamazaki, D., Suzuki, A., Mibe, K., Kikegawa, T., Shimomura, O., 1998b. An in situ X ray diffraction study of the ␣– transformation kinetics of MgSiO3 . Geophys. Res. Lett. 25, 695–698. Kubo, T., Ohtani, E., Funakoshi, K., 2004. Nucleation and growth kinetics of the ␣– transformation in Mg2 SiO4 determined by in situ synchrotron powder X-ray diffraction. Am. Miner. 89, 285–293. Kubo, A., Akaogi, M., 2000. Post-garnet transitions in the system Mg4 Si4 O12 –Mg3 Al2 Si3 O12 up to 28 GPa: phase relations of garnet, ilmenite and perovskite. Phys. Earth Planet. Inter. 121, 85–102. Marton, F.C., Shankland, T.J., Rubie, D.C., Xu, Y., 2005. Effects of variable thermal conductivity on the mineralogy of subducting slabs and implications for mechanisms of deep earthquakes. Phys. Earth Planet. Inter. 149, 53–64. Mosenfelder, J.L., Marton, F.C., Ross, C.R., Kerschhofer, L., Rubie, D.C., 2001. Experimental constraints on the depth of olivine metastability in subducting lithosphere. Phys. Earth Planet. Inter. 127, 165–180. Oguri, K., Funamori, N., Uchida, T., Miyajima, N., Yagi, T., Fujino, K., 2000. Postgarnet transition in a natural pyrope: a multi-anvil study based on in situ X-ray diffraction and transmission electron microscopy. Phys. Earth Planet. Inter. 122, 157–186. Panero, W.R., Benedetti, L.R., Jeanloz, R., 2003. Transport of water into the lower mantle: role of stishovite. J. Geophys. Res. 108 (B1), 2039, doi:10.1029/2002JB002053. Paterson, M.S., 1982. The determination of hydroxyl by infrared absorption in quartz, silicate glasses and similar materials. Bull. Mineral. 105, 20–29.
Ringwood, A.E., 1967. The pyroxene–garnet transformation in the earth’s mantle. Earth Planet. Sci. Lett. 2, 255–263. Riedel, M.R., Karato, S., 1997. Grain-size evolution in subducted oceanic lithosphere associated with the olivine–spinel transformation and its effects on rheology. Earth Planet. Sci. Lett. 148, 27–43. Rubie, D.C., 1984. The olivine–spinel transformation and the rheology of subducting lithosphere. Nature 308, 505–508. Rubie, D.C., Ross II, C.R., 1994. Kinetics of olivine–spinel transformation in subducting lithosphere: experimental constrains and implications for deep slab processes. Phys. Earth Planet. Inter. 86, 223–241. Shinmei, T., Tomioka, N., Fujino, K., Kuroda, K., Ifrifune, T., 1999. In situ X-ray diffraction study of enstatite up to 12 GPa and 1473 K and equations of state. Am. Miner. 84, 1588–1594. Speziale, S., Zha, C., Duffy, T.S., Hemley, R.J., Mao, H., 2001. Quasi-hydrostatic compression of magnesium oxide to 52 GPa: implications for the pressure–volume–temperature equation of state. J. Geophys. Res. 106 (B1), 515–528. Saltykov, S.A., 1958. Stereometric Metallography. Metallurgizdat, Moscow, pp. 446. Schwandt, C.S., Cygan, R.T., Westrich, H.R., 1996. Ca self-diffusion in grossular garnet. Am. Miner. 81, 448–451. Sung, C.M., Burns, R.G., 1976. Kinetics of high pressure phase transformations: implications to the evolution of olivine–spinel transition in the downgoing lithosphere and its consequences on the dynamics of the mantle. Tectonophysics 31, 1–32. Ulmer, P., Stalder, L., 2001. The Mg(Fe)SiO3 orthoenstatite–clinoenstatite transitions at high pressures and temperatures determined by Raman-spectroscopy on quenched samples. Am. Miner. 86, 1267– 1274. Vielzeuf, P., Baronnet, A., Perchuk, A.L., Laporte, D., Baker, M.B., 2007. Calcium diffusivity in alumino-silicate garnets: an experimental and ATEM study. Contrib. Mineral. Petrol. 154, 153–170.
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Survival of pyropic garnet in subducting plates Masayuki Nishi a,∗ , Takumi Kato b , Tomoaki Kubo b , Takumi Kikegawa c a
Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan c Photon Factory, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan b
a r t i c l e
i n f o
Article history: Received 29 June 2007 Received in revised form 1 March 2008 Accepted 26 March 2008 Keywords: Majoritic garnet Majorite Transition zone Subducting plate Kinetics
a b s t r a c t An experimental study has been conducted to clarify the conditions and kinetic aspects of the pyroxene–garnet transformation by using synthetic polycrystals (two pyroxenes + garnet) in the CaO–MgO–Al2 O3 –SiO2 system. The run products recovered from the in situ X-ray diffraction experiments in a double-stage multi-anvil system at 15.5–20.4 GPa and 800–1600 ◦ C were carefully analyzed by scanning electron microscopy. Our results show that the pyroxene–garnet transformation proceeds by overgrowth of garnet absorbing the surrounding pyroxene components above 1550 ◦ C. At 18.4 GPa and 1320 ◦ C, pyroxenes (enstatite + diopside) directly decomposed to the assemblages (wadsleyite + stishovite + Ca-perovskite) without dissolving in garnet. The pyroxene–garnet transformation requires much higher temperatures than those needed for transformation of olivine to wadsleyite (∼1000 ◦ C) and of pyroxenes to their high-pressure phases (∼1300 ◦ C) in laboratory time (∼103 s) and grain (∼10−5 m) scales. Therefore, the pyroxene–garnet transformation is likely to be kinetically inhibited at low temperatures in the subducting plate, and the pyropic garnet could survive in the transition zone instead of equilibrium majoritic garnet. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Untransformed metastable minerals possibly exist under the low-temperature conditions of the subducting plates (e.g., Sung and Burns, 1976; Rubie and Ross, 1994). Their presence has significant implications for the dynamics of plate subduction, by changing the density relation and mechanical properties (e.g., Rubie, 1984; Karato et al., 2001; Bina et al., 2001). In order to clarify the survival of metastable minerals, several kinetic studies have been extensively carried out to date. Most of the studies, under transition zone conditions, are focused on high-pressure transformations in olivine. For example, the kinetics of the olivine to wadsleyite transformation have been examined using high-pressure experiments (e.g., Kubo et al., 1998a,b), and the depths of the transformation occurrence were estimated in slab models with various thermal structures (e.g., Rubie and Ross, 1994; Kirby et al., 1996; Mosenfelder et al., 2001; Marton et al., 2005). Pyroxenes (enstatite and diopside) and garnet are the secondary abundant minerals in the Earth’s upper mantle. In the subducting plate, pyroxenes and garnet constitute about 40 vol.% in the peridotitic layer, and almost 100 vol.% in the basaltic layer (Irifune and
∗ Corresponding author. Tel.: +81 92 642 2652. E-mail address: [email protected] (M. Nishi). 0031-9201/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2008.03.013
Ringwood, 1987). Pyroxene components progressively dissolve into the garnet structure with increasing pressure, mainly at 14–16 GPa in the transition zone, resulting in a single phase of aluminumdeficient garnet (majoritic garnet) becoming stable above 16 GPa under the equilibrium state in typical models of mantle compositions (Ringwood, 1967; Akaogi and Akimoto, 1977; Irifune, 1987; Gasparik, 1992). This reaction is called the pyroxene–garnet transformation. According to Hogrefe et al. (1994), the transformation of enstatite to wadsleyite + stishovite requires a higher temperature than the olivine to wadsleyite transformation, and enstatite transforms directly to the ilmenite structure above 20 GPa at low temperatures. They proposed that the direct transformation of enstatite to ilmenite causes a large decrease in the volume, which increases the stresses and changes the buoyancy force in the subducting plate. However, as mentioned above, the pyroxene components are gradually dissolved into the garnet structure above 16 GPa in equilibrium. Therefore, it is important to investigate the kinetics of the pyroxene–garnet transformation to clarify the possible presence of metastable phases under subduction zone conditions. In spite of these issues, there have been very few studies on the mechanisms and kinetics of the pyroxene–garnet transformation. In this paper, we report experimental results on the pyroxene–garnet transformation in the CaO–MgO–Al2 O3 –SiO2
M. Nishi et al. / Physics of the Earth and Planetary Interiors 170 (2008) 274–280
system. The in situ X-ray diffraction method was applied to observe the transformation process. Textural changes in the recovered sample were carefully analyzed to obtain a consistent interpretation with the X-ray observations. The implications of the present results for metastable phase transformations in the subducting plates are briefly discussed.
2. Experimental procedures We synthesized polycrystals of pyroxenes (enstatite + diopside) + garnet as starting materials for the transformation experiments from a glass corresponding to the simplified composition of the mantle pyrolite–minus–olivine (Irifune, 1987). The chemical composition of the glass is 52.6 wt.% SiO2 , 11.9 wt.% Al2 O3 , 25.9 wt.% MgO, and 9.7 wt.% CaO. FeO was replaced by MgO in order to avoid the complexity arising from the valence state of Fe. The sample assembly for the synthesis experiments is composed of a fired pyrophyllite pressure medium, Mo electrodes, and a graphite heater which also served as the sample capsule. The temperature was monitored with a W3%Re–W25%Re thermocouple. The synthesis experiments were carried out at 3.5 GPa and 1180 ◦ C for 180 min using a MAX-90 multi-anvil high-pressure apparatus installed at Kyushu University. The recovered cylindrical samples were cut into disks with 1.2 mm diameter and 350-m thickness and these were used in the transformation experiments. In situ X-ray diffraction experiments were carried out to observe the transformation process by using a MAX-III multi-anvil high-pressure apparatus installed at the Photon Factory (BL14C2, KEK-PF), Tsukuba, Japan. Pressure was generated by the doublestage method. One sintered diamond and seven WC cubes with 14 mm edge length were used as the second-stage anvils. The truncated edge length of the anvil is 3 mm. The sample assembly for the transformation experiment is composed of a sintered (Mg,Co)O pressure medium, a cylindrical LaCrO3 heater, Mo electrodes, and a graphite sample capsule. The starting material and pressure marker (MgO and Au) were enclosed in a capsule with a graphite separation disk of 30-m thickness. Graphite and MgO rods were placed along the X-ray beam path through the pressure medium and the heater in order to increase X-ray transmissivity. The pressure medium and heater were dried at 900 ◦ C, whereas the sample and capsule were dried at 120 ◦ C in air for a few hours prior to the high-pressure experiments. Temperature was monitored with a W3%Re–W25%Re thermocouple located in the furnace, and pressure was calculated based on the equation of state of MgO (Speziale et al., 2001). White X-rays from synchrotron radiation were used as the incident X-ray beam which was collimated to 0.1 mm × 0.2 mm by a slit just before the high-pressure apparatus. The diffracted beam was collected through a 0.05 mm horizontal collimator by the energy dispersive method with a fixed 2 angle of 5.0◦ using a Ge solidstate detector. The 2 value was calibrated precisely on the basis of the unit cell parameters of MgO and Au observed under ambient conditions before and after the experiments. The sample was pressurized to about 16–21 GPa at room temperature and heated to 800 ◦ C at a constant oil pressure. There were no signs of transformation after several tens of minutes at 800 ◦ C in all the runs. Then, the temperature was rapidly increased to the target value, and X-ray diffraction data was collected every 30–600 s. After the experiments, the recovered samples were processed into polished thin sections. Textures and chemical compositions of the phases were examined by Scanning Electron Microscopy (SEM, JEOL JSM-5800) equipped with an Energy Dispersive X-ray Spectrometer (EDS) at Kyushu University. Unpolarized infrared absorption spectra of the polycrystalline recovered samples were obtained in air using Fourier-transform
275
infrared spectrometry (FTIR, JASCO FTIR-4100 combined with IRT3000). The samples were double-side polished polycrystalline disks with a thickness of 130–170 m. The aperture size was 100 m × 100 m. The water content in the samples was determined on the basis of the Paterson calibration (Paterson, 1982). Background corrections of the absorbance spectra were carried out by a linear fit of the baseline defined by the spectra of the OH-free region. An anisotropy factor of 1/3 was used assuming a random orientation of the polycrystalline samples. 3. Results and discussion Five transformation experiments were carried out at 15.5–20.4 GPa and 800–1600 ◦ C lasting from 3–90 min. The experimental conditions and equilibrium boundaries of the constituent phases reported in the present study are summarized in Fig. 1. Diffraction profiles obtained at high pressures and high temperatures are shown in Fig. 2A–D. The product phase assemblages at various conditions were identified using X-ray diffraction profiles with an acquisition time longer than 10 min. The time-resolved sequences of the X-ray diffraction data were not good enough for quantitative analysis of the transformation kinetics because of the weak intensities and overlapping of peaks among the multiple phases. Table 1 lists the experimental conditions, phase assemblages, the average atomic numbers of Si relative to 12 oxygens (the Si number) of garnet, average grain sizes of garnet, and the water content of the samples. Orthoenstatite (enstatite in orthorhombic form) would actually have transformed to high-pressure clinoenstatite above 7 GPa (e.g., Shinmei et al., 1999; Ulmer and Stalder, 2001). Ca-perovskite was amorphized during the release of the pressure. Table 2 shows the chemical compositions of product garnets at various conditions with a grain size of more than about 5 m. Back-scattered electron images (BEI) of the starting material and recovered samples are shown in Fig. 3A–H. The starting material shows a uniform microstructure, in which round-shaped
Fig. 1. Pressure–temperature diagram showing the experimental conditions and results. Solid circles show that the pyroxene–garnet transformation proceeded partially. Open circles show virtually no reaction. The open diamond shows that pyroxenes decompose to stishovite + wadsleyite + Ca-perovskite without the pyroxene–garnet transformation. In pyrolite composition under an equilibrium state, pyroxene components completely dissolve into garnet above 16 GPa forming majoritic garmet (Mj) (bold line; Irifune, 1987), and exsolution of a small amount of Ca-perovskite from majoritic garnet occurs above 20 GPa (gray bold line; Irifune, 1987). The dotted and dashed thin lines indicate equilibrium phase boundaries of MgSiO3 enstatite (En) to wadsleyite (Wd) + stishovite (St) (Gasparik, 1990) and CaMgSi2 O6 diopside (Di) to wadsleyite + stishovite + Ca-perovskite (Capv) (Akaogi et al., 2004).
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Table 1 Experimental conditions and results Run no.
Pressure (GPa)
Temperature (◦ C)
Start. MA23 MA11 MA12 MA22 MA21
3.5 19.9 15.5a 17.1 18.4 20.4
1180 800 1300 1550 1320 1600
Duration (min) 180 20 3 60 90 80
Present phases
Si number
Grain size (m)
OH contents (wt. ppm H2 O)
Ga, Di, En Ga, Di, En Ga, Di, En Ga(Mj), Di, En(tr) Ga, St, Capv, Wd Ga(Mj), Capv, St(tr)
3.13 (0.03) 3.12 (0.02) 3.14 (0.03) 3.17 (0.05) 3.14 (0.04) 3.31 (0.20)
6.7 (2.0) 5.9 (2.9) 6.7 (2.0) 7.9 (2.6) 6.6 (1.9) 9.3 (3.9)
3010 (600) – 3510 (40) 660 (50) 3840 (210) 1510 (240)
Ga, garnet; Di, diopside; En, enstatite; Mj, majoritic garnet; St, stishovite; Capv, Ca-perovskite; Wd, wadsleyite; (tr), trace amount. The Si number is the average atomic number of Si relative to 12 oxygens. Three-dimensional average grain sizes were calculated by using Schwartz–Saltykov method (Saltykov, 1958) under the assumption that the spherical grains are randomly distributed. The numbers in the parentheses are the standard deviation of 10–30 analyses (the Si number), 100–400 analyses (grain size), and 3–7 analyses (OH contents). Pressure was calculated based on the equation of state of MgO (Speziale et al., 2001). Estimated uncertainty is ±0.2 GPa. a We measured unit cell volume of MgO up to 800 ◦ C in this run for the pressure calculation. Above 800 ◦ C, pressure was estimated based on the similar P–T path in other runs.
Fig. 2. In situ X-ray diffraction observations of the pyroxene–garnet transformation. (A) Starting material; (B) Run MA12, 17.1 GPa and 1550 ◦ C for 60 min; (C) Run MA22, 18.4 GPa and 1320 ◦ C for 90 min; (D) Run MA21, 20.4 GPa and 1600 ◦ C for 80 min.
garnet grains are surrounded by pyroxenes with smaller grain sizes (Fig. 3A and B). After heating at 15.5 GPa and 1300 ◦ C for 3 min (Run MA11), no significant changes were observed in the X-ray diffraction patterns. The volume fraction, the average grain size, and the chemical composition of garnet in the recovered samples remained unchanged compared to those of the starting material (Tables 1 and 2).
After heating at 17.1 GPa and 1550 ◦ C for 60 min (Run MA12), the X-ray diffraction pattern showed an increase in the peak intensities corresponding to garnet (Fig. 2B). The Si number and the average grain size of the garnet slightly increased (Tables 1 and 2) from those of the starting material. Most pyroxenes remained unreacted in the recovered sample (Fig. 3C and D). Although the equilibrium assemblage in this condition should be a single phase of majoritic garnet (Fig. 1), the pyroxene–garnet transformation proceeded slightly. After heating at 18.4 GPa and 1320 ◦ C for 90 min (Run MA22), decomposition of pyroxenes to stishovite + wadsleyite + Caperovskite was observed in the X-ray diffraction profile (Fig. 2C). On the other hand, the volume fraction, grain size, and chemical composition of garnet did not change from those of the starting material (Fig. 3E and F, Tables 1 and 2). These results suggest that the decomposition of pyroxenes to their respective high-pressure phases proceeded much faster than the pyroxene–garnet transformation. For the 20.4 GPa and 1600 ◦ C run (Run MA21), the X-ray diffraction pattern indicated that pyroxenes rapidly decomposed to their high-pressure phases during elevation of the temperature. The pyroxene–garnet transformation proceeded significantly during 80 min heating at this condition, which was confirmed by the decrease in the peak intensities of the transformed high-pressure phases compared to those of garnet (Fig. 2D). The average grain size of garnet clearly increased in the recovered sample (Table 1, Fig. 3G and H), suggesting that the pyroxene–garnet transformation proceeds by overgrowth of the original garnet. In addition, the direct conversion of enstatite into Al-deficient majorite may also
Table 2 Chemical compositions of garnet Run no.
Start. mater.
MA23
MA11
MA12
MA22
MA21
Duration (min) Temperature (◦ C) Pressure (GPa)
180 1180 3.5
20 800 19.9
3 1300 15.5
60 1550 17.1
90 1320 18.4
80 1600 20.4
Wt. (%) SiO2 Al2 O3 MgO CaO Total Cation based on 12 oxygens Si Al Mg Ca Total
45.6 (0.5) 22.0 (0.7) 24.1 (0.4) 8.2 (0.3) 100.1
45.0 (0.4) 22.2 (0.4) 23.6 (0.2) 7.9 (0.5)
45.1 (0.4) 21.7 (0.6) 24.1 (0.3) 7.6 (0.4)
46.1 (0.7) 20.9 (1.3) 25.0 (0.7) 7.7 (0.7)
45.5 (0.7) 21.7 (0.9) 24.1 (0.8) 8.0 (0.8)
48.2 (3.1) 18.1 (4.3) 26.0 (2.3) 7.2 (0.8)
98.66
98.49
99.69
99.21
99.42
3.13 (0.04) 1.78 (0.05) 2.47 (0.03) 0.64 (0.03)
3.12 (0.02) 1.82 (0.03) 2.44 (0.03) 0.58 (0.04)
3.14 (0.03) 1.78 (0.05) 2.50 (0.03) 0.57 (0.03)
3.17 (0.05) 1.69 (0.10) 2.56 (0.06) 0.57 (0.06)
3.14 (0.04) 1.76 (0.07) 2.48 (0.08) 0.59 (0.06)
3.31 (0.19) 1.46 (0.36) 2.66 (0.22) 0.53 (0.06)
7.98
7.97
7.98
7.99
7.98
7.96
The numbers in the parentheses are the standard deviation of 10–30 analyses.
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Fig. 3. Back-scattered electron images showing textures of the starting material synthesized at 3.5 GPa and 1180 ◦ C for 180 min heating (A and B), and samples recovered from transformation experiments at various conditions: (C and D) Run MA12, 17.1 GPa and 1550 ◦ C for 60 min; (E and F) Run MA22, 18.4 GPa and 1320 ◦ C for 90 min; (G and H) Run MA21, 20.4 GPa and 1600 ◦ C for 80 min. At relatively high temperature conditions (C, D, G, and H), the volume fraction of garnet grains increased. Pyroxenes decomposed rapidly to their high-pressure phases at high-pressure conditions above the stability limit of pyroxenes (∼17 GPa) (E, F, G, and H). Although it is difficult to quantify the chemical compositions of the decomposed assemblages of pyroxenes (wadsleyite, Ca-perovskite, and stishovite) because of fine grains, the presence of these phases were inferred from the additional elemental mapping images of Ca, Mg, Si, and Al.
occur because the MgSiO3 majorite becomes stable above 16 GPa and 1600 ◦ C (e.g., Gasparik, 1996). We measured the unpolarized infrared absorption spectra of the starting material and the recovered samples using microFTIR to constrain the hydrous conditions of the experiments. The representative infrared spectra of the starting material and the
recovered sample of MA21 are shown in Fig. 4. FTIR spectra of the starting material show a broad absorption band in the range of 2500–3700 cm−1 with a relatively sharp peak around 3450 cm−1 . These features have also been observed in the previous study on cubic garnet (Bolfan-Cassanova et al., 2000). Small absorption peaks around 2900 cm−1 may be derived from acetone treatment
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Fig. 4. Unpolarized infrared spectra of the starting material and the recovered sample of Run MA22 (18.4 GPa and 1320 ◦ C for 90 min heating).
during sample preparation (e.g., Kubo et al., 2004). FTIR spectra of the recovered sample of run MA21 display a sharp absorption band around 3120 cm−1 . This peak is likely derived from the OH vibration of stishovite (Panero et al., 2003). The water content of the starting material and the recovered samples is in the range of 700–4000 wt. ppm H2 O (Table 1). Samples recovered from higher temperature conditions (Run MA12, MA21) had lower water content than the starting material. This result suggests that dehydration occurred mainly above 1550 ◦ C for 60–90 min. Fig. 5 shows the Si numbers and average grain sizes of garnet as a function of temperature. Al3+ ions in the garnet are replaced by M2+ and Si4+ ions (M = Mg + Ca) by the pyroxene–garnet transformation. In the equilibrium state, the Si number of garnet is 3.0 (equal to that of the ideal M3 Al2 Si3 O12 garnet) at low pressures. It increases with pressure above about 5 GPa and remains constant at around 3.5 between 16 and 20 GPa in the stability field of a single phase of majoritic garnet. The temperature dependence on the equilibrium Si content in garnet is relatively small in pyrolitic compositions (Irifune, 1987). Most of the present experiments have been carried out in the stability field of a single phase of majoritic garnet. Small
Fig. 5. Plots of atomic numbers of Si relative to 12 oxygens (A) and the grain sizes (B) of recovered garnets as a function of temperature. The Si number and the average grain size of the starting material garnet (probably containing small inclusions of enstatite) are 3.13 and 6.7 m, respectively (dotted lines). The gray area shows the Si number of equilibrium majoritic garnet (3.4–3.5) in the pyrolite model composition. The numbers indicate the run durations (min).
Fig. 6. Plots of the Si number with the Ca/(Mg + Ca) ratio of majoritic garnets formed in the run of MA21 (solid circles) and of the garnet in the starting material (open squares). Majoritic garnet with high Si number contains larger amounts of enstatite components compared to the equilibrium single phase of majoritic garnet calculated from the bulk composition (solid star sign). The open star sign indicates calculated chemical composition of majoritic garnet assuming coexistence with Ca-perovskite of 6 vol.%.
amounts of pyroxene and Ca-perovskite may coexist with majoritic garnet in equilibrium in the lowest and highest pressure conditions of the present experiments, respectively. However, because of their limited volume fractions, their presence only slightly reduces the Si number to about 3.4. Therefore, the Si number is a useful indicator of the progression of the pyroxene–garnet transformation. Fig. 5 shows the correlation between the Si number and the grain size of garnet. The garnet of the starting material has a relatively high Si number (3.07–3.15) compared to ideal garnet, probably because of the analytical interference by beam-overlapping of the small pyroxene inclusions. The Si number and the grain size increase only at high temperatures of 1550–1600 ◦ C in the time range of 60–90 min. This suggests that the pyroxene–garnet transformation requires a temperature higher than ∼1550 ◦ C in the time and grain scales of the present experimental condition. Intra-grain zonal heterogeneity may exist in garnet, as a result of the slow volume diffusion in garnet. However, we could find no evidence for grain chemical zonation or for a systematic relation between the Si number and the grain size. This is probably because the heterogeneous outer growth rim is limited to within the detection limit of a few m. Dissolution of pyroxene inclusions into garnet would also occur together with the separate grain interactions. Fig. 6 shows the compositional variation of the Si number and the Ca/(Mg + Ca) ratios in majoritic garnet recovered at 20.4 GPa and 1600 ◦ C (Run MA21, solid circles). The constituent minerals (garnet, enstatite and diopside) in the starting material are expressed by the reversed triangle in this figure. The compositions of garnet in this run product lie on the trend line between the original garnet and enstatite. The Si-rich majoritic garnet clearly has a higher Mg component compared to the expected equilibrium single phase of majoritic garnet calculated from the bulk composition (solid star sign). The compositional trend of garnet displayed in Fig. 6 could be interpreted to be the result of a combination of three transformation processes: (1) Pyropic garnet preferentially has absorbed the enstatite components relative to the diopside components immediately after two pyroxenes had transformed to their respective high-pressure phases (wadsleyite + stishovite and Ca-perovskite) at the initial stage of the pyroxene–garnet transformation, due to
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faster atomic diffusion of Mg relative to that of Ca in garnet (e.g., Schwandt et al., 1996; Carlson, 2006; Vielzeuf et al., 2007). (2) Some MgSiO3 majorite grains possibly have been formed from the direct conversion of enstatite, and the expected bimodal compositional distribution could have been smeared out by the reaction with pyropic garnet afterwards. (3) Some Ca-perovskite transformed from pyroxene may have existed without reaction of garnet because a small amount of Ca-perovskite could be a stable phase at this condition (Irifune and Ringwood, 1987). The composition of majoritic garnet shifts to a Mg-rich composition by coexisting the equilibrium Ca-perovskite (∼6 vol.%), as indicated by the open star sign in Fig. 6. In any of these cases, the elementary mechanism in the initial stage of the pyroxene–garnet transformation is likely to be the growth of the pyropic garnet controlled by the diffusion. The pyroxene–garnet transformation requires long-distance diffusion of atoms comparable to the grain size scale. It is known that cation diffusion is slow in garnet (e.g., Carlson, 2006; Vielzeuf et al., 2007). Therefore, it is likely that the transformation is kinetically inhibited by slow chemical diffusion in garnet although the quantitative diffusion kinetics of Si and Al has not been known. Exact rate-controlling process applicable to this transformation remains for future research. The present study demonstrated that the pyroxene–garnet transformation requires a much higher temperature (∼1550 ◦ C) than the olivine to wadsleyite transformation (∼1000 ◦ C) (Kubo et al., 2004), the enstatite to ilmenite transformation (∼1250 ◦ C) (Hogrefe et al., 1994), and the pyroxenes to wadsleyite + stishovite + Ca-perovskite transformation (∼1300 ◦ C of this study) in laboratory time (∼103 s) and grain size (∼10−5 m) scales. The temperatures within the cold subducting plates are estimated to be as low as 600 ◦ C even at a depth of 500 km corresponding to 16 GPa. It has been proposed that metastable olivine survives to depths of 500 km or more in the subducting slabs (Mosenfelder et al., 2001). Our experimental results suggest that the pyroxene–garnet transformation is much more kinetically inhibited than the transformations of olivine to wadsleyite and pyroxenes to their high-pressure phases. Moreover, as pyroxene and garnet coexist with a large amount of olivine and they are separated by olivine grains in the peridotite layer of the slabs, direct reaction between pyroxene and garnet is possibly prevented by the olivine. This also causes a delay in the pyroxene–garnet transformation. Therefore, the nonequilibrium phase of pyropic garnet could be present in the mantle transition zone in the subducting plate. The volume fraction of garnet increases slowly by overgrowth absorbing the surrounding pyroxenes. The remnant pyroxene phases, which kinetically failed to be accommodated in the garnet, transform to their own high-pressure phases while keeping their original chemical compositions. The transformation of pyroxenes (enstatite and diopside) to their high-pressure phases (wadsleyite, stishovite, and Ca-perovskite) could become major transformations in the subducting plate. In the coldest central part of the subducting plates, enstatite may directly transform to ilmenite as suggested by Hogrefe et al. (1994). In these cases, grain size reduction due to the transformation of pyroxenes may lead to rheological weakening of the subducting plates by the same mechanism as that of the olivinespinel transformation (Rubie, 1984; Riedel and Karato, 1997; Karato et al., 2001). This effect would be more important in the basaltic layer because pyroxene and garnet are major constituents. In addition, the survival of pyropic garnets affects the depth of the garnet–perovskite transformation, because the depth increases with an increase in the aluminum content of majoritic garnet (Irifune et al., 1996; Akaogi and Ito, 1999; Kubo and Akaogi, 2000). If the pyropic garnet survives in a metastable form, it then decomposes into four phases of Mg-perovskite, Ca-perovskite, stishovite, and aluminous phase (Oguri et al., 2000). Thus, the metastable min-
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eralogy in the peridotite layer is largely affected by the kinetics of the pyroxene–garnet transformation. The present study shows the importance of slow kinetics of the pyroxene–garnet transformation in the subducting slabs. Further quantitative determination of the kinetic parameters and their temperature dependence are needed for modeling the mineralogy and density of the subducting plate in the transition zone conditions. Moreover, the pyroxene–garnet transformation is a major transformation in the basaltic (MORB) layer. It is very important to investigate the kinetics of this transformation in the MORB composition including FeO and other minor components. Also, it has been suggested that water has great influence on the transformation kinetics (e.g., Hosoya et al., 2005). Although the present experiments were carried out under relatively hydrous conditions (∼700–4000 wt. ppm H2 O), the pyroxene–garnet transformation was very slow and difficult to be completed. It is indispensable to examine the effects of water on the pyroxene–garnet transformation kinetics for better understanding the realistic mineral constitution in the cold subducting plates. Acknowledgements We thank A. Tominaga for assistance with in situ X-ray diffraction experiments and S. Uehara and A. Shimojuku for their helpful discussions and useful comments. We also acknowledge D.C. Rubie, E Ohtani, and two anonymous referees for valuable comments. In situ X-ray diffraction experiments were carried out using the MAXIII system at BL14C of the Photon Factory (Proposal no. 2004G225). This work was supported by a Front Researcher Development program of Kyushu University, and by a Grant-in-Aid for Scientific Research to T.K. from the Japan Society for the Promotion of Science. References Akaogi, M., Akimoto, S., 1977. Pyroxene–garnet solid-solution equilibria in the systems Mg4 Si4 O12 –Mg3 Al2 Si3 O12 and Fe4 SiO12 –Fe3 Al2 Si3 O12 at high pressures and temperatures. Phys. Earth Planet. Inter. 15, 90–106. Akaogi, M., Ito, E., 1999. Calorimetric study on majorite-perovskite transition in the system Mg4 Si4 O12 –Mg3 Al2 Si3 O12 : transition boundaries with positive pressuretemperature slopes. Phys. Earth Planet. Inter. 114, 3–4. Akaogi, M., Yano, M., Tejima, Y., Iijima, M., Kojitani, H., 2004. High-pressure transitions of diopside and wollastonite: phase equilibria and thermochemistry of CaMgSi2 O6 , CaSiO3 and CaSi2 O5 –CaTiSiO5 system. Phys. Earth Planet. Inter. 143, 145–156. Bina, C.R., Stein, S., Marton, F.C., Van Ark, E.M., 2001. Implications of slab mineralogy for subduction dynamics. Phys. Earth Planet. Inter. 127, 51–66. Bolfan-Cassanova, N., Keppler, H., Rubie, D.C., 2000. Water partitioning between nominally anhydrous minerals in the MgO–SiO2 –H2 O system up to 24 GPa: implications for the distribution of water in the Earth’s mantle. Earth Planet. Sci. Lett. 182, 209–221. Carlson, W.D., 2006. Rates of Fe, Mg, Mn, and Ca diffusion in garnet. Am. Miner. 91, 1–11. Gasparik, T., 1996. Melting experiments on the enstatite-diopside join at 70–224 kbar, including the melting of diopside. Contrib. Mineral. Petrol. 124, 139–153. Gasparik, T., 1992. Melting experiments on the enstatite–pyrope join at 80–152 kbar. J. Geophys. Res. 97, 15181–15188. Gasparik, T., 1990. Phase relations in the transition zone. J. Geophys. Res. 95, 15751–15769. Hogrefe, A., Rubie, D.C., Sharp, T.G., Seifert, F., 1994. Metastability of enstatite in deep subducting lithosphere. Nature 372, 351–353. Hosoya, T., Kubo, T., Ohtani, E., Sano, A., Funakoshi, K.-I., 2005. Water controls the fields of metastable olivine in cold subducting slabs. Geophys. Res. Lett. 31, L17305, doi:10.1029/2005GL023398. Irifune, T., 1987. An experimental investigation of the pyroxene–garnet transformation in a pyrolite composition and its bearing on the constitution of the mantle. Phys. Earth Planet. Inter. 45, 324–336. Irifune, T., Ringwood, A.E., 1987. Phase transformations in primitive MORB and pyrolite compositions to 25 GPa and some geophysical implications. In: Manghnani, M.H., Shono, Y. (Eds.), High-Pressure Research in Mineral Physics, Terrapub, Tokyo, pp. 231–242. Irifune, T., Koizumi, T., Ando, J., 1996. An experimental study of the garnet–perovskite transformation in the system MgSiO3 –Mg3 Al2 Si3 O12 . Phys. Earth Planet. Inter. 96, 147–157.
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